In situ observations of reconnection Hall magnetic fields at Mars:Evidence for ion diffusion region encounters

摘要

We present Mars Global Surveyor measurements of bipolar out-of-plane magneticfields at current sheets in Mars' magnetosphere. These signatures match predictions fromsimulations and terrestrial observations of collisionless magnetic reconnection, and couldsimilarly indicate differential ion and electron motion and the resulting Hall currentsystems near magnetic X lines. Thus, these observations may represent passages throughor very near reconnection diffusion regions at Mars. Out of 28 events found at 400 kmaltitude with well-defined current sheet orientations, 26 have magnetic fields consistentwith the expected polarities of Hall fields near diffusion regions. For these events, wefind an average ratio of Hall field to main field of 0.51 ± 0.13, and an average ratio ofnormal to main field (reconnection rate) of 0.16 ± 0.09, consistent with terrestrialobservations of reconnection. These events do not consistently correlate with the locationof crustal fields or with IMF reversals, indicating that magnetic field draping alone(perhaps enhanced by high solar wind dynamic pressure) may generate current sheetscapable of reconnection. For some events, we observe field-aligned electrons that maycarry parallel currents that close the Hall current loop. Electron distributions aroundcurrent sheets often indicate magnetic connection to the collisional exosphere. Forcrossings sunward of the X line, we usually observe an electron flux minimum at thecurrent sheet, consistent with the resulting closed magnetic structure. For crossingsantisunward of the X line, we do not observe flux minima, consistent with field lines opendownstream. Collisionless reconnection, if common at Mars, could represent a significantatmospheric loss process.