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