Click Here for Full Article STELLAR ABLATION OF PLANETARY ATMOSPHERES

T. E. Moore1 and J. L. Horwitz2 Received 21 December 2005; revised 22 October 2006; accepted 3 January 2007; published 9 August 2007.

[1] We review observations and theories of the solar electric fields and by convection pickup processes also known ablation of planetary atmospheres, focusing on the as ‘‘centrifugal acceleration.’’ We consider also the global terrestrial case where a large magnetosphere holds off the circulation of ionospheric plasmas within the magnetosphere, solar wind, so that there is little direct atmospheric impact, their participation in magnetospheric disturbances as absorbers but also couples the solar wind electromagnetically to the of momentum and energy, and their ultimate loss from the auroral zones. We consider the photothermal escape flows magnetosphere into the downstream solar wind, loading known as the polar wind or refilling flows, the enhanced reconnection processes that occur at high altitudes near the mass flux escape flows that result from localized solar wind magnetospheric boundaries. We consider the role of planetary energy dissipation in the auroral zones, and the resultant magnetization and the accumulating evidence of stellar ablation enhanced neutral atom escape flows. We term these latter of extrasolar planetary atmospheres. Finally, we suggest and two escape flows the ‘‘auroral wind.’’ We review discuss future needs for both the theory and observation of the observations and theories of the heating and acceleration planetary ionospheres and their role in solar wind interactions, to of auroral winds, including energy inputs from precipitating achieve the generality required for a predictive science of the particles, electromagnetic energy flux at magnetohydrodynamic coupling of stellar and planetary atmospheres over the full range and plasma wave frequencies, and acceleration by parallel of possible conditions. Citation: Moore, T. E., and J. L. Horwitz (2007), Stellar ablation of planetary atmospheres, Rev. Geophys., 45, RG3002, doi:10.1029/ 2005RG000194.

1. INTRODUCTION [3] Some nomenclature explanations are in order. We refer to the region dominated by terrestrial material [2] This review has a focus upon the terrestrial iono- (‘‘geogenic’’) as the ‘‘geosphere,’’ and its outer interface, sphere and upper atmosphere, and their expansion into beyond which lies mainly solar material, as the ‘‘geopause,’’ geospace and loss from the Earth, owing to energy inputs following Moore and Delcourt [1995]. Solar energy depos- from the Sun, both electromagnetic and mechanical. The ited in the geosphere, particularly the third (gas) and fourth term ‘‘geospace’’ is used here in the sense suggested by the (plasma) geospheres, produces expansion of the gas and NSF Geospace Environment Modeling (GEM) Program, to plasma, initially producing what we term as ‘‘upflow’’ encompass the coupled atmosphere, ionosphere, magneto- against gravitational confinement. With sufficient energy, sphere, and heliosphere. Over the past 40 years, the Sun- upflow becomes ‘‘outflow,’’ escaping gravity, but the plasma Earth system has been the subject of observations and is still trapped in the magnetosphere. When geospheric theoretical analyses of increasing sophistication, far beyond plasma finds its way to the outer magnetospheric boundary what has been possible for the other planets of our solar and escapes downstream in the solar wind, we call it system, or what will be possible for planets of other astro- ‘‘ablation’’ because material has then been completely spheres in the foreseeable future. Geospace studies are removed from the Earth and can never return, given the directly applicable to other planets, solar and extrasolar, supersonic expansion of the solar wind to fill the heliosphere. and to the Earth during early solar system formation, or [4] Planets and their early atmospheres are thought to geomagnetic reversals. These studies will contribute to our condense out of nebulae in parallel with their central stars preparedness for space exploration, as well as our under- [Jeans, 1902]. By the time stellar ignition occurs, however, standing of our home planet in space. In addition, the results condensation competes with the process of nebular and of geospace studies are essential to advancing the frontiers atmospheric ablation, powered by an enhanced early stellar of space weather prediction. wind thought to prevail during the T-Tauri phase for stars in the size class of our Sun [Balick, 1987]. Volatiles may be acquired by planets through impacts of long-period comets 1NASA Goddard Space Flight Center, Greenbelt, Maryland, USA. [Chyba et al., 1990], but ablation continues and may 2 Department of Physics, University of Texas at Arlington, Arlington, eventually dominate for planets sufficiently close to their Texas, USA.

Copyright 2007 by the American Geophysical Union. Reviews of Geophysics, 45, RG3002 / 2007 1of34 8755-1209/07/2005RG000194$15.00 Paper number 2005RG000194 RG3002 RG3002 Moore and Horwitz: ABLATION OF ATMOSPHERES RG3002

Figure 1. Noon-midnight cross section of ionospherelower magnetosphere summarizing phenomena involved in the heating and escape of ionospheric plasmas. After Moore et al. [1999a] with kind permission of Springer Science and Business Media. central star [Vidal-Madjar et al., 2004]. The terrestrial charged particles are accelerated and in turn generate the planets of our solar system all bear evidence of long-term aurora [Ergun et al., 1998]. atmospheric evolution to which solar photon or solar wind [6] This high-latitude ionosphere-magnetosphere region ablation or erosion must have contributed. Mercury barely is one of the richest and most interesting regions within the has an exosphere [Hunten et al., 1988]. Venus’ atmosphere space plasma universe that is accessible to direct measure- has been desiccated of water [Kasting, 1988]. Mars has lost ments. The many energetic processes occurring there con- most of an atmosphere that increasingly appears to have tribute to the expansion and escape of the ionosphere contained substantial water vapor [Squyres and Kasting, beyond the outer regions of geospace. Figure 1 shows a 1994]. Earth is unique among these planets in the amount of schematic illustration of the range of processes within the water that it acquired and retains. Intriguingly, of these ionosphere and lower magnetosphere that propel these out- rocky inner planets, the Earth has by far the strongest flows upward and outward. planetary magnetic field [Stevenson et al., 1983]. [7] As depicted by Figure 1, ionospheric outflows [5] The presence of a magnetic field has a strong influ- emerge from the high-latitude ionospherelower magneto- ence on the interaction between a planet and the solar wind sphere generation regions on magnetic field lines that guide [Stern and Ness, 1982], and thus the global distribution of these flows into magnetospheric regions such as the tail energy dissipated into atmospheric gases by the solar wind. lobes and the plasma sheet, as shown in Figure 2. If these A planet without an intrinsic magnetic field might seem ionospheric plasmas remained on polar lobe field lines, more exposed to atmospheric ablation, with direct impact of which are ultimately connected to the interplanetary mag- the solar wind on the entire upper atmosphere. However, as netic field, they would be lost from the magnetosphere. we will discuss further below, the net loss may be compa- However, these plasma outflows join the convective circu- rable with or without a magnetic field, because its presence lation of plasma in the magnetosphere as they expand out of concentrates electro-mechanical energy dissipation at the the ionosphere proper. Hence these emerging ionospheric planetary end of flux tubes that link with the boundary layer plasmas flow across the magnetic field into the closed field between the solar wind and its magnetic field and the line region of the plasma sheet, which carries the current planetary magnetic field and its enclosed plasma. This is responsible for the stretched magnetotail. Ionospheric flows the region known as the auroral zone, where strong electri- from the nightside auroral regions are injected directly into cal currents link the solar wind to the ionosphere, and the plasma sheet, and since the plasma sheet itself is the

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Figure 2. Global circulation of plasmas in Earth’s magnetosphere, in the noon-midnight meridian. After Hultqvist et al. [1999] with kind permission of Springer Science and Business Media. primary source of plasmas for the quasi-trapped hot plasmas hybrid, broadband, and ion cyclotron waves, whereas the of the inner magnetosphere, these ionospheric plasmas beam ion distributions are often caused by parallel electric ultimately populate the ring current in considerable quantities fields, either in quasi-DC or wave or impulsive forms. Also, as well. An ongoing controversy for the magnetospheric in addition to the light (H+, He+) ions and heavier, often + + + + community, particularly since the provocative article of dominant O , ions, at times, molecular ions (NO ,N2,O2) Chappell et al. [1987], has been the question of how much are observed in the 1000- to 4000-km regions over the of the plasma density and/or pressure within various regions auroral zones and the cusp, as well as over subauroral ion of the magnetosphere originates in the ionosphere, as drifts [e.g., Peterson et al., 1994; Wilson and Craven, 1998, opposed to the other principal plasma source, the solar 1999]. wind. [9] The overall process of generating ionospheric out- [8] Figure 1 portrays several of the driving processes flows often occurs in a multistage sequence, particularly in and features of ionospheric upflow and outflows in a noon- terms of the escape of normally gravitationally bound midnight cross section of the high-latitude ionospherelower heavier ions such as O+. One set of processes operates magnetosphere region. Various types of ion conics and within and just above the principal ionospheric production beams [e.g., Øieroset et al., 1999], rings [Moore et al., region, at altitudes from 300 to 1000 km, to either produce 1986], electron beams and conics [e.g., Menietti and ionization or propel initial upflows, or both. Frictional Weimer, 1998], counterstreaming and modulated field- or Joule heating (FH in Figure 1), caused by the cross field aligned electrons [e.g., Carlson et al., 1998; McFadden et E B drift of ionospheric ions through the neutral al., 1998], and other highly non-Maxwellian particle distri- atmosphere at these altitudes, enhances the ion pressure bution functions have been consistently observed, often and produces an initial upflow [e.g., Wahlund et al., 1992; in conjunction with a variety of wave and strong field Korosmezey et al., 1992; Ho et al., 1997]. Soft electron disturbances [e.g., Hirahara et al., 1998; Norqvist et al., precipitation produces both ionization and collisional heat- 1998; Vaivads et al., 1999]. The ion conics and rings are ing of thermal electrons, which increases the local ambipo- often the results of perpendicular heating or acceleration of lar electric field and lifts the ions upward [e.g., Wahlund et ionospheric ions by various types of waves, including lower al., 1992; Seo et al., 1997; Su et al., 1999]. The wave-

3of34 RG3002 Moore and Horwitz: ABLATION OF ATMOSPHERES RG3002 particle driven ion heating, parallel electric field, and other promising remote energetic neutral atom imaging results processes occurring at altitudes from 1000 km upward from the IMAGE spacecraft, in parallel with strides in [Andre´etal., 1998] then supply the additional energy simulation techniques and applications utilizing fluid and needed, particularly for O+ to escape the Earth’s gravita- kinetic and combined fluid-kinetic computer codes over one tional well (about 10 eV). Wu et al. [1999] has systemat- or more dimensions and different timescales. The aim of ically investigated the effects of the different stage effects this review is to present this progress within the organiza- on overall ion outflows, focusing on soft electron precipi- tional context described above. The last article within tation effects in the lower (<1000 km) ionospheric region, Reviews of Geophysics that covered the topical purview of and ion cyclotron wave–based ion heating above 1000 km the present review appears to be one by Ganguli [1996], altitude. which had a focus primarily on the polar wind. Therefore [10] During the 1960s, ionospheric expansion into upper much of the detailed emphasis in the present review will be geospace was viewed in terms of the so-called (classical) on progress on understanding ionospheric expansion into polar wind [Dessler and Michel, 1966; Axford, 1968; Banks geospace since 1996, and with more emphasis on the and Holzer, 1968, 1969]. This classical polar wind outflow auroral wind, as defined above. was predicted as a light ion (H+ and He+) outflow along open magnetic field lines threading the polar cap, in which 2. GEOSPACE MODELING IMPLICATIONS the principal upward driving forces are the plasma pressure gradients and the ambipolar electric field sustained by [13] The Heliophysics research community aspires to charge balance requirements involving the light electrons incorporate its understanding of space plasma physics into and the heavy, gravitationally bound majority ion species, theoretical models and global simulations [Winglee, 1998; + O . The first definitive measurements of this classical light Lyon et al., 2004; Siscoe et al., 2002b; Gombosi et al., ion polar wind outflow were obtained by a polar orbiting 2003; Raeder et al., 1997; Ashour-Abdalla et al., 1997]. satellite above 1000 km altitude in the early 1970s [Hoffman The preference is clearly for simulations that run faster than et al., 1974]. However, as a result of satellite measurements real time on available computer hardware, leading to of ions during the 1970s, the importance of outflows driven limitations that are gradually relaxed as hardware capabil- by the stronger auroral processes, including the wave- ities develop. Criticism of global models tends to focus on particle interactions and parallel electric fields mentioned the lack of kinetic or microphysical dissipative processes in above, became apparent. Furthermore, the term ‘‘polar critical current carrying regions best exemplified by the wind’’ has become somewhat imprecise in community reconnection diffusion zone. Practical computer codes are usage. Some authors include outflows driven by auroral subject to numerical effects that give rise to diffusive processes within the polar wind purview, while others tend processes, independent of the true microphysics. A promi- to confine use of the term mostly to the ‘‘classical,’’ thermal, nent objective of the code developers is to answer these nonauroral processes and their natural extensions within the criticisms through reduction of the numerical effects to a paradigm arising from the analyses of Banks and Holzer as level that is negligible compared with the true physical suggested by Axford. dissipative effects or their implementation in code. This is [11] In view of the above, we will thus organize the first an important goal and deserves all the attention it receives. portion of this review into the physics considerations on two [14] However, there is another assumption that has tradi- relatively distinct classes of ion expansion into geospace. tionally been made to obtain practical global simulation We introduce the term ‘‘photothermal outflows’’ to describe codes. This is an assumption that the dissipative medium of those outflows driven chiefly by photon energy from the terrestrial (ionospheric) plasmas is confined exclusively Sun. We introduce the term ‘‘auroral wind’’ for those within a thin layer of scale height less than 100 km, close outflows driven by solar wind mechanical energy. Essen- to the Earth. This thin layer is generally taken to lie tially, the ‘‘photothermal outflows’’ will be congruent with completely outside the global simulation space and is what many refer to as the ‘‘classical polar wind,’’ while the lumped into the boundary condition. From the perspective ‘‘induced outflows’’ will be mostly consistent with ‘‘auroral of electro-dynamic coupling, this is equivalent to assuming wind.’’ The remainder of the review will describe the that all solar wind-driven current systems close outside the morphologies and larger-scale physics involved in high- simulation space, or that the magnetosphere is exclusively latitude ‘‘fountain’’ and ‘‘plume’’ flows, the relationship of an electro-dynamic dynamo and transmission system with the ionospheric expansion in terms of (lower) boundary substantial dissipative or inertial elements limited to the conditions set on the dynamics of the magnetosphere, ionospheric thin layer, beyond the system boundaries. desirable directions for future theory and measurements Exceptions may include the bow shock region where shock investigations, and discussion and conclusions. heating accompanies compression, and the magnetosheath, [12] Progress in compiling and understanding the char- where the shocked solar wind is reaccelerated downstream. acteristics and significance of ionospheric expansion into Not surprisingly, in view of this assumption, an outstanding geospace has advanced in the past decade or so chiefly problem of global simulations is their inability to produce through ongoing in situ measurements by sophisticated sufficient plasma pressure in the inner magnetosphere hot particle instrumentation on such polar-orbiting spacecraft plasma, or ring current region. as Akebono, Polar, Freja, FAST, and Cluster, augmented by

