MAGnetosphere-Ionosphere Connector (MAGIC): Investigation of Magnetosphere-Ionosphere Coupling from High-to-Low Latitudes

A White Paper submitted to 2013 Solar and Space Physics Decadal Survey

Shing F. Fung ([email protected]) Robert F. Benson Joseph M. Grebowsky George V. Khazanov NASA/Goddard Space Flight Center, Greenbelt, Maryland

Donald L. Carpenter (Emeritus) STAR Laboratory, Stanford University, Stanford, California

Abstract An outstanding challenge in Heliophysics is to understand the physics of systemic coupling between the magnetosphere and ionosphere from hemisphere to hemisphere, and the roles played by the intervening plasmasphere. Measurements from past missions like the dual Dynamics Explorers (DE), Polar, and IMAGE have shown the complexities of the inner magnetosphere. Yet, we still do not have a coherent understanding of the multi-scale processes and their interrelationships that couple the ionosphere and magnetosphere from high to low latitudes. DE, Polar and IMAGE clearly demonstrated the importance of global-scale imaging in providing the context for understanding local measurements. Except for DE limited to high latitudes, imaging observations from Polar and IMAGE were seldom accompanied by coordinated in situ observations of targeted regions. For example, plasmasphere imaging by IMAGE EUV were performed at L > 4 and near apogee (~ 8 Re), but in situ or radio-sounding measurements of the plasmasphere were best when the IMAGE Radio Plasma Imager was nearer the plasmasphere (L < 4). Plasmaspheric mass loss through large-scale plumes has been confirmed by IMAGE, although quantitative knowledge of such losses and their impact on the ionosphere and magnetosphere are still lacking. IMAGE also revealed numerous previously unknown plasmapause structures, such as notches, fingers, and shoulders, indicating much unknown about the coupling processes of ionosphere- plasmasphere-magnetosphere system. Other topics that need investigation in the context of magnetosphere-ionosphere coupling include the plasmasphere boundary layer, plasmaspheric thermal structure, ring current interaction, plasmaspheric refilling as revealed from field-aligned density profiles, field-aligned irregularities, ionospheric outflows, equatorial spread-F plumes, etc. We propose the MAGnetosphere-Ionosphere Connector (MAGIC) mission consisting of a high- altitude, high-inclination satellite for auroral and plasmaspheric imaging and multiple lower-orbiting spacecraft for simultaneous in situ and radio sounding measurements. Complemented by ground-based observations, MAGIC will investigate magnetosphere-plasmasphere-ionosphere coupling processes from high-to-low latitudes.

Introduction Magnetosphere-ionosphere coupling is an age-old problem in heliophysics. Except for the direct influence of solar irradiance on the underlying ionosphere and atmosphere, magnetosphere-ionosphere (M-I) coupling processes mediate almost all effects of solar wind input into the ionosphere and its

Figure 1. Sequences of IMAGE EUV plasmasheric images showing that the plasmasphere can change from fully filled (featureless), to having a large-scale drainage plume, and to highly structured and irregular on timescales of days, thus demanding the question about the corresponding ionosphere conditions and M-I coupling processes. feedback on the magnetosphere above through the plasmasphere. Understanding M-I coupling processes is critical to understanding the solar-terrestrial relationship. Since its discovery in the 1950’s, the plasmasphere has been regarded as a passive body of cold plasma with little activity or consequences. The predication in the 1970’s of a large-scale drainage plume as a result of a preponderance of magnetospheric convection electric field over the plasmaspheric co- rotational electric field [Grebowsky, 1970] was only verified after the 2000-launch of the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite. Figure 1 shows different 5-hour sequences of global-scale images of the plasmasphere obtained by observing the resonantly scattered 304- nm solar radiation by He+ ions. The highly variable plasmaspheric structures clearly imply the presence of correspondingly significant changes in ionospheric conditions mediated by M-I coupling processes. To fully comprehend M-I coupling we will need to investigate not only the physics of individual coupling processes, but also their inter-relationships, so that we can arrive at an understanding of M-I coupling at the Sun-Earth system level.

