Mercury's Magnetospheric Cusps and Cross-Tail Current Sheet: Structure and Dynamics

Abstract

Mercury has proven to be a unique natural laboratory for space plasma processes. Mercury’s magnetosphere is formed by the interaction between its intrinsic planetary magnetic field and the supersonic solar wind. The structure of Mercury’s magnetosphere is very similar to Earth’s; yet the results from the MESSENGER mission to Mercury have shown that the spatial and temporal scales of magnetospheric processes are very different at Mercury. In this thesis, we analyze in situ observations from the MESSENGER spacecraft to characterize and understand the dynamic physical plasma processes occurring in Mercury’s magnetosphere. We identified and analyzed 345 plasma filaments in Mercury’s northern magnetospheric cusp to determine their physical properties. Cusp plasma filaments are magnetic structures that are identified on the basis of their characteristic 2‒3 seconds long decrease in magnetic field intensity. Our analysis indicates that these cusp filaments are cylindrical flux tubes filled with plasma, which causes a diamagnetic decrease in the magnetic field inside the flux tube. MESSENGER observations of flux transfer events (FTEs) and cusp filament suggests that cusp filaments properties are the low-altitude extension of FTEs formed at Mercury’s dayside magnetopause. We examined 319 central plasma sheet crossings observed by MESSENGER. Using a Harris model, we determined the physical properties of Mercury’s cross-tail current sheet. Analysis of BZ in the current sheet indicated that MESSENGER usually crossed the current sheet sunward of the Near Mercury Neutral Line. Magnetohydrodynamics-based analysis using the MESSENGER magnetic field and plasma measurements suggests that heavy planetary ions and/or ion temperature anisotropy may be important in maintaining radial stress balance within Mercury’s central plasma sheet. We report the observation of significant dawn-dusk variation in Mercury’s cross-tail current sheet with thicker, lower plasma β dawn side current sheets than the dusk side. Using the Harris current sheet model, we determined the peak current sheet current density and reported an asymmetry pattern for peak current density consistent with observed asymmetries in current sheet thickness. We propose that enhancement of heavy ions in the dusk side current sheet, due to centrifugal acceleration and gradient-curvature drift of ions from the cusp and current sheet, provides a partial explanation of the dawn-dusk current sheet asymmetries observed in this study. Furthermore, our results are consistent with earlier studies on reconnection-related structures and phenomenon, which suggest that the asymmetries observed in this study are associated with the asymmetric nature of magnetotail reconnection at Mercury. We also report the possible observation of an Earth-like substorm current wedge in the near-Mercury magnetotail. We calculate the total current in the Hermean substorm current wedge and found that the current close via the conductive planetary core. The current closure mechanism may be similar to the Region 1 currents observed in an earlier study. From the above results, we conclude that the plasma processes occurring at Mercury are different from those at Earth due to difference in internal plasma composition, relative size of Mercury’s magnetosphere and solar wind conditions at small heliospheric distances, despite many structural similarities in both magnetospheres. The results reported in this thesis have far-reaching implications for the physical processes in Mercury’s magnetospheres and those of the other planets.