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3 2 [15] However, the ‘‘thin shell’’ assumption leads to a (1 10 J/s-m ) greatly exceeds the mechanical energy flux number of problems. One fundamental problem is perhaps of its stellar wind (3 1017 eV/s-m2 or 5 102 J/s-m2). that codes based on this assumption can never include the Thus the predominant interaction between the Sun and Earth plasmasphere because its ionospheric origin violates the is the deposition of the photon energy. However, the vast assumption of a thin shell ionosphere. The plasmasphere majority of that is unabsorbed until it reaches the ground. is an important region of the inner magnetosphere that The UV component of the photon flux is absorbed in the was discovered early in the space age [Carpenter, 1963; upper atmosphere and creates the ionosphere, maintaining its Carpenter et al., 1993]. Recent advances in imaging of temperature at a few thousand K in sunlight, by removing space plasmas have shown that the plasmasphere is a electrons from atmospheric atoms to create a partially dynamic participant in magnetospheric plasma circulation, ionized plasma. This plasma is substantially warmer than producing global-scale drainage plumes that supply plasma the parent gases from which it is created and hence contains to the dayside subsolar magnetopause region [Sandel et al., a larger fraction of particles with enough energy to escape 2001; Goldstein et al., 2002, 2003a], and to the polar cap from the Earth’s gravity. Photothermal escape of neutral gas topside ionosphere [Foster et al., 2002]. It is clear from (that is, neutral escape which is driven by the photon flux these recent developments that the ionosphere, via the absorbed by the upper atmosphere) is for Earth limited plasmasphere, is at times an important source of plasma mainly to the lightest species, H and to some degree He. mass density to the dayside magnetosphere. Given this new The same is true of the plasma species H+ and He+,the validation of global ionospheric circulation at high altitudes, escape of which is called ‘‘polar wind,’’ following the a credible global simulation must evidently include these nomenclature of Axford [1968] and Banks and Holzer plasmas to get the bulk characteristics of the magnetosphere [1968, 1969]. correct. The minimum requirement to include them is that plasma be allowed to flow across the boundary between the 3.1. Jeans Escape thin shell ionosphere into the magnetosphere at large. [18] ‘‘Jeans’’ escape [Jeans, 1902] refers to the escape of [16] As important as the photothermal polar wind out- an atmosphere from a gravitating body, owing simply to the flows may be, the auroral outflows at latitudes higher than thermal motions of the gas atoms or molecules. Calculations that of the plasmapause have greater potential impact upon of the expected escape as a function of the temperature of magnetospheric dynamics, given sufficient available energy, the gas have been made [Chamberlain and Hunten, 1987]. owing to their much higher escape flux limits and more Loss of the more energetic particles into space constitutes a massive ions. As solar wind mechanical energy is much heat loss from the atmosphere, which must clearly be more variable than the solar photon flux, so is the auroral balanced by other energy sources, such as incoming radia- wind much more strongly modulated than is the polar wind. tion or upward heat conduction from below (normally The area from which auroral wind is emitted is substantially negligible or even negative). smaller than the area outside the plasmasphere that emits [19] Analogous escape of ions is also expected from an light ion polar wind, but it is substantially more productive ionosphere or partially ionized atmosphere. The simplest of mass flux than the polar wind regions, when the solar possible ionosphere is formed when ultraviolet radiation wind interaction is strong. Owing to recent observational from a nearby star partially ionizes an upper atmosphere, and theoretical advances, it is now becoming feasible to giving rise to electron-ion pairs, and creating a plasma that relax the assumption of a thin shell ionosphere in global is far from thermodynamic equilibrium, reflecting the pres- magnetospheric simulations either by allowing flow of a ence of a significant flux of photons with spectrum that single plasma fluid into the system from the ionosphere, or extends above the electron binding energy of the gaseous by introducing a separate ionospheric fluid (or fluids). We atoms. Where the photoelectron energy is partly thermalized can distinguish the outflows beyond the thin shell into by collisions with ions, the ion gas may be hotter than the ‘‘photothermal’’ and ‘‘auroral wind’’ categories. We further neutral gas, and hence have a larger than expected kinetic consider and summarize the global outflow structures that escape rate, under this simple model. The enhancement of result from interactions between structured outflows and mass flux is attributable to the energy flux of photons magnetospheric circulation. Then we consider the role of absorbed by the gas in the processes of ionization and planetary magnetization, and the evidence that extrasolar thermalization. planets also experience atmospheric ablation by their stellar 3.2. Polar Wind and Plasmasphere photon flux and plasma winds. Finally, we summarize the [20] In practice, ionization that occurs in the presence of a outstanding problems, new observations required, and the- magnetic field produces a more complex situation than that oretical developments that will allow us to incorporate our described by Jeans escape. Moreover, the presence of the new knowledge into our global simulations to further solar wind in addition to the solar photon flux complicates improve their predictive capability and generality. the situation substantially. Initial exploration of the Earth’s magnetosphere revealed the presence of an extended iono- 3. PHOTOTHERMAL OUTFLOWS sphere within the inner magnetosphere, and of plasma circulation throughout the outer magnetosphere. Soon, the [17] Our Sun is at a stage of its evolution during which implications for ionospheric escape flows were appreciated the electromagnetic energy flux of its photon emission

5of34 RG3002 Moore and Horwitz: ABLATION OF ATMOSPHERES RG3002 and a theory of polar outflows was outlined and subse- zero-pressure upper boundary condition, introducing light quently developed [Dessler and Michel, 1966; Axford, ion plasma to the flux tube at a rate limited by the available 1968; Banks and Holzer, 1968, 1969]. This theory, as source of plasma. The net result is a very low density, further developed and reviewed by others [Raitt et al., supersonic flux of cold light ions through the polar caps and 1978; Schunk, 1988; Gombosi et al., 1985; Ganguli, into the magnetospheric lobes. This outflow corresponds 1996; Moore et al., 1999a], has been confirmed in most closely to the polar wind and has been observed from respects [Abe et al., 1996; Su et al., 1998b; Drakou et al., ionospheric topside heights [Brinton et al., 1971; Hoffman 1997; Huddleston et al., 2005], the main exception being et al., 1974; Hoffman and Dodson, 1980] up through that the initial theory made no account of auroral zone altitudes around 1 RE [Abe et al., 1993a, 1993b, 1996], energetic phenomena that affect the nature of the iono- around 3–4 RE [Nagai et al., 1984; Olsen et al., 1986; Su et spheric outflows. In recent studies of the polar wind, global al., 1998b], and at altitudes up to 9.5 RE [Moore et al., circulation has been considered [Demars and Schunk, 1997; Su et al., 1998b; Liemohn et al., 2005], extending into 2002], and complex structures have been shown to result the lobes of the magnetosphere. However, as has been noted that cannot be resolved well using statistical surveys of by many of these authors, the cold polar wind is often spacecraft data. accompanied by comparable or larger fluxes of O+ ions, [21] Because plasma electrons and ions are linked by even under conditions of low solar activity. The high- electrostatic forces, they are not free to separate and are altitude, cold supersonic outflows into the magnetotail, required to maintain very nearly equal densities by an including O+ as well as light ions, have recently been ambipolar electric field that swiftly forms if they begin to studied as the ‘‘lobal wind’’ [Liemohn et al., 2005]. separate. Since electrons are so much faster and more [24] The polar wind has recently been considered as a mobile than ions, and are negligibly bound by gravity, the three-dimensional system, in the context of global iono- topside ionosphere forms an approximately radially directed spheric convection, with Joule heating and auroral electron ambipolar electric field that holds down the electrons, precipitation effects on the ambipolar electric field [Demars exerting a balancing upward force on the ions. The strength and Schunk, 2002; Coley et al., 2003]. A complex three- of the field depends on the mass of the dominant ion dimensional picture emerges of light ion outflow variations, species. When the field is strong enough to hold O+ in especially in velocity. Moreover, heavy ion ‘‘breathing’’ equilibrium, it is strong enough to accelerate lighter ions motions inflate and deflate the topside ionosphere with upward strongly, and this is an important correction to the mixed upflows and downflows, in response to these pro- theory of the light ion polar wind. Such ambipolar electric cesses. Particularly in the case of the heavy O+, what goes fields are important wherever differential transport of elec- up in regions of Joule heating or precipitation for the most trons and ions tend to make them separate because of either part falls back into the ionosphere in the polar cap, or gravity or inertia. Moreover, the ambipolar field links subauroral regions that are downstream of the aurora in the electrons and ions such that heating of either species adds ionospheric circulation pattern. This leads to complex energy to the system and enhances escape of both species as patterns of counterstreaming in which the light ions are an ambipolar plasma, though gravitational stratification by flowing upward in the same region where O+ downflows are mass will occur. occurring. The authors cite good agreement with observa- + [22] Within high-latitude magnetospheric flows, as par- tional studies of O downflow [Chandler et al.,1991; tially depicted in Figure 2, plasmas undergo a circulation Chandler, 1995]. cycle whose timescale ranges from diurnal to much shorter [25] During the relaxation part of the flux tube convec- than diurnal, depending on the strength of the solar wind tion cycle, stretched flux tubes contract through the mag- interaction. As specified by Alfve´n’s [1942] theorem, mag- netotail toward the Earth. Disconnected flux tubes also netized plasmas move on large scales as elastic ‘‘flux tubes’’ reconnect there and circulate back toward the dayside linked along their length by magnetic field lines of force. subsolar region. However, these flux tubes do not pass During the course of this circulation cycle, a plasma flux through the inner magnetosphere, except during conditions tube is first stretched from 10 RE to >100 RE in half-length of strong magnetospheric circulation. When the interplane- as it is convected antisunward, or it may be disconnected tary magnetic field (IMF) tends northward, convection from the conjugate hemisphere and reconnected into the stagnates in the inner magnetosphere, while the polar wind solar wind plasma during part of the cycle, permitting direct escapes downstream as flux tubes are peeled off the plasma exchange between the two media. Later, after magnetospheric lobes by high-latitude reconnection. This reconnection in the tail (if disconnection has occurred), is illustrated in Figure 3 (left), from a global simulation that the flux tube relaxes back to its starting point. During each tracks ionospheric fluids in addition to the solar wind fluid circulation cycle, the flux tube volume changes by a factor [Winglee, 1998]. The surface plotted in Figure 3 bounds the on the order of 104. region dominated by polar and auroral wind plasmas [23] During the stretch part of the cycle, or when the flux and excludes the region dominated by solar plasmas. tube is connected to the downstream solar wind (which Polar wind outflow at lower, stagnant latitudes, closer to evacuates rather than filling the flux tube), ionospheric the Earth, continues until large densities are accumulated plasma expands freely into the flux tube as if there were a (up to >1000 cm3), forming the region of extended iono-

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Figure 3. A simulated configuration of ionospheric plasmas in the magnetosphere [Winglee, 1998]. The highlighted surface represents the limits of the region within which the plasma is predominantly of ionosphere origin, regardless of its absolute density, that is, the geopause of Moore and Delcourt [1995]. sphere known as the plasmasphere. In Figure 3 (left), region, is eroded to smaller size during the strong circula- intermediate latitudes are dominated by solar wind plasmas tion driven by southward IMF. that circulate inward from the low-latitude boundary layers in this case. 3.3. Plasmasphere Filling, Erosion, and Refilling [27] The basic paradigm is that the outer portion of the [26]Fornormal(BZ 0, where Z points northward normal to the ecliptic plane) or southward IMF, as shown dense plasmasphere is eroded away by enhanced magneto- in Figure 3 (middle and right), magnetospheric circulation is spheric circulation and then refills via upward ionospheric excited by a combination of dayside and nightside recon- plasma flow during the recovery phase of magnetospheric nection that grows increasingly stronger with more south- storms. This conceptual framework dates from the seminal ward IMF, penetrating deeper into the inner magnetosphere papers by Nishida [1966] and Brice [1967]. In this picture, a and eroding the outer parts of the dense plasmasphere (see period of enhanced magnetospheric convection during a also section 3.3). Polar wind outflows are convected into storm’s main phase causes the boundary between closed the plasma sheet and then sunward, replacing the denser (Earth-encircling) flux tube trajectories and circulating flux plasmaspheric material being removed toward the subsolar tube trajectories to shrink. The plasma on the flux tubes that magnetosphere. The surface separating solar and geogenic are on open trajectories (in which the flux tubes link with plasmas, the geopause [Moore and Delcourt, 1995], the interplanetary magnetic field at some point during the changes in shape and grows in size with increasing mag- cycle) may then vent to the interplanetary medium, resulting netospheric convection. In contrast, the classically defined in low densities. In steady state conditions, this boundary plasmapause, which bounds the high-density light ion

Figure 4. Images of the plasmasphere in scattered He+ resonant sunlight [Goldstein et al., 2003a], illustrating the observed and modeled plasmasphere as it is eroded by a period of enhanced magnetospheric convection, from left to right. Sun is to the right.