M-I Coupling: Plasmaspheric Perspective The plasmasphere boundary layer can change from featureless to appearing highly structured on timescales of hours to days (Figure 1). Such dynamic variability must be accompanied by changes in plasmaspheric and ionospheric structures. Figure 2 upper panel shows the ion composition measurements by the OGO 5 satellite on March 9, 1968, revealing significant differences at times in the H+ and He+ plasmapause locations. Since EUV observation depends on He+ distributions, had EUV imaging been available during the time of OGO 5 observations it would have led to the conclusion of a much smaller plasmasphere than that indicated by the H+ observations. In situ ion composition measurements are thus critical for relating internal plasmaspheric structures with magnetospheric dynamics and interpreting global EUV images (Figure 1). Figure 2 lower panel shows the overlapping region (shaded) between the plasmasphere (PS) at L ≤ 3.5 (represented by the red He+-density curve in the upper left portion) and the ring current (RC) at L > 2 [demarcated by the smooth energetic neutral atom (HENA and MENA) profiles in the lower left portion]. While electron precipitation fluxes (light blue curve in upper portion) are generally low inside the PS as shown by the simultaneous DMSP magnetic conjunction measurements, electron precipitation can be more significant in the higher latitude plasma trough and subauroral regions, where the electron temperature (purple curve) can also be higher. Inside the PS-RC interaction region, however, localized electron heating occurred apparently due to plasma instabilities or wave-particle interaction processes. Understanding the local wave and wave-particle processes may also hold a key to understanding the plasmapause irregularities [Carpenter et al., 2002] and their roles in mediating systemic (larger-scale) M-I coupling. In order to properly investigate wave propagation and wave-particle interaction processes in the presence of thermal structure in the plasmasphere and in the PS-RC interaction region as shown in Figure 2 lower panel, we must have adequate knowledge of Figure 2. (Upper) Sometimes discrepant plasmapause plasmaspheric electron and ion distributions. locations between He+ and H+ observations and (lower) While ion distributions can be obtained from thermal structure associated with RC-PS interaction region composition measurements, nearly illustrate the importance of having coordinated global instantaneous field-aligned electron imaging (EUV, ENA) and in situ (e.g., DMSP) observations. distribution, critical for understanding plasmasphere erosion and refilling processes [Reinisch et al., 2004], can only be obtained by radio sounding technique [Huang et al., 2004]. Figure 3 shows an example of ducted echo traces (upper left) and the corresponding inverted field- aligned electron density (FAED) profile (lower left) from one set of swept- frequency sounding observation near L = 3 in the plasmasphere. By combining successive soundings through the plasmasphere (Figure 3 Figure 3. Global plasmaspheric electron density model (lower right) can be upper right), it is possible to constructed from a sequence of FAED profiles obtained over an L range (~ 22 construct a plasmaspheric min; upper right). FAED (lower left) are obtained by inverting echo traces electron model distribution resulting from swept-frequency soundings (~1-2 min; upper left) taken within (Figure 3 lower right) on half- hemisphere-to-hemisphere FAED irregularities [Huang et al., 2004]. hour time scale [Reinisch et al., 2009], sufficient to resolve storm or substorm dynamics. The existence of an annual density variation in the plasmasphere has been known for many years [e.g., pp. 89-92 in Lemaire and Gringauz, 1998]. Past work on the annual density variation found a peak density in December a factor of ~3 above levels in June over a broad L range inside L ~ 4. Limited modeling work that has been done on this phenomenon has suggested that the annual density variation may be related to the asymmetry in the conjugate ionospheres (associated with the large offset in Earth’s spin and magnetic axes near the 75°-west longitude) and thus is an issue of ionosphere-plasmasphere coupling. It seems possible that the same conditions that favor the occurrence of the annual density variation could also affect the subauroral ionosphere and contribute to locally deeper or more efficient penetration of convection electric fields to lower latitudes, thus giving rise to certain temporal and/or longitudinal preference in plume formation. It is therefore quite possible that there are previously unappreciated plasmasphere-ionosphere coupling processes that may lead to both temporal (seasonal) as well as longitudinal variations in the erosion of the plasmasphere and associated plume formation.

