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Pickup Ion-Mediated Plasma Physics of the Outer Heliosphere and Very

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Pickup Ion-Mediated Plasma Physics of the Outer Heliosphere and Very Zank Geosci. Lett. (2016) 3:22 DOI 10.1186/s40562-016-0055-2 REVIEW Open Access Pickup ion‑mediated plasma physics of the outer heliosphere and very local interstellar medium G. P. Zank* Abstract Observations of plasma and turbulence in the outer heliosphere (the distant supersonic solar wind and the subsonic solar wind beyond the heliospheric termination shock) made by the Voyager Interstellar Mission and the energetic neutral atom observations made by the IBEX spacecraft have revealed that the underlying plasma in the outer helio- sphere and very local interstellar medium (VLISM) comprises distinct thermal proton and electron and suprathermal pickup ion (PUI) populations. Estimates of the appropriate collisional frequencies show that the multi-component plasma is not collisionally equilibrated in either the outer heliosphere or VLISM. Furthermore, suprathermal PUIs in these regions form a thermodynamically dominant component. We review briefly a subset of the observations that led to the realization that the solar wind–VLISM interaction region is described by a non-equilibrated multi-compo- nent plasma and summarizes the derivation of suitable plasma models that describe a PUI-mediated plasma. Keywords: Solar wind, Interstellar medium, Plasma, Pickup ions, Neutral gas Introduction interstellar boundary explorer (IBEX) of and in our very The Voyager 1 (V1) spacecraft crossed the heliopause, the local neighborhood of the LISM. boundary separating matter of solar origin from inter- It is now recognized that the interstellar medium stellar matter, and entered the local interstellar medium and heliosphere are coupled intimately through charge (LISM) during August 2012 (Stone et al. 2013; Krimi- exchange of neutral H and protons, and that the physics gis et al. 2013; Burlaga et al. 2013; Gurnett et al. 2013), of the outer heliosphere and neighboring LISM cannot be an event of enormous historical import for humankind. understood independently of each other. Voyager 1 is the first human-made object to leave the The heliosphere is the region of space filled by the confines of the heliosphere and enter interstellar space. expanding solar corona; a region extending >120 astro- With a working set of instruments, Voyager 1 begins nomical units (AU) in the direction of the Sun’s motion an epoch of extraordinary in situ discovery science in through the interstellar medium and perhaps tens of the interstellar medium. We now have the opportunity thousands of AU in the opposite or heliotail direction. to study in situ basic plasma physical processes in the Neutral interstellar hydrogen is the dominant (by mass) interstellar medium (ISM). We review briefly our under- constituent of the solar wind beyond an ionization cav- standing of the basic plasma physics model that is begin- ity of ∼6−10 AU in the upwind direction (the direction ning to emerge as a result of observations made by the antiparallel to the incident interstellar wind), and is cou- Voyager interstellar mission (Voyagers 1 and 2) and the pled weakly to the solar wind plasma via resonant charge exchange. Charge exchange produces pickup ions (PUIs) that eventually dominate the internal energy of the solar wind. *Correspondence: [email protected] If, for simplicity, we adopt initially a perspective that the Center of Space Plasma and Aeronomic Research and Department of Space Science, University of Alabama in Huntsville, Sparkman Ave, plasma can be described as a single-fluid or magnetohy- Huntsville, AL 35899, USA drodynamic (MHD) system, then the heliospheric-LISM © 2016 The Author(s). This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Zank Geosci. Lett. (2016) 3:22 Page 2 of 17 −15 2 plasma environment is composed of essentially three σc = 5 × 10 cm and a total LISM number density −3 thermodynamically distinct regions: (i) the supersonic of 0.2 cm ), in the IHS (region 2) ∼2500 AU for a num- −3 solar wind, with a relatively low temperature, large radial ber density of 0.005 cm , and >200 AU in the supersonic speeds, and low densities, bounded by the heliospheric solar wind beyond 10 AU (region 3). With the excep- termination shock (HTS). The outer heliosphere is that tion of the local interstellar medium region, the charge- region of the solar wind influenced dynamically by physi- exchange mfps are so large that they exceed the expected cal processes associated with the LISM. (ii) The transition scale size of the boundary regions separating the helio- of the supersonic solar wind to a subsonic flow through sphere and LISM. The interaction of the solar wind with the HTS creates a region of heated subsonic solar wind, the LISM therefore requires the modeling of plasmas called the inner heliosheath (IHS). The IHS has much and non-equilibrated H atom gas. Despite the very large higher temperatures and