7of34 RG3002 Moore and Horwitz: ABLATION OF ATMOSPHERES RG3002 coincides with the density gradient known as the plasma- low-altitude shocks stem from the same physics as the pause [Carpenter, 1963]. At the onset of the storm recovery equatorial thermalized plasma slab formation in the equa- phase, when the intensity of magnetospheric convection is torial region for the single-stream formulation. reduced, there is an outer region of initially low density [30] An alternative to these single-stream and two-stream between the quiet and active closed/open flux tube trajec- hydrodynamic approaches is to use a kinetic treatment, in tory boundaries. This outer region of newly closed or Earth- which the ions are treated as particles. Wilson et al. [1992] encircling trajectories is subject to plasma upflow from the considered a refilling flux tube simulation in which ions ionosphere below on the same field lines. The reality of this were injected above the conjugate ionospheres onto an description was confirmed when the IMAGE mission made initially empty flux tube. The electrons were treated as a possible global imaging of the plasmasphere, showing that Boltzmann neutralizing fluid. No shocks were formed in the it expands during quiet times and is then eroded by ensuing development, but rather a more gradual isotropiza- convection, producing pronounced sunward extending tion of the counterstreaming polar wind flows, caused plumes, as well as other dynamic features, as illustrated in primarily by small-angle Coulomb collisions, until eventu- Figure 4. ally a diffusive equilibrium condition was achieved. Under [28] A particularly influential theoretical paper on how the same kinetic framework, the effect of equatorially the refilling could proceed was that of Banks et al. [1971]. concentrated ion heating perpendicular to the magnetic field They considered a single-stream hydrodynamic model treat- by ion cyclotron waves was considered by Lin et al. [1992, ment in which equal supersonic polar wind streams from 1994]. These processes, in combination with a presumption conjugate ionospheres flowed onto initially empty flux of a heated, field-aligned or isotropic electron component tubes and interacted at the magnetic equator. The interaction around the equator, produced an electric potential peak at in that treatment led to a ‘‘slab’’ of thermalized denser the magnetic equator of order 1 V, an electric field directed plasma at the equator, whose edges were shock fronts which away from the magnetic equator. Since the refilling flows propagated back down the field lines toward the ionosphere. from each conjugate ionospheric source region had (polar When these shock front slab edges reach the flow critical wind) energies below this potential level, this led to a points near the topside ionosphere, the overall flow character ‘‘hemispheric decoupling’’ in the refilling. That is, the ion changes to subsonic flow throughout. The flow proceeds streams in each hemisphere reflected off this potential layer through this second, subsonic flow stage until the flux tube back toward the ionospheric end, to be either isotropized via density distribution attains diffusive equilibrium or is inter- collisions, mirrored, or lost to the atmosphere below. These rupted by a new storm or gust of enhanced magnetospheric simulation results were in accord with observations about convection. the magnetic equator on L 4.5 flux tubes by Dynamics [29] One of the principal issues with this scenario concerns Explorer 1 (DE-1) [Olsen et al., 1987]. the single-stream restriction and the interaction between the [31] Other kinetic aspects of refilling that have been opposing ionospheric flows. In the above picture, the studied with similar models include velocity dispersion modeling requirement that the ion plasma be described as [Miller et al., 1993], other ion heating by waves [Singh, a single stream everywhere, with a single bulk velocity (as 1996], and further examination of reflection of incoming well as temperature and density), necessarily forces a ionospheric polar wind streams by potentials created stationary slab around the equator as a consequence of the through resulting differential ion-electron anisotropies interaction. However, as Banks et al. [1971] acknowledged, [Liemohn et al., 1999]. However, a serious limitation with the densities of the interacting streams would be sufficiently these kinetic-focused studies is that the altitudinal boundary low that such a strong interaction should not occur purely on conditions typically involved presumed plasma injection the basis of Coulomb collisions. Other processes, such as into the simulation regions at topside altitudes (generally wave-particle interactions, would be required in order to 10002000 km). Thus there has been no true self-consistent sustain such an interaction. This issue led to treatment by coupling of the ionospheric plasma and energy sources and two-stream models [e.g., Rasmussen and Schunk, 1988]. losses and chemistry to be synergistic with the intriguing Here the streams originating from the Northern and Southern kinetic aspects of these simulations. Hemispheres are treated as identifiably separate populations. [32] There are a number of intriguing features of the outer Owing to the low collision frequencies involved, the plasmasphere and trough regions that have been observed streams typically would pass through each other largely by spacecraft but are not presently understood, and probably undisturbed as interpenetrating streams, until each would require incorporation of high-altitude kinetic processes and interact with the conjugate ionosphere. Each stream would self-consistent coupling of ionospheric processes to model then reflect from that conjugate ionosphere, but would now and understand quantitatively. For example, in the Horwitz create a shock front that would propagate back up the field et al. [1984] DE-1 examination of plasmaspheric refilling line toward the equator. The reverse shock in this frame- during the recovery of a magnetospheric storm, it was found work was presumably a consequence of the fact that above that O+ densities were greatly elevated and actually the topside ionospheres of each hemisphere, the incident appeared to be temporarily the dominant ion species at stream and its reflection were required by the treatment to mid magnetic latitude refilling plasmasphere. Horwitz et al. be considered as a single, largely stationary stream. The [1984] also showed that the O+ in this event was flowing

8of34 RG3002 Moore and Horwitz: ABLATION OF ATMOSPHERES RG3002 along the field lines out of the northern ionosphere. It is [36] The IMAGE spacecraft, launched in 2000, opened a possible that such brief strong impulsive O+ upward flows new window into understanding the large-scale dynamic may represent the incipient formation of the heavy ion response of the plasmasphere to magnetospheric electric density enhancements characteristically observed in the field changes. For example, Goldstein et al. [2003b] exam- outer plasmasphere in the vicinity of steep plasmapause ined observations by the IMAGE EUV imager that showed density gradients and associated with enhanced ion and erosion of the nightside plasmasphere occurring in two electron temperatures at both ionospheric and plasmaspheric bursts during one interval in July 2000, and indicated that altitudes [e.g., Horwitz et al., 1986; Roberts et al., 1987; these erosion bursts were correlated with solar wind south- Horwitz et al., 1990]. ward turnings of about 30 min prior (after correcting for [33] Another intriguing feature is the apparent distribution transit time from observing satellite). Foster et al. [2002] of trapped light ions and heavy ion density enhancements on examined total electron content (TEC) measurements from outer plasmaspheric field lines. Olsen et al. [1987] showed GPS satellites in conjunction with Millstone Hill incoherent that perpendicularly peaked fluxes of light ions (H+ and He+) scatter radar observations of storm-associated ionospheric were predominantly observed by DE-1 within about 10° of electron densities during IMAGE EUV observations of the magnetic equator on L = 3 4.6 flux tubes. Craven plasmasphere erosion and formation of elongated plasma- [1993], on the other hand, showed that DE-1 measured heavy spheric tails formed by convection of the main plasma- N+ ion fluxes were detectable only at midlatitudes. Although sphere, including the dusk bulge region. Foster et al. [2002] O+ fluxes were not specifically represented, N+ is typically a found that enhanced electron densities in the incoherent ‘‘proxy’’ for O+ with N+/O+ density ratios typically around scatter radar and TEC measurements corresponded to 0.10 in the plasmasphere [Craven et al., 1995]. The combi- regions of these plasmaspheric plumes mapped down along nation of these and other observations may suggest that magnetic field lines into the ionosphere. the heavy ion density enhancements occur at midlatitudes and the light ion ‘‘pancake’’ or trapped distributions occur 4. AURORAL WIND around the equator, on field lines threading the ring current-plasmasphere overlap region near the plasmapause, [37] The photon flux of the sun is uniformly distributed possibly also related to SAR arc types of phenomena [e.g., over the dayside of the Earth, and the mechanical energy Horwitz et al., 1986, 1990]. flux of the solar wind is similarly distributed over the [34] Other species-dependent behavior on outer plasma- boundary of the magnetosphere. However, that mechanical spheretrough flux tubes has been observed, particularly energy is by no means uniformly transmitted to the upper + for He , which is especially significant in view of the atmosphere. Ring-like concentrations of energy input to the + remote large-scale plasmasphere He observations by the atmosphere exist in the regions of bright auroras, known as magnetospheric IMAGE mission via the 30.4-nm EUV the auroral ovals. Indeed, the auroral ovals can usefully be + scattering line of He , and others bearing on this issue, thought of as the footprint of the magnetospheric boundary including the refilling process, for example by Yoshikawa et layer in the ionosphere, defined by the conduction of al. [2003]. Chandler and Chappell [1986] examined the electrical currents connecting the boundary layer with the + + field-aligned flow velocities of He and H from DE-1 ionosphere. The largest energy inputs to the atmosphere observations in the plasmasphere, and found instances when occur in these twin halos roughly centered on each magnetic + + the H and He were counterstreaming toward opposite pole. These mark the cleft between magnetic field lines that hemispheres. Anderson and Fuselier [1994] examined low- connect directly between hemispheres and those which have + + energy H and He in the presence of electromagnetic ion been recently reconnected to the solar wind or have been cyclotron waves, in the dayside morning trough region with swept back into the magnetotail by earlier connection to the AMPTE observations. They found that ‘‘X’’ distributions solar wind, and may still be so connected. Plasmas + (or bidirectional conics) of He were observed near the connected to different parts of the auroral ovals are sub- magnetic equator with energies around 35 eV on average, stantially separated in space and time and may have quite + while colocated H perpendicularly peaked had temper- different histories, so the ovals are not monolithic. They atures of 5 eV. Anderson and Fuselier [1994] argued that may be characterized by quite different behavior at any + these He ‘‘X’’ distributions were gyroresonantly heated by given instant, their dynamics revealing to some degree the the EMIC waves at off-equatorial latitudes. different dynamics going on upstream, downstream, at the [35] Still further intriguing features reported by Olsen north or south polar regions, or on the dawn or dusk side of and Boardsen [1992] were not only interesting density the Earth, in the solar wind and boundary layers. Neverthe- minima near the magnetic equator, but also latitudinally less, the energy fluxes delivered to the ionosphere are asymmetrical densities around the magnetic equator, on outer substantial. A small fraction of those energy fluxes, in the plasmasphere field lines. If such latitudinal asymmetries in form of thermal energy of the electrons and ions, is the densities do exist, it is possible they could involve the sufficient to greatly enhance the escape flux of plasma into above-noted electrostatic hemispheric decoupling [Lin et al., space, even for the heavier ions O+,N+, and at times also + + + 1994] of asymmetric Northern and Southern Hemisphere NO ,O2, and N2 [Wilson and Craven, 1999]. As noted ionospheric flows. above, this heavy ion outflow (with accelerated light on ion outflows) will herein be referred to as the ‘‘auroral wind.’’

