Is Mercury's Magnetosphere Driven By Flux Transfer Events?

Abstract:

Mercury's magnetosphere closely resembles that of Earth in terms of its topology and structure, but major differences are found when their dynamics are compared. The strong interplanetary magnetic fields at 0.3 to 0.5 AU result in low Alfven Mach numbers, weak bow shocks and low plasma β magnetosheaths at Mercury. These conditions support the development of strong plasma depletion layers adjacent to the magnetopause and intense magnetopause reconnection. MESSENGER observations indicate that reconnection occurs for all non-zero shear angles across the magnetopause with magnetosheat β being the primary factor controlling its rate. Flux transfer events (FTEs) with ~ 1-2 s durations and flux rope topology are observed during nearlly all magnetopause crossings. In contrast with the Earth where FTEs are typically observed every ~ 8 min, FTE ecounters at Mercury are separated on average by only ~ 10 s. At lower altitudes near the cusp MESSENGER observes ~1-2-s-long strong decreases in mgnetic field intensity that are termed cusp plasma filaments. These filaments are beleived to be formed by the inflow of magnetosheath plasma associated with flux transfer events. Mercury's magnetotail exhibits magnetic flux loading/unloading events similar to those observed at Earth during substorms. The Dungey cycle durations and lobe flux loading amplitudes are ~ 2 – 3 min and ~ 30 to 50% at Mercury as compared to ~ 1 - 2 hr and ~ 10 to 25% at Earth. However, FTEs at Earth account for only a few per cent of the magnetic flux carried by the Dungey cycle, while the contribution of FTEs at Mercury is estimated to be ~ 30 to 50%. Mercury also differs from Earth in that it lacks an ionosphere, but possesses a large, highly conducting iron core. The strong IMF and lack of an ionosphere results in a relatively large dawn-to-dusk cross-magnetosphere potential drop of ~ 30 kV at Mercury. Inductive coupling between Mercury's magnetosphere and its large iron core stiffens the dayside magnetosphere against compression by the strong solar wind pressure increases that accompay coronal mass ejections and high-speed streams. The effect of this inductive coupling on FTEs and other aspects of magnetospheric dynamics remains to be determined.