M-I Coupling: Ionospheric Perspective With myriad plasma processes having a wide range of spatial and temporal scales, the ionosphere is a complex system in its own right. Many of the processes involving Earth’s magnetic field extend to altitudes well beyond the ionosphere, coupling the ionosphere to the thermosphere below and magnetosphere above. Figure 4 depicts many of the processes that occur from the equator to the poles. In

Figure 4. Complex ionospheric/thermospheric processes occurring from equator to poles, many of which (highlighted by red boxes) extend and couple to the plasmasphere and magnetosphere above, contributing to systemic magnetosphere-ionosphere coupling. recognizing their potentially important contributions to systemic M-I coupling, the red boxes in Figure 4 highlight the pertinent processes that collectively link the ionosphere/thermosphere to the plasmasphere. As noted in the last section and in Figure 4, plasmaspheric density structure and plume formation are closely affected by plasmasphere-ionosphere coupling processes. Shortly after the plasmaspheric plume observation by IMAGE EUV, it was quickly realized that plumes are high-altitude extensions of storm-enhanced density (SED) signatures often seen in ionospheric TEC observations during storms [Foster et al., Figure 5. Model calculations of equatorial fountain originating 2002]. The equatorial fountain (Figure from the thermosphere/ionosphere, showing its possible 4), long recognized to result from the E- significant extension into the equatorial plasmasphere [Lin et al., and F-region dynamo (eastward electric 2005]. field) driven by thermospheric neutral winds, may help supply the plume plasmas [Schunk and Nagy, 2009]. This is supported by recent model calculations showing that the equatorial fountain may reach nearly 1.3 Re in altitude at the equator (as shown in Figure 5), well into the plasmasphere. After analyzing the “Bastille Day storm” using multiple satellite data, Lin et al. [2007] used Figure 6a to describe the plasma flows on the Earth's corotating frame. The solid line near L = 2 is the plasmaspheric boundary in the afternoon and evening sectors. The boundary extends to higher latitudes near the plume. Arrows indicate subauroral polarization stream (SAPS) flows in the evening sector, initially westward, but diverted poleward due to the reduction of poleward electric fields inside the plasmasphere at the dusk terminator. The proposed flow pattern is consistent with Figure 6(b) that depicts the meridional view of the large-scale flows in which the equatorial fountain can be seen as a possible source of the plume plasmas.

(a) (b) Figure 6. (a) Scenario of plasma flow patterns in the topside ionosphere in the vicinity of the plasmaspheric drainage plume [adapted from Lin et al., 2007]; and (b) Sketch of meridional view of the plasmaspheric plasma flows, showing the plasma supply of SED from the equatorial fountain [courtesy of J. Foster in Schunk and Nagy, 2009]