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In this section we consider the detailed causes and con- Though originally based on the subsonic flow regime of sequences of the auroral wind. plasmaspheric filling, in steady state, this concept describes a real limitation on the average rate at which ions of a 4.1. Overview specific species can flow upward out of the ionosphere. [38] While photothermal outflows originate from the Higher transient fluxes are possible but cannot be sustained, entire sunlit globe, only the lighter ion species have signif- so a longer term limit is ultimately enforced. The limit icant escape fluxes at Earth, since only they have thermal results from a limit either on how rapidly the species in energies exceeding their escape energy. The gravitational question is created through ion-neutral chemistry or a limit escape energy for the C-N-O group is of order several eV, on the rate at which a species can diffuse through the and a negligible fraction of these ions can escape for typical frictional medium of other species or neutral gas atoms ionospheric temperatures. Thus the photothermal polar wind [Barakat et al., 1987]. For current purposes, it is sufficient contains relatively small amounts of these heavy ions, to note that H+ in the polar wind is limited by typical owing to gravitational confinement. ionospheric conditions to a flux on the order of 2 [39] The auroral zones are defined as regions in which 108 cm2 s1 (at 1000 km altitude). On the other hand, the solar wind energy dissipation is frequent and substantial, flux of O+ has a substantially higher limit on the order of often leading to auroral light emissions. This is equivalent 3 109 cm2 s1, depending somewhat on local conditions. to saying that the auroral zones are magnetically conjugate [43] In summary, depending on the amount of energy with the boundary layers and associated region-1 field- available in either imposed magnetospheric convection aligned current system, along and inside the magnetopause, (Poynting or electromagnetic energy flux), or imposed or with the hot plasmas created inside the magnetosphere, electron precipitation (heat flux), auroral wind fluxes of and their associated region-2 field-aligned current system. heavy ions will range from negligible to more than an order For our purposes, the auroral zones represent a localized of magnitude greater than polar wind fluxes. In this section, source energy deposited in the topside ionosphere. This we consider the various types of energy and their contribu- energy comes in various forms, with the net effect of tions to localized heavy ion escape from the ionosphere. enhancing ionospheric expansion, outflow and escape, or in short, ablation. 4.2. Polar Rain Precipitation [40] The altitude distribution of solar wind energy dissi- [44] One of the most basic sources of energy to the pation is also important and relevant. The theory of astro- ionosphere, erroneously cited by most encyclopedia physical winds [Leer and Holzer, 1980] states that energy accounts as the cause of the aurora, is the direct ‘‘polar added below the supersonic outflow transition (critical rain’’ of solar particles entering along reconnected magnetic height) increases the escaping mass flux, while energy field lines linking the Earth and the solar wind [Gosling et added above the critical height increases the velocity of al., 2004]. The ion polar rain is confined largely to the outflow without increasing the mass flux. We thus anticipate cusp proper, where freshly injected magnetosheath plasma this same effect in the auroral wind, with energy added low penetrates quite low and precipitates into the conjugate down producing enhanced mass flux, while energy added ionosphere, where it can be imaged as proton aurora, and higher up will mainly accelerate the flux determined lower has been by the IMAGE mission [Frey et al., 2002]. Most down. However, some level of high-altitude acceleration such solar wind ions are mirrored and reflected back into the may be required to sustain escape, turning upflow into boundary layers, forming the plasma mantle flow along the outflow. flanks of the magnetosphere, following the convective flow [41] Two types of low-altitude heating have been identi- of plasma along the magnetopause. fied as relevant to ionospheric upflows [Banks and Holzer, [45] Solar wind thermal electrons, though much faster 1968, 1969; Wahlund et al., 1992]. First, ion heating by than the thermal ions, are required to stay with those ions frictional collisions with neutral gas is an important heating unless replaced by addition of electrons from the Earth. A mechanism in the topside F layer. It is often referred to as subset of them is able to move along reconnected field lines ‘‘Joule heating’’ even though that term is usually reserved all the way to the ionosphere, barring adverse electrostatic for the heating of current conduction electrons by electric potentials. The region of polar rain can include much of the fields. In the ionosphere, horizontal currents are carried polar cap when linked to the upstream solar wind, because largely by ions, owing to their demagnetization by colli- the solar wind electron gas often includes a highly field sions with neutrals, so this may be appropriate for that aligned, superthermal component known as the ‘‘strahl’’ medium. Second, electron heating by the impact of precip- [Fitzenreiter et al., 1998], the presence of which will itating energetic auroral electrons is a substantial source of enhance the energy flux of electrons precipitating into the energy for the ionospheric electron gas. Owing to electro- atmosphere in the polar regions. static forces, heating of the electron gas is just as effective in [46] Polar rain precipitation has been noted and studied producing expansion and upflow as heating of the ion gas, [Fairfield and Scudder, 1985], but it does not appear to be but clearly has a different signature when observed, for associated with geophysically significant ionospheric out- example, by upward looking ionospheric radars. flows, at least in the observations obtained to date. It is [42] Another important concept for ionospheric outflows possible that polar rain precipitation is responsible for is that of limiting flux [Hanson and Patterson,1963]. certain O+ outflows that have been suggested to originate

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Figure 5. Electron precipitation as a driver of outflows via electron heating. (left) Ion outflow velocities in the auroral topside ionosphere (850950 km altitude) from DE-2 versus electron temperature. Straight line fits and correlation coefficients are based on binned data. (middle) Ion upflow velocities versus average energy of soft electron precipitation. (right) Ion upflow fluxes versus average energy of soft electron precipitation. From Seo et al. [1997].

from the polar cap region rather than from the auroral zones [49] Modeling of specific auroral ionospheric upflow/ [Tam et al., 1995, 1998], but this needs more study. outflow observations in which effects of soft electron precipitation were incorporated was performed by Liu et 4.3. Auroral Precipitation al. [1995] and Caton et al. [1996], while Su et al. [1998a] [47] A major driver for F region/topside upward flows in performed systematic fluid-based ionospheric modeling of the topside ionosphere appears to be the effect of soft the effects of electron precipitation on upflows by examining auroral electron precipitation. Seo et al. [1997] examined the effects of a range of electron precipitation parameters on field-aligned flows at topside ionospheric altitudes ionospheric parameters, such as the upflows versus time (850950 km) from Dynamics Explorer 2 (DE-2) and found and altitude, the electron and ion temperatures. Wu et al. strong correlations between the upward flows observed in the [1999, 2002] incorporated the effects of soft electron auroral zones and elevated electron temperatures, as indicated precipitation into a fluid-kinetic transport model that exam- in Figure 5. They also found significant correlations of the ined the synergistic effects of soft electron precipitation and electron temperatures themselves with the soft electron transverse ion heating on the transport of ionospheric plasma precipitation energy fluxes, and anticorrelation with the through the auroral ionosphere-magnetosphere region. characteristic or average energy of the precipitating electrons as indicated in Figure 5. Moreover, they found that unusually 4.4. Auroral Heating 10 2 large upward ion fluxes exceeding 10 ions/cm -s were only [50] Transverse ion heating in the auroral zone was observed during periods when the average soft electron observed well before high-altitude observations of the polar precipitation energy was less than 80 eV, or ‘‘ultrasoft’’ wind were made [Sharp et al., 1977; Whalen et al., 1978; precipitation cases (Figure 5). Klumpar, 1979]. This is owed at least in part to the fact that [48] These results were interpreted by Seo et al. [1997] as it is relatively easier to observe, as directional hot tails of the indicative that soft electron precipitation played a signifi- ionospheric energy distribution, extending readily up to tens cant role in driving upward F-region/topside ionospheric of eV and often up to hundreds of eV or even several keV, plasma flows. The principal ionospheric effects of the depending on event intensity and instrument sensitivity soft electrons in this regard were to directly enhance the [Hultqvist et al., 1991; Yau and Andre´, 1997; Andre´etal., F-region ionization and to heat the thermal electrons, either 1998; Norqvist et al., 1998]. via Coulomb collisions or more indirectly through wave- [51] The basic observation of transversely accelerated particle processes. Elevating the ionospheric electron ions (TAI) clearly implies energy gain principally transverse temperatures increases the local ambipolar electric field to to the local magnetic field. This was often envisioned as lift the ionospheric ions. The fact that unusually high taking place in a narrow altitude range, and it was assumed ionospheric ion upward fluxes were observed only during that the subsequent behavior of the ions would be essen- periods when the average electron precipitation energy was tially that of a free particle gyrating and mirroring up out very low is consistent with the concept that ultrasoft of the divergent magnetic field of the auroral zone, leading precipitation is most efficient at both ionization and heating to a ‘‘conical’’ distribution whose cone angle would vary at upper F-region and topside altitudes. Higher-energy with altitude according to the conservation of the first precipitation penetrates to lower ionospheric altitudes, in adiabatic invariant and total energy. However, it soon which ionization tends to remain in local equilibrium and became apparent that transverse heating is in general not diffuse upward.

11 of 34 RG3002 Moore and Horwitz: ABLATION OF ATMOSPHERES RG3002 widely distributed along auroral flux tubes in altitude [e.g., with theoretical predictions, which predict preferential Moore et al., 1999a]. heavy ion acceleration, can be understood through a version [52] Transverse heating also occurs through multiple of the ‘‘pressure cooker’’ mechanism. With simple single different physical processes. At the lowest altitudes, in the particle computations, Lund et al. [1998] showed that for a F region, the dominant mechanism is ion-neutral friction as significant downward electric field within the transverse magnetospheric motions drag the ionized plasma through heating region, ions of different masses are energized to the embedded neutral gas. The collisions become infrequent approximately the same energy as required to escape the relative to the gyro frequency at some altitude, with charge downward electric field ion trapping region. exchange interactions persisting to the greatest heights. [56] It seems likely that all these processes contribute to Subsequent to a collision, the scattered or newly formed the inferred wide altitude range of auroral ion transverse ion is picked up by the convection electric field, imparting heating, beginning in the F region with frictional heating, a perpendicular energy that exceeds the ion thermal extending through the topside with EMIC and LHR wave energy for strong convection. Transverse ion heating is heating of ions, with the latter extending to altitudes of the relatively straightforward result, but it is limited to several RE. All of the processes derive energy from the energies corresponding to a few km/s or a few eV for O+ dissipation of MHD energy in the form of medium frequency ions, and much less for H+ [e.g., Moore et al., 1996]. (approximately cyclotron) waves or turbulence, including [53] Plasma waves with substantial power in the electro- the auroral current systems and wave driven by them. magnetic ion cyclotron range (EMIC waves) are frequently observed in the auroral ionosphere, with frequencies well 4.5. E// Acceleration above those associated with MHD instabilities. These are [57] Parallel electric fields occur in the auroral region in thought to result from turbulent cascades of MHD wave both upward and downward current regions. Their presence energy, or to be associated with free energy of auroral and significance for auroral acceleration had been discussed current systems, or possibly with the occurrence of certain for many years, certainly in Scandinavian perspectives, and types of ion anisotropy, for example ion shell distributions. evidence for their existence had been found in rocket [e.g., EMIC waves or broadband extreme low frequency (BBELF) Evans, 1974] and satellite [e.g., Frank and Gurnett, 1971] waves have sufficient power in the cyclotron range to raise electron measurements. However, the first systematic and transverse tails on the ion velocity distributions and are relatively conclusive measurements of such parallel electric thought to do so frequently. Some of these are Alfve´n waves fields probably came from the measurements in the mid- propagating into the ionosphere along flux tubes, which are 1970s by the S3-3 spacecraft near 1 RE altitude, an altitude thought to be effective in heating ionospheric ions, and have region hitherto largely unsampled. Direct measurements of been associated with the most productive region of nightside parallel electric fields [Mozer et al., 1977] and upgoing ion outflows at the poleward boundary of the midnight auroral beams [Shelley et al., 1976] confirmed the existence of oval [Tung et al., 2001; Keiling et al., 2001]. These low- strong parallel electric fields somewhere below 6000 km frequency waves are thought to be generated by boundary altitude in the auroral zones. Moreover, in addition to the layer instabilities or other sources of turbulent motions, such upward parallel electric field observations, associated with as reconnection. upward current regions and generally linked with discrete [54] At higher plasma wave frequencies, ion transverse auroral arcs, S3-3 observations also indicated evidence of motions are though to be coupled with parallel electron downward electric fields having important effects ion out- motions by means of lower hybrid waves [Retterer et al., flows by trapping ions in transverse heating regions for 1994; Kintner et al., 1992]. These waves are not thought extended periods of time and thus leading to higher trans- capable of influencing the core of the ion velocity distribu- verse energization [e.g., Gorney et al., 1985]. tions, but are thought to create or extend transverse tails of [58] An example of a recent simulation investigation the distribution to energies well above thermal. The waves which sought to elicit the effects of parallel electric fields are thought to be current-driven, deriving energy from the on ionospheric ion outflows is the work of Wu et al. [2002], parallel motion of current-carrying electrons. mentioned earlier. As noted above, Wu et al. [1999] used a [55] Recent observations from the FAST Mission have Dynamic Fluid Kinetic (DyFK) code to simulate the effects added many details to our knowledge of auroral transverse of soft electron precipitation and transverse ion heating on ion heating. These include events in which He+ ions are ionospheric outflows. Wu et al. [2002] added to these resonantly accelerated perpendicular to the magnetic field to influences a distributed field-aligned potential drop pro- energies of several keV [Lund et al., 1998]. These He+ duced self-consistently by a high-altitude (magnetospheric) energizations occur during periods of EMIC waves and population of hot ion and electron populations. When conic angular distributions of up to a few hundred eV in H+ the presumed upper boundary magnetospheric population and a few keV in O+. This was identified by Lund et al. had significant differential anisotropy between the ion [1998] as a process similar to that which may occur in solar distribution and the electron distribution, significant parallel flares. Lund et al. [1999] also examined FAST observations potential drops resulted. When the upper boundary ion indicating that for ion conics produced by interactions with population was more perpendicularly peaked than the BBELF waves, the energies of different ion species are electron distribution, such as a trapped type of ion distri- often nearly equal. They suggested that this discrepancy bution with a more isotropic electron distribution, this led to

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Figure 6. Observed relationship between ionospheric outflow flux in the dayside auroral zone and the hourly standard deviation of the solar wind dynamic pressure [Fok et al., 2005].

a positive potential peak and downward directed parallel [60] More recently, instruments aboard the FAST space- electric fields. When the upper distribution of magneto- craft have been used to observe effects of parallel electric spheric electrons was more perpendicularly peaked than that fields on ion beams and other ion outflows. For example, of the magnetospheric ions, an upward electric field distri- McFadden et al. [1998] exploited very high time resolution bution resulted. In both cases, the upward flow of iono- measurements of ion distributions to show kilometer-scale spheric plasma into these hot plasma regions gradually spatial structures, which involve ‘‘fingers’’ of parallel neutralized and reduced the potentials over timescales of electric potential structures which extend hundreds of kilo- the order 30 min. meters along the magnetic field line but are only a few to [59] In the case of the hot plasma differential anisotropy tens of kilometers wide. McFadden et al. [1998] were able circumstances associated with upward electric fields, these to show that integrations of the electric field along the fields of course accelerated the ionospheric ions, which spacecraft track to calculate the parallel potential below the were first transversely heated above the topside by trans- spacecraft were typically in reasonable agreement with verse waves, to parallel beam energies of the order of the observed ion energy. In another paper based on FAST 100200eVofenergy.Inthecaseofthehotplasma measurements in the cusp region, Pfaff et al. [1998] found a differential anisotropies leading to downward electric fields, complex mixture of DC electric fields and plasma waves in various types of ionospheric distributions were produced, association with observed particle accelerations as mani- including classical conics as well as ‘‘butterfly’’ distribu- fested by upgoing ion beams, ion conics, and related electron tions, in which occurred simultaneous upward and down- distributions. In a particular case, injections of magneto- ward conical distributions, at times with ‘‘holes’’ or sheath ions were associated with upgoing ion beams and absences of particles at the low-energy part of phase space. electrons going earthward, indicative of potential structures These holes presumably were the result of exclusion of set up by the magnetosheath injections. ionospheric ion access by the positive potential barrier. [61] In another result from FAST, Ergun et al. [2000] Some observations of toroidal or quasibutterfly distributions analyzed data from the upward current region of the auroral which were observed by POLAR [e.g., Huddleston et al., zone, which inspired Vlasov simulations of the static 2000] could possibly be interpreted along such lines, particle distributions and potential structures. From although other alternative explanations might be involved the observed electron distributions, they inferred density as well. While anisotropies clearly require parallel potential cavities, and regions in which the densities were dominated drops, it is field-aligned currents that are thought to by either plasma sheet electrons or cold ionospheric elec- ultimately determine their necessity [e.g., Lyons, 1980]. trons. Ergun et al. [2000] conducted Vlasov calculations with one-spatial and two-velocity dimensions to obtain