densities, larger magnetic fields, charge-exchange mfps in both the supersonic solar wind and lower flow speeds than does the distant supersonic and the boundary regions, the structure of the global solar wind (SW). The IHS is bounded by a contact or tan- heliosphere is determined in large part by the non-equil- gential discontinuity called the heliopause (HP). (iii) The ibrated coupling of neutral interstellar H to supersonic HP is the boundary that separates plasma of solar ori- and subsonic solar wind plasma (Zank 1999; 2015; Zank gin from plasma of interstellar origin. The LISM in our et al. 2009; McComas et al. 2011). This makes the mod- neighborhood possesses a small plasma flow speed and eling of the solar wind interaction with the LISM very temperature, but the density is higher than in regions challenging. Nonetheless, despite these complications, (i) and (ii). A bow shock may or may not exist ahead of the basic structure illustrated in the cartoon Fig. 1 (left) the heliosphere due to the relative motion of the Sun and emerges from simulations that include the basic physics interstellar medium. The three regions are illustrated of the plasma–H charge-exchange coupling. An illustra- in Fig. 1 (left panel), where region 3 corresponds to the tive simulation of a 2D coupled model of the heliospheric supersonic solar wind, region 2 to the hot inner heli- interaction with the LISM is shown in Fig. 1 (right panel). osheath, and the interstellar region is subdivided into The top plot shows the 2D plasma temperature distribu- region 1b between the HP and a possible bow shock/ tion, clearly identifying the three distinct regions and the wave, sometimes called the outer heliosheath (OHS), and overall topology and boundaries that can exist (together region 1a beyond a bow shock or bow wave. Of course, with a further sub-division of region 1 into pre- and post- as we discuss below, the plasma system is vastly more bow shock regions 1a and 1b, respectively). The bottom complicated than that of MHD and the plasma itself pos- plot illustrates the neutral H density distribution. A more sesses multiple components coupled via charge-exchange extended summary that discusses the magnetic field and/or collisional and collisionless processes, with asso- observations in both the IHS and at the HP, together with ciated transfer of charge, momentum, and energy, and associated references and related theoretical modeling, thus thermodynamic coupling. Nonetheless, the zeroth- can be found in the review by Zank (2015). order distinction of thermodynamically distinct regions The coupling of plasma and neutral H occurs through provides a useful intuitive guide to the underlying physics the creation of PUIs via charge exchange between the of the global solar wind–LISM interaction. charged and neutral gases. Over suitably large distances, Each of the thermodynamically distinct regions is the neutral H and protons are fully equilibrated, both the source of a distinct population of hydrogen (H) possessing the same temperature and velocity. Charge atoms produced by charge exchange between the ambi- exchange in a fully equilibrated partially ionized plasma ent plasma and neutrals entering the region (Zank et al. has no essential dynamical effect, with charge exchange 1996). These three distinct neutral H populations include effectively doing no more than relabeling protons and the “splash” component produced in the fast or super- H atoms (assuming that the dominant neutral gas com- sonic solar wind, i.e., fast neutrals that acquire high ponent is H atoms—in the LISM, this is a reasonable 400 750 radially outward speeds (∼ − km/s) with a rela- assumption, although He atoms are approximately 9 % tively small thermal spread, very hot neutrals produced of the neutral gas and the remaining heavy atom neu- in the inner heliosheath with comparatively high speeds tral gas is about 1 %). However, in regions 2 and 3, the 100 (∼ km/s) and a large thermal spread (which can pro- interstellar H drift speed is different from the plasma duce ENAs with speeds even that exceed 100 km/s), flow velocity ∼( 20 km/s for H versus ∼100−750 km/s and decelerated heated atoms originating in the outer for the plasma), and H originating from regions 3 and 2 heliosheath. that splashes back into the LISM has flow speeds rang- The charge-exchange mean free path (mfp) of neutral ing from ∼100−>400 km/s, which is quite different from hydrogen atoms in the LISM (region 1) is approximately the ∼15−26 km/s speed of region 1. Thus, throughout 100 ∼ AU (assuming a charge-exchange cross-section the boundary regions and in the LISM within several 100 Zank Geosci. Lett. (2016) 3:22 Page 3 of 17 a b BS(?) T (K) 7 × 103 1.6 × 104 3.8 × 104 8.7 × 104 2.0 × 105 4.7 × 105 1.1 × 106 HP Plasma temperature 400 Region 1a 2 B Region 1b HTS ow shoc Heliopause Region Region 3 200 k VLISM VSW Termination shock AU 0 Hy drogen wall drogen 200 400 Hydrogen density –500 –300 –100100 300 AU n (cm–3) 0.00 0.04 0.08 0.12 0.16 0.20 0.24 0.28 Fig.
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