MAGnetosphere-Ionosphere Connector (MAGIC) Mission Concept Global EUV and ENA imaging carried out by the IMAGE satellite (see Figures 1 and 2) have revived interest in plasmasphere research and revealed the important roles the plasmasphere plays in multiple processes [Figures 4, 5, 6] that contribute to systemic M-I coupling. While global imaging can clearly provide the overall context of magnetospheric dynamics, it cannot replace the ability of in situ observations in yielding accurate details of plasmaspheric core structures (Figures 2 and 3) that are required for understanding the physics of individual coupling processes. The MAGIC mission will employ coordinated global imaging, in situ space observations and ground- based measurements to provide the empirical data needed to investigate M-I coupling processes. Data assimilation, theory and modeling techniques will be used to validate the processes and integrate the knowledge to arrive at an understanding of systemic M-I coupling. MAGIC will consist of: MAGIC Components Instrumentations Observational variables 1) High-altitude (≤ 9 Re), Imaging: EUV, FUV, ENA; Global imaging of plasmasphere, high-inclination (~ 90°) Vector magnetic field; auroral and ring current dynamics; satellite; primarily for global Plasma wave instrument magnetic field and ULF waves, plasma imaging of aurora, Relaxation sounder wave and electron density environment plasmasphere and ring current 2) Two or more lower- Long-range radio sounder; Field-aligned electron density profiles; altitude (≤ 4.5 Re), lower- Plasma wave instruments; Passive wave observations; inclination (≤ 50°) satellites, Vector electric ans magnetic Vector E & B field; phased in time and local time; fields; Ion mass spectrometer; Plasma electron density and temp primarily for plasmaspheric Retarding potential analyzer; Plasma ion density, composition, temp “ground-truth” observations Langmuir probe Energetic ring-current particles 3) Ground-based ionospheric Ground magnetometer; All- ULF waves; “ground-truth” observations sky camera; Incoherent Ionospheric density structures and and extended local-time scatter radar; profiles, temperature, flows, electric coverage Ionosonde/digisonde field, SAPS, SED, TIDs, TEC 4) Data assimilation, theory CCMC, Virtual Modeling Model development and validation and modeling for science Repository (VRM), Guest with data closure Investigator Program Temporal resolutions of satellite observational variables: Imaging: FUV & ENA ~ 1-2 min, EUV ~ 5-10 min (may be improved with technology) Vector magnetic and electric fields: 0.1 s Plasma wave: Swept-frequency receiver, 0.1 – 800 kHz, 2 min; waveform capture, 0.1-1 ms Electron plasma density and temperature: 0.1 s Ion plasma density and temperature: 0.1 s Field-aligned electron density profile: 1-2 min While IMAGE has confirmed plasmaspheric mass loss through large-scale plumes, it has also revealed numerous previously unknown structures in the plasmasphere boundary layer, i.e., the plasmapause, such as notches, fingers and shoulders, and has clearly demonstrated the fibrous irregular nature of this boundary through the scattering and ducting of radio waves [Figure 3] and the ability to quantify plasmaspheric refilling rates based on FAED profiles obtained from radio sounding in hemisphere-to-hemisphere FAED irregularities [Reinisch et al., 2004]. These observations indicate that much is unknown concerning the magnetosphere-ionosphere coupling processes [Lemaire and Gringauz, 1998; Darrouzet et al., 2009]. Understanding these processes, and the important plasmasphere/ring- current interaction, will require IMAGE-type measurements in addition to in situ measurements of plasmaspheric thermal structure and ion composition. A major advantage of MAGIC is that it will offer campaign-like simultaneous, multi-point observations throughout the mission. In situ measurements will always be analyzed within the context of corresponding inner magnetospheric dynamic conditions from global images and in terms of ionosphere- plasmasphere-ring current interaction processes. This capability will address a major limitation of the IMAGE mission where, with only one satellite, the plasmaspheric hemisphere-to-hemisphere field-aligned electron-density profiles were separated in time from the plasmaspheric EUV and ENA images by many hours and no in-situ measurements, such as ion-composition, were available. MAGIC will provide simultaneous multi-point observations, including global EUV, FUV and ENA images, FAED profiles at multiple locations in the plasmasphere/plasmapause, ground-based observations, and will enable quantitative studies to be made of wave-particle interaction processes, plasmaspheric erosion and refilling, and other ionosphere-plasmasphere-ring current interactions. Data collected by MAGIC will allow proper posing of boundary conditions for developing and validating physical models. Given that all proposed MAGIC instrumentations have well documented flight heritage (OGO-5, Alouette-ISIS, DE, Polar, IMAGE, TWINS, Cluster, etc.), there should be little or no cost associated with new instrument technology development. In addition, since only a relaxation sounder is needed for the high-altitude satellite and radio sounding on the lower-altitude satellites will only be operating near and within the plasmasphere, much shorter antennas than those on IMAGE will be needed for MAGIC. With the lower-altitude satellites being identically built, the rough estimated cost of MAGIC may be ~$750M. If launched within the next 10 years, MAGIC can become a significant part of the next Heliophysics System Observatory and enhance the scientific returns of other NASA and international space missions such as RBSP (NASA), GEC (NASA), ORBITALS (Canadian), and ERG (Japanese). The last Solar and Space Physics Decadal Survey (2003) already recognized the significant contributions and potential of global EUV and ENA imaging in addressing the system-coupling complexity confronting Heliophysics today. By combining multi-satellite observations, ground-based measurements and a robust theory and modeling program, MAGIC will implement effectively the recommendations of the Panels on Solar Wind-Magnetosphere Interactions (SWM), Atmosphere- Ionosphere-Magnetosphere Interactions (AIM), as well as Theory, Modeling and Data Exploration (TMDE). MAGIC will be well positioned to directly address the outstanding science questions enumerated in the SWMI and AIM Panel Reports. It will help mobilize the efforts and integrate the results of national (ROSES, LWS, SHINE, GEM, CEDAR, Heliophysics Virtual Observatories-VxOs) and international (e.g., SuperDARN, ILWS, ISWI [Davila et al., 2010]) research and instrumentation programs, and contribute directly to satisfying the national needs to advance the knowledge of Space Sciences and promoting a technically trained and increasingly diverse workforce, as well as a scientifically literate citizenry (Education and Society Panel Report).

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