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Figure 7. A flowchart depicting the relationships between energy inputs to the ionosphere and ionospheric ion outflows detected by FAST near 4000 km altitude. From Strangeway et al. [2005]. large-scale self-consistent solutions of the parallel electric clearly suffer from a large amount of scatter indicating that field. Depending on the ionospheric H+ density level and multiple processes mediate the interaction with the solar other parameters, the electric potential along the magnetic wind. Any attempt to relate global outflow with typical field lines shows three different potential and plasma upstream solar wind conditions begs the question of how population regions, and includes distinct electron and ion auroral wind outflows respond to the detailed local distri- ‘‘transition’’ layers. Such potential distributions could then bution of energy inputs into the topside ionosphere gas, as be responsible for the upward ion beams emerging from determined by magnetospheric global dynamics [Andersson these types of regions. et al., 2004]. Only when such detailed outflows can be specified as functions of global dynamics can a realistic 4.6. Specifying Global Outflows global simulation be constructed. Even then, the solar wind [62] To compute and appreciate their consequences for is far from uniform across the magnetopause and the system the solar wind interaction with Earth, as well as their responds to prior history as well as instantaneous condi- ultimate loss into the downstream solar wind, it is essential tions, and no solar wind sequence is ever repeated exactly. to specify how these outflows are driven by that interaction. [63] A significant advance in our ability to observe global Prior to the advent of upstream solar wind observations, patterns and time variability of ionospheric outflows and efforts focused on the dependence of outflows upon atmospheric escape came with the IMAGE mission and its internal magnetic indices such as Kp (planetary activity), neutral atom imagers, including especially the Low Energy AE (auroral electrojet), or Dst (storm time disturbance) Neutral Atom (LENA) Imager [Moore et al., 2000]. LENA [Yau and Lockwood, 1988]. The results were usually stated provides a new capability to both remotely sense ion plasma in statistical terms of global aggregate outflow (ions/s), heating and acceleration, and to observe the escape of fast sometimes treating the Earth as a point source, but usually neutral atoms from the atmosphere [Moore et al., 2003]. sorted by longitude and latitude [e.g., Giles et al., 1994]. Using LENA it was confirmed that heavy ion outflows are a More recently, outflows have been regarded as directly prompt response to solar wind dynamic pressure enhance- driven by solar wind parameters [Pollock et al.,1990; ments [Fuselier et al., 2001, 2002; Khan et al., 2003]. It was Moore et al., 1999b; Cully et al., 2003a; Lennartsson et also confirmed that the creation of outflows is strongly al., 2004]. It has become clear from these studies, as shown related to internal geomagnetic activity of the magneto- in Figure 6, that the dynamic pressure of the solar wind is a sphere [Wilson and Moore, 2005]. A hot neutral oxygen very strong driver of outflows, particularly from the dayside exosphere was shown to be created by auroral activity auroral zone where the solar wind interaction is quite direct. [Wilson et al., 2003]. The related outflows of fast neutral Somewhat surprisingly, the IMF Bz appears to be more atoms were initially surprisingly intense [Moore et al., weakly correlated with auroral wind outflows [Pollock et 2003], but recent theoretical work suggests that this is al., 1990]. While illuminating, these observed relationships

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Figure 8. (top) Hemispheric distribution of NeHot, Poynting energy flux, and parallel current density, mapped to ionospheric heights in the Northern Hemisphere, for a snapshot of the LFM MHD simulation of the magnetosphere during SBZ. (bottom) The hemispheric distribution of ion Tperp, V//, and escape flux corresponding to the conditions in the top row (simulation results courtesy of S. Slinker (personal communication, 2005); ionospheric outflow conditions computed per Strangeway et al. [2005]; R. J. Strangeway (private communication, 2005) for ETH). expected when ion acceleration begins sufficiently low in However, the soft electron precipitation and the Poynting the ionosphere [Gardner and Schunk, 2004]. flux appear to have somewhat separable effects on the [64] Recently, Strangeway et al. [2000, 2005] examined outflows. If so, this relationship is consistent with the ionospheric outflows near 4000 km altitude using the FAST concept of ‘‘two-stage’’ processes driving auroral iono- spacecraft in order to understand the factors that drive the spheric outflows, as modeled for instance by Wu et al. outflows. Zheng et al. [2005] performed a similar study for [1999, 2002]. In this paradigm, auroral outflows consist the Polar data set. Both found high correlations between the typically first of ‘‘upwelling’’ in the F-region and topside logarithms of the observed outflow ion fluxes and both ionosphere (driving the mass density of outflows), followed the density of the soft electron precipitation as well as by stronger energization at higher altitudes (driving the the electromagnetic energy flux. Both concluded that the velocity and therefore escape of outflows). The processes principal effects were Poynting flux and electron precipita- that are thought to drive ‘‘upwelling’’ at low altitudes, such tion inputs in the low ionosphere, which would be broadly as soft electron precipitation-driven ionospheric electron consistent with incoherent scatter radar observations by heating and Joule/frictional ion heating, may propel signif- Wahlund et al. [1992] and others, and the DyFK simulations icant fluxes to higher altitudes but only to relatively small of Wu et al. [1999]. A summary ‘‘flowchart’’ of their energies (typically below 1 eV), which are too small alone findings is indicated in Figure 7. to cause O+ escape. Thus a second energization stage, for [65] There is a strong internal correlation between the instance involving wave-driven transverse ion heating pro- precipitation and Poynting energy fluxes themselves, and cesses, at altitudes down extending toward 1000 km or even therefore it is somewhat difficult to separate their effects on lower, is then required to produce significant fluxes of the outflows purely from such statistical observational escaping O+ ions that could overcome the gravitational methods. Recently, Moore et al. [2007] and Zeng and barrier, approximately 10 eV from the Earth’s surface, and Horwitz [2007] concluded that both are necessary at some attain magnetospheric altitudes. level for outflow to occur. Owing to the transit times of ions [66] Another important factor in the acceleration of ions from F-region/topside ionosphere to the observation alti- to magnetospheric energies and altitudes is the parallel tude, as the observation altitude increases, spatiotemporal electric field, generally considered to be significant in the discrepancies between responsible energy inputs and out- altitude range of 1 RE, as discussed above. From a global flow plumes are likely to be more significant. Such transit magnetospheric perspective, E// is a relatively low altitude time–produced uncertainties may well have reduced possi- influence on the properties of outflowing auroral ions. E// is ble correlations, especially at Polar altitudes of 5000 km. thought to be determined largely by the sense and magni-

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outflows was introduced by Horwitz [1987a], while the basic effect itself was identified by Cladis [1986], Mauk [1986], and Delcourt et al. [1988] as a curvature drift. The effect may be described [e.g., Horwitz, 1987b] as similar to the acceleration of a bead moving easily on a rotating stiff wire, in which the bead is flung radially outward along the wire as a result of the rotation. In the case of diverging magnetic field lines of the Earth, convection can be viewed as a quasirotation of charged particles along these field lines, again effectively accelerating them outward toward higher altitudes in the magnetosphere. [69] Several authors have included and investigated the significance of this effect on high-latitude and magneto- spheric particle motion. For example, Horwitz et al. [1994] examined the possible effect of centrifugal acceleration on the polar wind in a kinetic ionospheric transport simulation, and found that such acceleration can in principle explain observed polar electron density profiles and elevated O+ densities for moderate convection speeds. However, Demars et al. [1996] calculated that the effect of centrifugal + Figure 9. Velocities of H+,He+, and O+ during a substorm acceleration on the polar wind H may be slight for normal dipolarization event observed by DE-1 in the middle convection values. Centrifugal acceleration has been incor- magnetosphere [Liu et al., 1994]. porated in numerous single particle trajectory studies of ionospheric and other particle flow into and through the tude of the local parallel or Birkeland current density [e.g., magnetosphere [e.g., Delcourt et al., 1990, 1994]. Also, the Lyons, 1980; Carlson et al., 1998]. High-altitude flow shear one current global MHD model of magnetospheric dynamics (or vorticity) producing J// away from the ionosphere, and [Winglee, 1998; Winglee et al., 2002] that incorporates the exceeding a threshold current density that is easily supplied effects of ionospheric plasma outflow into the magneto- by low-density magnetospheric electrons, implies parallel spheric dynamics at present relies chiefly on centrifugal potential drop that increases with the current density. In acceleration as the agent for energization of ionospheric contrast, when the current J// flows toward the ionosphere, it plasma to be ejected into the magnetosphere. is relatively easily carried by ionospheric electrons, and [70] Arguably the most compelling discrete observational implies quite small potential drops. evidence for centrifugal acceleration of ionospheric + + [67] In Figure 8, we exhibit the inner boundary condi- ions comes from the observations of energized H ,He, tions relevant to auroral wind outflows, mapped to iono- and O+ ions in the middle magnetosphere during substorm spheric heights, from the LFM simulation as a snapshot ‘‘dipolarization’’ events [Liu et al., 1994], as illustrated in during an interval of southward IMF. In Figure 8 (bottom), Figure 9. Liu et al. [1994] observed energized ionospheric we have translated these boundary conditions into ion ion species having similar parallel (and convection) veloci- outflow conditions using relations based on those of ties, and thus characteristic energies roughly in the proportion Strangeway et al. [2005] and Zheng et al. [2005]. Here H+:He+:O+ = 1:4:16. This is a telltale characteristic of we have assumed that the total ion flux has superposed centrifugal acceleration and should be observed when it contributions from the Poynting energy flux and from the dominates other processes. hot electron precipitation density, but a geometric mean has 5.2. Fountain Flows proven more realistic [Moore et al., 2007; Zeng and Horwitz 2007]. We have taken the Poynting flux to drive the ion [71] The concept of a localized fountain of outflowing temperature, and we have taken the parallel current density ionospheric ions ejected into a horizontal wind of convec- to drive the parallel ion velocity according to the modified tion was introduced in the form of the ‘‘cleft ion fountain’’ Knight relationship of Lyons [1981]. It can be seen that the (CIF) during the mid-1980s [Lockwood et al., 1985; result is highly structured outflow distribution, in both flux Horwitz and Lockwood, 1985]. These formulations were and velocity, and it can readily be appreciated that this based on observations of low-energy ionospheric ions from pattern is highly variable in response to system dynamics as DE-1 in the region of the cleft and on into the polar cap simulated by a global MHD simulation. lower-intermediate altitude magnetosphere. Observations and corresponding simulations depicted a cleft-associated source of upward moving ions that spread into the polar 5. GLOBAL CIRCULATION cap magnetosphere under the influence of antisunward 5.1. Centrifugal Acceleration (or sunward) convection. [72] Among the interesting phenomena observed and [68] The terminology ‘‘centrifugal acceleration’’ for simulated were the effects of separation of ion species, convection-driven acceleration of high-latitude ionospheric 16 of 34 RG3002 Moore and Horwitz: ABLATION OF ATMOSPHERES RG3002

Figure 10. Simulated variations of the O+ densities and velocities along a noon-midnight oriented convection trajectory with the horizontal distance and altitude. Vertical bars on the bottom x axis mark the locations of the equatorward and poleward boundaries of the cusp/cleft region. (Reprinted/modified from Tu et al. [2005], with permission from Elsevier.) termed the ‘‘geophysical mass spectrometer’’ [Moore et al., 1994] Ashour-Abdalla et al. [1992], Peroomian [2003], and 1985], or ‘‘geomagnetic mass spectrometer’’ [Lockwood et Fok et al. [1999], seeking to account for observed beams al., 1985] and ‘‘parabolic’’ heavy ion flows [e.g., Horwitz, and other specific ionospheric ion distributions in the larger 1984, 1986; Horwitz and Lockwood, 1985]. The picture of a magnetosphere and also under the influence of centrifugal ‘‘fountain’’ followed from the discovery of the geophysical acceleration and related effects. An example from a recent mass spectrometer effect, in which midaltitude spacecraft effort [Tu et al., 2005] is used to visualize the auroral observe, in sequence ‘‘downstream’’ from the cleft source, plasma fountain in Figure 10. + + + first H , then He , and then O as the spacecraft moves [74] One of the ongoing issues concerning ‘‘fountain antisunward from near cleft threading magnetic field lines flows’’ is the question of whether or not the cleft ion into the polar cap region. This results because, for similar fountain, or more generally the auroral fountain, is the thermal energies, light H+ ions have larger field-aligned dominant source of observed O+ in the polar cap magneto- velocities than do heavier He+ and O+, while for all energies sphere. Additional observations of frequent downward and all species the horizontal convection velocity is the flows of O+ in the main polar cap under southward IMF same. Thus the faster H+ ions tend to remain closer to the conditions [e.g., Chandler, 1995] have led others [e.g., source field lines, while the slower O+ ions are transported Horwitz and Moore, 1997] to at least tentatively conclude further into the polar cap by convection during the transit that indeed the cleft ion fountain, and not the polar cap time required to attain similar spacecraft altitudes. ‘‘Para- ionosphere, must be the source of the O+ in the polar cap bolic’’ flows, involving upward and then downward motion magnetosphere proper, at least during southward IMF when of O+ ions with energies below the gravitational escape the polar cap is not normally threaded by transpolar arcs that barrier, were indicated in the trajectory modeling of Horwitz might be sources of energized upflowing O+. More recently, [1984], and the downward O+ flows in the polar cap were papers by Tu et al. [2004, 2005] have demonstrated that observed near 1 RE altitude by Lockwood et al. [1985]. sophisticated ionospheric plasma transport modeling based [73] Soon thereafter, Chappell et al. [1987] incorporated on the CIF paradigm can produce close agreement the cleft ion fountain concept, together with polar wind and with observed polar cap density profiles and from observed ‘‘auroral ion fountain’’ sources, to consider whether circu- O+ field-aligned velocity and density profiles seen near lation of ionospheric origin plasma into the magnetosphere 5000 altitude from cross-polar spacecraft passes. However, could account for the observed magnetospheric plasmas. other authors [e.g., Abe et al., 2004] maintain that the solar- They concluded that indeed this was the case and in illuminated polar cap ionosphere can directly supply the O+ particular that the ionosphere was a ‘‘fully adequate source’’ observed in the midaltitude polar cap magnetosphere, for the plasma sheet. Several other efforts have employed through effects such as photo-electron acceleration of the large-scale trajectory models to track ionospheric particles polar wind and/or elevated effective electron temperatures through the magnetosphere, including Delcourt et al. [1989,

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(such as through effects of the polar rain [e.g., Barakat and transformed into bidirectional interpenetrating streams from Schunk, 1983]). the two lobes as they convect onto closed field lines. These [75] As indicated previously, the idea that photo-electron bidirectional streams are isotropized and strongly heated effects can accelerate relatively low altitude heavy iono- when the neutral sheet thins during periods of higher spheric ions to escape energies comes in large part from activity. This behavior is associated with the onset of papers by Tam et al. [1995, 1998], whose modeling calcu- spatially nonadiabatic trajectories as the plasma sheet thins. lations suggested that O+ could attain supersonic speeds Temporal nonadiabatic behavior is also triggered when at altitudes as low as 500 km. However, as also noted reconnection or other effects change the neutral sheet on earlier, these predicted low-altitude photo-electron acceler- gyroperiod timescales for specific ion species. ation effects have never been identified in observations, and other efforts on modeling the photo-electric polar wind by 5.4. Geospace Storms Khazanov et al. [1997], Wilson et al. [1997], and Su et al. [79] Downstream of the midtail plasma sheet, during [1998a] have instead indicated that photo-electron accelera- normal slow magnetospheric convection, lies the magneto- pause along the dawn and dusk flanks of the magneto- tion occurs at relatively high altitudes of 2–3 RE, in con- junction with a thin double layer of tens of volts. Hence we sphere. When dayside reconnection complements nightside maintain that the cleft, or more broadly the auroral, ion reconnection, the plasma circulation is drawn deeply fountain, is the principal source of O+ in the polar cap through the inner magnetosphere toward the subsolar magnetosphere ‘‘proper,’’ that is, on magnetic field lines magnetopause region. Then heated ionospheric and solar not threaded by polar cap/transpolar arcs such as occur during plasmas supply the inner magnetosphere and ring current northward IMF conditions. However, the most powerful region, and a lesser or greater geospace storm results, evidence that photothermal outflow cannot in itself supply depending upon the strength and persistence of these significant heavy ion plasmas at high altitudes is the well- effects, combined with the number and frequency of sub- known light ion dominance of the plasmasphere, a region storm dipolarizations that heat and compress plasma sheet that is supplied almost exclusively by the sunlit low latitude plasmas into the inner magnetosphere. ionosphere. [80] Mitchell et al. [2003] have shown that substorms, in particular, modulate primarily the heavy ion component of 5.3. Lobal Wind ring current hot plasmas, while the proton component is [76] The polar and auroral winds are easily distinguished relatively unresponsive to them. Moore et al. [2005a] have in terms of their causative agents, but they are not so easily argued that this is a natural consequence of the different distinguished when observed from a given point in geo- circulation paths of solar wind and auroral wind contribu- space. Owing to the fountain flow effects discussed in tions to the ring current. In the global test particle simu- section 5.2, ions are dispersed according to their differing lations they performed in MHD fields, solar wind proton parallel velocities, and are thus selected for velocity by entry occurred principally through the low-latitude flank the geophysical mass spectrometer effect. The result is a magnetopause boundary layers, with direct transport into mixture of polar wind and auroral wind that varies substan- the ring current region. In contrast, auroral wind O+ was tially with location within the high-latitude regions of transported via the traditional route through the polar caps magnetospheric circulation, but tends to have a restricted and lobes, into the plasma sheet, and then back to the inner range of parallel velocities in any particular spot [Horwitz et magnetosphere via the midnight plasma sheet. They argued al., 1985]. This effect is accentuated for auroral wind ions that this implies a greater substorm dipolarization effect on that are emitted from a region of relatively narrow extent plasmas of ionospheric origin. Recent work with simulated embedded within magnetospheric convective flow, as substorms has confirmed this conclusion [Fok et al., 2006]. + described above, creating the geophysical mass spectrometer [81] The amount of auroral wind O in the energetic effect. plasmas of the ring current is a direct function of the [77] The resultant mixture flows outward through the magnitude of that current [Sharp et al., 1985, 1999, 1994; magnetospheric lobes, in part supplying the plasma sheet Daglis and Axford, 1996; Daglis, 1997; Moore et al., 2001, with relatively cold source plasmas, and in part escaping 2005b]. Recently, Nose´etal.[2005] have studied the downstream beyond the region convecting earthward. To October 2003 superstorm and reviewed its place in the preserve the distinction between polar and auroral winds, existing observations of storm plasma composition, as we have adopted the term ‘‘lobal wind’’ for the resultant shown in Figure 11. They show that a trend line can be mixed plasmas that flow out of the polar cap into the lobes identified in which the ratio of O+ to H+ pressure increases of the magnetosphere, and then convect into the plasma exponentially with the magnitude of storms as measured by sheet or escape beyond. Dst. The pressure ratio rises from near zero for very small [78] Recently, Liemohn et al. [2005] have surveyed the Dst and passes through unity at Dst 200, making the properties of the lobal wind observed from the Polar origin of the largest geospace storm plasmas primarily spacecraft and determined that it is a pervasive feature of ionospheric. the lobes, providing a continuous supply of cool plasmas to [82] Energetic neutral atoms (ENA) imaging from the the central plasma sheet inside the location of any persistent IMAGE mission has confirmed that the principal mecha- reconnection X line. The unidirectional outward flows are nism of ring current decay is charge exchange production

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Figure 11. Relationship between ring current composition and magnitude of the ring current as measured by Dst or Sym-H (limited Dst). The ‘‘Halloween 2003’’ event appears at the top right of the plot [Nose´etal., 2005]. of fast neutral atoms that immediately escape from the through the innermost magnetosphere. Then plasma built magnetosphere [Burch et al., 2001]. Thus even that part up on outer flux tubes of the plasmasphere is entrained in of the auroral wind outflow that is trapped within the the flow and transported to the dayside magnetopause where magnetosphere and fails to escape to the downstream solar it enters the reconnection regions there, or becomes part of wind eventually manages to escape through charge exchange the low-latitude boundary layer flows. (on cold geocoronal hydrogen) back to a neutral state. Since [85] This process was hypothesized in the late 1960s and the Earth is such a small object within the realm of the ring early 1970s [Grebowsky, 1970], and shown then to create current, a small fraction of such fast atoms will precipitate a plume of plasma leaving the outer plasmasphere and back into the atmosphere, and most of them will escape into moving toward the postnoon sector of the dayside magne- the solar wind. topause. Borovsky et al. [1997] and Elphic et al. [1997] further hypothesized that such transient release of plasma 5.5. Plume Flows from the plasmasphere could create superdense polar wind, [83] The polar wind fills the plasmasphere to pressure and possibly a superdense plasma sheet. Evidence of this equilibrium with the ionosphere and then shuts off in the was observed during events when the magnetopause was stagnant, corotating, inner magnetosphere. Beyond the compressed to geosynchronous orbit [Su et al., 2000, 2001], plasmasphere, magnetospheric circulation carries flux tubes but our understanding of this process was greatly expanded away to the dayside magnetopause and boundary layers. when the IMAGE spacecraft was able to image the forma- Some of these flux tubes are opened up to the solar wind by tion of such plumes [Sandel et al., 2001], permitting dayside reconnection and then dragged downstream through detailed studies of their formation and variability [Goldstein the polar caps or low-latitude boundary layers. Viscosity in et al., 2002]. the ionosphere spreads this motion to neighboring flux [86] More recently, direct observations of cold convecting tubes even if they are not actually opened up by reconnec- plasmas have been obtained at the normal equatorial mag- tion, and these flux tubes are also stretched dramatically as netopause [Chandler and Moore, 2003]. Chen and Moore they flow down the low-latitude boundary layers. [2004] found further that sunward flow bursts approaching [84] Magnetospheric circulation varies greatly in its depth the local Alfve´n speed were observed in these flows, of penetration into the inner magnetosphere, depending on correlated with southward IMF in the upstream solar wind. the synchronization, or lack thereof, between dayside and More recently, statistical studies [Chen and Moore, 2006] nightside reconnection. When reconnection is weak, and have shown that the occurrence and sense of convection circulation is restricted to the outer magnetosphere, the of these flows is fully consistent with global convection plasmasphere grows larger as flux tubes fill with plasma, as driven by dayside reconnection, and its variation with longer timescales for the outer, longer flux tubes at with interplanetary magnetic field, Bz. Figure 12 gives an greater radius. When reconnection increases simultaneously overview of these results in terms of the distribution of cold on the dayside and on the nightside, the night-day plasma convecting plasmas observed in the dayside magnetopause pressure gradient increases and deep convection occurs region.

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Figure 12. Distribution of cold convecting ions at the dayside magnetosphere for (a) northward and (b) southward IMF. After Chen and Moore [2006].

[87] The significance of these plasmas is that they have well described by friction in the topside ionosphere, that is, not been observed systematically before, yet they substan- that no ionospheric energy appears in the magnetosphere tially reduce the Alfve´n speed of the inflow to the low- proper, when the observed fact is that ionospheric pressure latitude reconnection region at the dayside magnetosphere. dominates the largest storms. Freeman et al. [1977] have previously noted that this [89] Nevertheless, the full task is more difficult than it could create a negative feedback on deep magnetospheric appears because heavy ions are nonadiabatic in many convection, whereby reconnection would be slowed if a magnetospheric regions. Even when they are reasonably sufficient amount of ionospheric or plasmaspheric plasma adiabatic, their transport departs substantially from the was convected to the dayside subsolar region. This may be convective paths for cold particles. We must move in the related to recent interest in the saturation of the transpolar direction of accounting for ion drifts and finite gyroradius potential [Siscoe et al., 2002a]. More recently, it has effects, which will ultimately require hybrid simulations, been shown that enhanced densities of plasma, especially with fluid electrons and particle ions. In the meantime, cold protons, will produce essentially a ‘‘cessation of recon- progress is being made in understanding and assessing the nection’’ [Hesse and Birn, 2004]. O+ ions are observed to be character of ionospheric circulation in the magnetosphere part of the hot magnetospheric plasma when arriving at the using global simulation fields to compute the motions of test dayside magnetosphere [McFadden et al., 2003]. This is particles [Winglee, 2003; Peroomian, 2003; Cully et al., consistent with most O+ outflow being driven by auroral 2003b; Moore et al., 2005a, 2005b]. processes, as part of what we have called the auroral wind. [90] In Figure 13, we illustrate this work with an example Such hot plasmas may also load the dayside reconnection result from Cully et al. [2003b]. Here the supply of iono- processes, especially for heavy ions. spheric plasmas to the plasma sheet has been assessed in aggregate as a function of time during two different step 5.6. Global Simulations changes in the IMF, one a northward turning and the second [88] We have learned from observations that ionospheric a southward turning. It can readily be seen that an abrupt plasma permeate the magnetosphere and come to dominate northward turning results in a sharp and relatively prompt even the energetic particle populations during great geo- cutoff in the supply of O+ ions to the plasma sheet. This is space storms. Many of the groups performing global sim- because convection in the lobes stops and reverses direction ulations have begun to modify their codes to take account of in response to this change. In contrast, when the IMF turns ionospheric plasmas as circulating dynamical elements of southward, convection through the lobes is enhanced, but the system, literally above and beyond their role as a enhanced ion outflow delivery to the plasma sheet increases resistive medium in the thin F layer. The most needed gradually and with a delay owing to the transport time of improvements are, first, the specification of ionospheric relatively slow O+ ions from their source regions. outflows in terms of energy inputs and, second, the simple [91] An important point made by Cully et al. [2003b] is inclusion of multiple fluids in global simulations [Winglee, that auroral wind outflow through the lobes is constant and 1998, 2000]. This will slow the calculations of such ongoing, though the magnitude of the flux can vary greatly simulation codes proportionately to the number of fluids. in response to ionospheric energy inputs. Even though the However, we have made the case earlier that it is a serious transit time from the ionosphere to the plasma sheet is long, oversimplification to assume that the ionospheric load is the outflow stream responds instantaneously to changes in

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+ Figure 13. Simulated supply of O to the CPS in the region 30 RE moons. region as a result, with possible implications for substorm These are generally well shielded from direct solar wind dynamics [Lyons et al., 1998]. Cully et al. did not consider impact by the large magnetospheres of these planets. Still, an abrupt compression of the magnetosphere, as would be substantial amounts of energy derived either from the solar produced by a sudden increase in solar wind dynamic wind or the rotation of these large magnetospheric systems pressure. Such compression would likely have an energizing is clearly present, and it generates robust populations of and intensifying effect on the plasma sheet and the auroral energetic particles comparable to or far exceeding the winds feeding it, with no significant transport delay. Simu- intensity of the Van Allen radiation belts at Earth [Bolton lation of this is best done with MHD fields for which the et al., 2004; Jun and Garrett, 2005]. The moons are global response can be calculated self-consistently, as themselves highly diverse, with or without their own reported by Moore et al. [2007]. magnetospheres. As this is written, a handful of flyby [92] In addition to supplying current carriers, the presence missions have briefly explored the gas giant magnetospheric of polar and auroral winds in the magnetosphere also systems in the past, and the Saturnian system is now being represents a moderating load on the system, and this appears explored by the orbiting Cassini Mission. New results are when they are treated as true dynamical elements of the appearing rapidly, so the detailed exploration of gas giants global simulation, as noted by Winglee et al. [2002]. While and their satellite systems is therefore beyond the scope of illustrating the processes of transport and energization that this review. operate on polar and auroral wind plasmas, current single [94] Among the terrestrial planets, only Earth has a particle simulations must in the future be adapted to run planetary magnetic field strong enough to hold the solar within self-consistently calculated global fields so that their wind off at about 10 times the planetary radius for typical responses to dynamical solar wind events can be properly solar wind conditions. Earth is also unique in having assessed. acquired and retained substantial amounts water in all forms. Mercury has a relatively weak magnetic field that 6. COMPARATIVE PLANETARY ABLATION holds the solar wind off the planet for average conditions, but stronger solar winds are directly incident on the planet, [93] It is interesting to compare the situation at Earth with beginning in the magnetic cusps and spreading with our best knowledge of other planets. Within the solar increasing solar wind dynamic pressure [Delcourt et al., system, the gas giant planets with large magnetospheres 2002]. Mars has scattered regions of remnant crustal mag- are so massive and cold that the loss rates of their atmos- netization [Connerney et al., 1999], which will produce pheres are negligible, even though hydrogen dominates features in the solar wind interaction, but do not hold the them, accounting for the tremendous thickness of their solar wind off Mars’ ionosphere. Venus has no measurable atmospheres compared with those of the inner terrestrial magnetodynamo so the solar wind is continuously incident planets. Replete with moons of varying sizes and composi- upon the ionosphere [Luhmann, 1991]. tions, the outer planet magnetospheres tend to be dominated

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[95] Mars and Venus are both are believed to have [99] Given that heavy ion expansion out of the gravita- become deficient in water relative to their earlier states. tional well of the planet is dependent upon auroral levels of Their solar wind interactions have been observed extensively energy flux, it might seem that an unmagnetized Earth [Lundin and Dubinen, 1992; Brace and Kliore, 1991] and would lose only lighter species such as H and He, while are known to involve considerable rates of loss of water the magnetized Earth loses substantial quantities of N, O, products into space. The Hermean solar wind interaction and the diatomic molecules of those species. However, has not been observed since Mariner 10 [Glassmeier, 1997], studies of Mars and Venus [Cravens, 1991; Luhmann and but is about to be revisited by the NASA Messenger Kozyra, 1991; Fok et al., 2004] (the latter based on the Mission. Mercury is believed to lack any significant water MHD simulation of Tanaka and Murawski [1997] with or water product escape and has an exosphere in which embedded aeronomy models) indicate that they suffer sodium is prominent [Killen and Ip, 1999]. nearly as much total heavy ion and heavy atom loss as [96] It is tempting to hypothesize causality in the mag- the Earth does, and more steadily over time. The interplan- netized Earth’s retention of water, but we should first etary magnetic field diffuses into the dayside ionosphere of consider what we have learned about the expansion of Venus and couples energy from the solar wind, even though Earth’s ionosphere into space. Water loss begins with there is no planetary magnetic field. Thus it appears that the photodissociation of water in the upper atmosphere. Photo- solar wind finds ways to carry off as much of a planetary thermal expansion of water products into space is oxidizing atmosphere as is possible (given its momentum flux), for larger planets that trap heavier species. Smaller planets independent of planetary magnetization, though this needs like Mars may be an exception. Solar wind ablation tends to much more investigation. balance the loss of light species, or to be reducing (when oxygen loss exceeds light species loss). Jeans escape and 7. EXTRASOLAR PLANETS photothermal escape via the creation of an ionosphere are common to the inner planets. Mars and Mercury are smaller [100] So far, more than 160 extrasolar planets have been + and can lose heavy species such as O photothermally. identified (J. Schneider, Interactive Extra-Solar Planets Venus (and Earth) loses only lighter species to any signif- Catalog, http://www.obspm.fr/planets, retrieved on 4 November icant degree owing to photothermal effects, but solar wind 2005), and almost all of them have masses comparable to ablation may be active at Venus, though different in (within a factor of 5) or larger than Jupiter. About 20% of character from Earth [Fok et al., 2004]. the planets are classified as close-in extrasolar giant planets [97] The solar wind is directly incident upon the iono- (CEGP) (or ‘‘Hot Jupiters’’), as they orbit their center stars spheres of the unmagnetized planets, Mars and Venus, within a distance of 0.10 AU or less, noting that the smallest exposing dayside ionospheres to an energy flux on the star-planet separation distance identified to date is 0.021 AU 2 order of 0.3 mW m . A number of processes are active [Rivera et al., 2005], equivalent to 4.5 solar radii. Magnetic in these cases, including charge exchange between atmo- interaction between CEGPs and their host stars was first spheric atoms and solar wind ions, impact ionization of postulated by Cuntz et al. [2000] and later confirmed by atmospheric atoms by solar wind electrons, and some high- Shkolnik et al. [2003] for HD 179949 through the detection altitude photoionization [Breus et al., 1991; Brecht et al., of temporal variations of Ca II H, K line emission in the 1993; Kallio et al., 1997, 2006]. Some atoms and ions are stellar plasma, which were found to be synchronized with sputtered to escape energy by direct collisions with solar the planetary orbit rather than the stellar rotation. wind ions, and ions are picked up by the interplanetary [101] Direct observations of planetary atmospheric plasma electric field. All of these processes contribute to the flows, revealed through hydrogen Lyman-a [Vidal-Madjar contamination of the planetary magnetosheath by planetary et al., 2003], oxygen and carbon [Vidal-Madjar et al., material that is lost from the planet. 2004], and sodium detections [Charbonneau et al., 2002] [98] Similar fluxes (scaled for distance from the Sun) are have been obtained for the planet of HD 209458, which incident upon the regions of the ionosphere conjugate with constitutes the ‘‘best studied to date’’ extrasolar star-planet the magnetospheric cusps for the magnetized Mercury and system. Vidal-Madjar et al. [2003] concluded that the Earth. On the other hand, an appreciable fraction of the solar Lyman-a absorption feature is formed outside the planetary wind energy incident upon the entire magnetospheric cross Roche limit, thus constituting evidence of escaping planetary section (15–20 RE radius) is dissipated in the terrestrial hydrogen plasma. As a way of visualizing the planet in auroral zones, with energy fluxes at the ionosphere ranging question, these authors developed an artist’s conception, 2 from 3 to 100 mW m . The lower end represents a typical which is shown in Figure 14, accompanied by their principle value for average conditions, while the upper end represents result plot. very active solar wind and aurora. Thus the energy flux [102] A further possible consequence of planetary plasma available for heating and ablation of ionospheric plasmas is flows and evaporation is the existence of ‘‘close-in hot- 10 to 300 times larger in the localized auroral zones than it Neptunes,’’ which have been found around GJ 436, r Cancri, would be if the solar wind were simply incident upon an and m Ara and are believed to be caused by extreme stellar- unmagnetized Earth, as it might possibly be during a induced mass loss [Baraffe et al., 2004, 2005] of formerly geomagnetic field reversal. intact Jupiter-mass planets. This interpretation is strongly supported by planetary evolution studies, which in particu-

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Figure 14. (left) Total intensity of lines as function of the orbital phase for extrasolar planet HD 209458. Circles, squares, triangles, and diamonds are for first to fourth transits, respectively. Vertical dotted lines correspond to the position of the first to fourth contacts of the planetary disk [Vidal-Madjar et al., 2004] (reproduced by permission of the AAS). (right) Artist’s conception of the extrasolar planet of HD 209458. lar take into account effects of stellar irradiation. An even of theory and simulation of ionospheric outflows that appear more extreme outcome of this type of process is suggested to be ready for near-term rapid advances include the by observational evidence that CEGPs can even be engulfed following. by their host stars. Israelian et al. [2001] reported the presence of the rare 6Li isotope in the stellar atmosphere 8.1. Removing Low-Energy Plasma Obstacles of HD 82943. This isotope is normally destroyed in the [105] Recent observations have largely overcome the early evolution of solar-type stars, but is preserved intact in problems of observing cold plasmas from spacecraft in the atmospheres of giant planets. In fact, the 6Li isotope has sunlight, owing to photoelectric charging. Another problem hitherto not reliably been observed in any metal-rich star has been the prevalent assumption that cold plasma densi- besides HD 82943. The authors interpret this finding as ties were so low that they were negligible, which made it evidence for a planet (or planets) previously engulfed by the difficult to justify the expenditure of resources to measure parent star. them. These problems delayed a beginning survey of [103] While most of the extrasolar planets discovered to the polar wind circulation throughout the magnetosphere date are in circumstances that are far more extreme than from the first theoretical predictions in the 1960s until those of the Earth in our own solar system, they serve as the Dynamics Explorer-1 (DE-1) mission in the 1980s reminders of the range of possible conditions over which we [Chappell, 1988]. Akebono, Polar, and Cluster have estab- would like our theories to be applicable. As observing lished the technology [Moore and Delcourt, 1995; Comfort capabilities are refined, future discoveries are likely to et al., 1998; Singh et al., 2001; Torkar et al., 2001] to reveal planets that are much more similar to the terrestrial actively control spacecraft floating potential, leading to planets. successful tracing of polar and auroral winds from the ionosphere to the magnetotail [Moore et al., 1997; Su et al., 1998b; Liemohn et al., 2005; Huddleston et al., 2005]. 8. FUTURE MEASUREMENT REQUIREMENTS On the Cluster mission, Active Spacecraft Potential Control (ASPOC) is an ion emitter that effectively neutralizes posi- [104] This section deals with the requirements for future progress in the observation of ionospheric outflows as tive spacecraft charging in sunlight. By avoiding emission driven by stellar wind energy dissipation. Some of the areas of electrons, it creates a benign amount of plasma wave

23 of 34 RG3002 Moore and Horwitz: ABLATION OF ATMOSPHERES RG3002 noise, and appears to improve the accuracy of double probe recently, when considerable information has been obtained electric field measurements [Eriksson et al., 2006] in low- on the drivers of such outflows by the IMAGE/LENA density polar wind regions. This technology is included in imager [Wilson and Moore, 2005; Fuselier et al., 2006]. plans for the Magnetospheric MultiScale Mission currently Plasmas in the magnetosphere are heated strongly by entering development. geospace storm events such that they substantially inflate the inner magnetosphere, producing a ring current that adds 8.2. Observing the Heating Mechanisms to the Earth’s dipole moment [Moore et al., 2001; Seki et al., [106] Sounding rockets offer direct low velocity probing 2001; Cully et al., 2003a, 2003b]. Recently, it has been of topside ionospheric heating processes, albeit with limited found that this ring current is overwhelmingly composed of scope and duration. They have contributed a great deal to O+ from the Earth during the largest geospace storm events. our knowledge and should certainly continue with new This surprising result grew out of a collaboration between strategies and observing technologies. The Polar Outflow the Geotail investigators and the IMAGE LENA Imager Probe (POP) [Yau et al., 2002] spacecraft is being devel- [Nose´etal., 2005; Moore et al., 2007]. oped for the Canadian Space Agency by the University of Calgary Institute for Space Research, and will explore 8.5. Imaging Missions ionospheric outflow origins in the altitude range from [109] The IMAGE mission, at nearly 6 years of age and 300 to 1500 km, where energy goes into creating mass flux long past its design lifetime of 2.5 years, unfortunately (and where photoelectric charging is tolerable). The science failed a few days before this paper was submitted to objectives of POP are to quantify the microscale character- Reviews of Geophysics. After pioneering many aspects of istics of plasma outflow in the polar ionosphere and probe- geospace imaging, including those based on energetic related microscale and mesoscale plasma processes at neutral atoms from the ring current, the Polar Mission is unprecedented resolution, and explore the occurrence scheduled to terminate its full solar cycle of operations in morphology of neutral escape in the upper atmosphere. the next month or two. Owing in large part to the transfor- This type of mission is much needed to better understand mation of NASA to revive human space flight, no geospace the fundamental causes of ionospheric outflows so they can imaging missions are currently in development or have firm be predicted on the basis of their causal physics rather than plans for the future, though several are ‘‘in the wings’’ and through empirical scaling. waiting for a suitable opportunity. There is a pressing need for future magnetospheric and auroral imaging to 8.3. Determining Exospheric Structure and Variability better understand the terrestrial atmospheric contribution [107] Recently, owing in large part to the IMAGE mission to geospace storms. Imaging has proven so fruitful in [Burch, 2005], it has been possible to better observe and understanding how our planetary atmosphere is disturbed understand the ionospheric topside, geocoronal, and plas- and ablated by solar wind energy inputs that it has become maspheric media that supply gas and plasma to magneto- an essential feature of the toolbin of both space meteorol- spheric reservoirs encompassing the plasmaspheric plumes, ogists and space weather forecasters. polar lobes, plasma sheet, and ring current regions. Both the gas and plasma of the ionosphere expand into the magne- 8.6. Extending Observations Beyond the Geosphere tosphere [Gardner and Schunk, 2004, 2005], impeded [110] An important priority for the space exploration mainly by gravity. The gas creates a relatively steady, nearly initiative will be the exploration of planetary atmosphere spherical geocorona of light hydrogen gas with a scale of a ablation by the solar wind, which is thought to have been an few Earth radii. The denser gas species form a smaller important factor in the evolution of the Martian atmosphere geocorona or exosphere with a height scale that about [Lundin et al., 2004; Vaisberg et al., 1995, 2005]. As noted 100 km at normal F-layer heights but increasing with above, Venus is thought to lose atmosphere at a rate altitude above that in a highly variable way that depends comparable to that of the Earth, but this is based largely on the amount of energy deposition into the auroral iono- on the Pioneer Venus Orbiter, which did not have instru- sphere. The actual configuration and response of the oxygen mentation designed to observe such loss. Moreover, this exosphere has been very poorly known, but considerable comparison needs an update for recent observations of the information has recently been obtained by the IMAGE Earth’s ablation rate. A number of strategic studies have mission and especially the LENA imager [Moore et al., placed high priority on aeronomy probes for Mars and 2003; Wilson et al., 2003; Shematovich et al., 2005]. Venus, but the current fascination appears to be with Mercury, which has a magnetosphere but very little in the 8.4. Resolving Dynamic Escape Processes way of an atmosphere. In the past, planetary exploration [108] The heavy ion plasmas are normally confined to the missions have understandably focused on the condensed F layer proper, similar to the heavy gas atoms. Exceptions parts of the planets. Two current missions, the ESA Mars are important in the auroral ionosphere, where oxygen flows Express and Venus Express, are now making strides toward locally in an auroral wind with fluxes greatly exceeding a better understanding of how those planets compare with those of the steadier and more widespread polar wind [Cully the Earth in terms of solar wind coupling. In the future, et al., 2003a, 2003b; Strangeway et al., 2005]. The time missions may devote more resources to the study of effects variations of auroral wind have been poorly known until that could have a long-term evolutionary impact on those

24 of 34 RG3002 Moore and Horwitz: ABLATION OF ATMOSPHERES RG3002 planets, now that those effects are better appreciated as a dynamics models. At the present time, the global magneto- result of closer study at Earth. spheric dynamics model of Winglee et al. [2002] has pioneered the incorporation of ionospheric plasma sources, 8.7. Extending Observations Beyond the Heliosphere but more and more researchers are reporting efforts to move [111] There is no foreseeable opportunity for in situ in a similar direction. There is room for improvement in the measurements of an extrasolar planet, but the planets being manner in which ionospheric plasma outflows occur in such remotely sensed are slowly becoming more Earth-like as models. As a first step, it would be possible to use empirical technology advances and smaller planets become more specifications of ionospheric response to magnetospheric detectable. It is hoped that the synergy between extrasolar inputs [Strangeway et al., 2005; Zheng et al., 2005], as planetary studies and in situ measurements of our own reported by Moore et al. [2007] and Fok et al. [2006]. planet and its nearby neighbors will be fully exploited by Ultimately, we will need an embedded model of the researchers. Though the techniques are vastly different at ionosphere and thermosphere, with all appropriate physics present, they clearly have the capability to inform each included, within a global dynamical simulation. other. [117] 5. The inner magnetosphere is centrally important to terrestrial space weather. It is common for global simulation 9. FUTURE THEORY REQUIREMENTS codes to omit the inner magnetosphere from the simulation space, setting an inner boundary at a radius of 3–4 RE. This [112] Theory and modeling progress in ionospheric out- is mainly for reasons of resolution, though curvilinear and flows, as with other phenomena, is in large part driven and adaptive calculation grids have largely rendered that reason inspired by new observational findings, such as those obsolete, and indeed, one of the codes now extends down projected in the previous section. For example, advances all the way through the ionosphere proper [Maynard et al., in spacecraft potential control may enable new comprehen- 2001], albeit with a single fluid that can originate in either sive findings on the characteristics and behavior of <2 eV the ionosphere or the solar wind. While this cannot readily ions in regions such as the high-altitude polar cap, tail lobes, distinguish the separate roles of plasma from the iono- and plasma sheet. This will inspire and suggest new sphere and solar wind, it allows for realistic momentum directions of theory and simulation to understand and and energy flows, which may ultimately be the most replicate those observational discoveries. Some of the areas important consideration. of theory and simulation of ionospheric outflows that appear to be ready for near-term rapid advances include the 10. CONCLUSIONS AND OUTSTANDING following. PROBLEMS [113] 1. Auroral field-aligned plasma transport can be better simulated by incorporating ionospheric plasma and [118] In this review we have focused upon observations energy production and loss and auroral and kinetic processes of the expansion of our ionized upper atmosphere into [e.g., Wu et al., 1999, 2002; Tu et al., 2004, 2005]. This geospace, and the theories that have been developed to would be done by implementing more sophisticated, self- explain those observations as well as their consequences. consistent treatments of auroral processes involving fine- We assert that observations and theory have reached suffi- structured parallel electric fields, for example, kinetic cient convergence to form a strong basis for predicting the Alfve´n waves and solitary waves [e.g., Ergun et al., 1998; behavior of similar systems under different conditions and Singh, 2002] and waves which cause transverse heating, in different contexts. In particular, the knowledge we have including ion cyclotron and lower hybrid waves [e.g., Andre´ reviewed can be used to anticipate and predict the behavior and Yau, 1997]. of the diverse other planets throughout our own solar [114] 2. Molecular ion outflows in the auroral regions system, and for conditions around other stars, to the degree [e.g., Peterson et al., 1994; Wilson and Craven, 1999] have that their stellar winds and intrinsic magnetic fields and thus far not been systematically treated with transport atmospheres have observed or inferred properties. This models, but can readily be added to multifluid codes. includes, for example, the practical exercise of anticipating [115] 3. Plasmaspheric drainage plumes, involving sub- terrestrial space weather changes during the next geomag- stantial ionospheric F-region plasma densities, are observed netic field reversal, which may well be underway in view of convecting toward the cleft region and across the polar caps recent trends in the internal field of Earth. following large magnetic storms [e.g., Foster et al., 2002]. [119] Thus this review should be understood in the These should be incorporated into ionospheric plasma general context of coupling between a stellar atmosphere transport models of the cleft ion fountain and global and the volatile matter embedded within it as part of magnetosphere. Foster [2005] has suggested that such large a planetary system. Prior to stellar ignition, the overall F-region densities in these drainage plumes could be situation is dominated by gravitational accretion with rota- sources of plasma content, in particular heavy ions, for tional dynamics reflecting the initial angular momentum of the cleft ion fountain at these times. the system. Once stellar ignition occurs, however, the [116] 4. Improved ionospheric plasma transport models emanated UV energy flux in general ionizes these atmos- and processes can be incorporated as ionospheric outflow pheres into fully or partially ionized plasmas, with higher plasma sources in global magnetospheric circulation and charge states and complete stripping of electrons from

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Figure 15. A schematic plasma flowchart of the heliosphere-geosphere interaction. Flow color coding runs from dotted line (energy) to solid line (mass), with dashed line for both (mass and energy). Flow dependences on IMF are indicated by labels above the arrows. atoms emitted from the star itself. Energetic photon fluxes strengthening the high-latitude flow cells in comparison then extend throughout the planetary system and ionize with the low-latitude cells. much of the diffuse gas surrounding the more condensed [121] Over the past 2 decades, considerable progress has objects. Once the system contains a substantial amount been made in the study of atmospheric ablation by helio- of plasma, electromagnetic effects and magneto-plasma spheric influences, leading to the following conclusions. dynamic forces become important in coupling the entire [122] 1. Both the solar wind and polar wind contribute system together, and we expect to see telltale signs of light ion plasmas to the magnetospheric coupling region. nonequilibrium activity, including reconnection, plasma During typical conditions (average solar wind properties; heating, and superthermal particle acceleration, together spiral IMF with small Bz) a substantial contribution of with neutral atom and photon emission. ionospheric plasma with significant heavy ion content + + [120] As an aid to visualizing the global circulation of the (mainly O but also N and molecular species during high various plasmas in the geosphere, we offer Figure 15 as a activity) is supplied via the auroral wind, to a degree that schematic diagram or plasma flowchart. The flows begin varies in response to energy dissipation as a strong function with the upstream solar wind and the creation of an of solar wind dynamic pressure. ionosphere and light ion plasmasphere by solar photons. [123] 2. For events with strong northward IMF, the The solar wind boundary layers carry the bulk of solar wind auroral wind is reduced and circulated into the low-latitude around the geosphere, but entry occurs in the cusp/cleft and boundary layers and downstream rather than over the poles low-latitude boundary layer regions. The high- and low- into the magnetotail. latitude features are divided top to bottom in this schematic [124] 3. For events with strong southward IMF, the to fit the three-dimensional structure on the page without eroded plasmasphere (supplied by polar wind) supplies adding unnecessary complexity. The overall circulation is significant amounts of plasma to the dayside afternoon controlled by IMF orientation via the distribution and effect magnetopause region, resulting in an interaction with of dayside magnetic reconnection. Northward IMF Bz magnetospheric global circulation as driven by reconnection. interrupts the normal flow of boundary layer flow away [125] 4. Substorm effects in the near-Earth midnight from the subsolar region, interrupting and diverting the sector significantly energize and enhance the auroral wind high-latitude flow cell to the low-latitude boundary layers, plasmas that return to the inner magnetosphere via the closing open flux from the polar lobes and shrinking the midnight plasma sheet. The solar wind and polar wind polar caps, while also thickening and drawing plasma from proton plasmas are less influenced by substorm effects. the plasma sheet, as indicated by arrow labels. This also [126] 5. The storm time ring current becomes increasingly interrupts the supply of the plasma sheet from the high- auroral wind dominated with increasing ring current energy latitude or auroral wind circulation, switching the source to density, implying that these enhancements of the quiet time the low-latitude circulation cells, which are fed by solar ring current are supplied with charge carriers largely wind plasmas. Southward IMF has the reverse effect of through atmospheric ablation by solar wind energy. Essen-

26 of 34 RG3002 Moore and Horwitz: ABLATION OF ATMOSPHERES RG3002 tially, the storm time ring current is produced by the models would need further development, since the iono- pressure of heated ionospheric plasmas seeking to escape spheric load is not confined to a thin boundary layer. from magnetospheric confinement. Accurate description of a magnetosphere in which the [127] 6. The expanding ionosphere is lost into the down- ionosphere extends throughout the system will require stream solar wind in both the dayside and nightside recon- models with ionospheric fluids or particles as active dynamic nection regions, where closed magnetospheric flux tubes components. are first opened up to the solar wind, stretched into the [131] We have emerged from this transformation with tail with acceleration of the plasmas residing in them, and ample evidence and community acceptance that the iono- then reconnected after substantial plasma expansion has sphere expands to the magnetospheric boundaries and occurred, beyond the nightside reconnection region. escapes continually into the downstream solar wind, its [128] 7. Ionospheric plasmas that are retained in the inner composition and partial pressure varying with solar wind magnetosphere are ultimately lost, mostly by escaping drivers. Updated ionospheric models now produce the outward after being freed by charge exchange. observed heavy ion outflows from solar wind energy inputs. [129] Among the outstanding problems stemming We also have promising new or revised global circulation from the expansion of ionospheric plasma into space, models that incorporate the ionosphere as an extended load the following pose the highest priority requirements for within the system, and we are learning that this load can be improved observations and theory: (1) continuing aware- felt all the way out to the boundary layer reconnection ness of the importance of relatively cold plasma populations regions. There is much additional work to be done to work in the magnetosphere, as reflected in attention to spacecraft out the consequences of this shift in our understanding, but floating potential measurement and control; (2) a global we are now well equipped for this work and also well theoretical and/or empirical model of ionospheric topside equipped to use our new simulation tools to predict and response to solar wind electromagnetic energy and particle understand the behavior of diverse other planetary systems precipitation inputs, including auroral potential structures within and beyond our solar system. associated with circulation shear and field aligned currents; (3) theory and observations of the dynamic role of [132] ACKNOWLEDGMENTS. We are indebted to Manfred the extended ionosphere in mass loading, wave propagation, Cuntz of the University of Texas at Arlington for valuable inputs to and particle acceleration within the magnetosphere; the section on extrasolar atmospheric ablation. This work was (4) theory and observations of the extended ionospheric supported by Polar/TIDE under UPN 370-08-43, IMAGE/LENA under UPN 370-28-20, and by NASA ROSES under UPN 432-01-34 pressure that inflates the storm time magnetosphere and at Goddard Space Flight Center, by NASA grant NNG05GF67G, escapes during the largest storms; and (5) theory and and NSF grant ATM-0505918 to the University of Texas at observations of the role of extended ionospheric density in Arlington. moderating plasma and particle acceleration, and enhancing [133] The Editor responsible for this paper was Peter Riley. 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