MESSENGER Observations of Magnetic Reconnection in Mercury’s Magnetosphere

Abstract

Solar wind energy transfer to planetary magnetospheres and ionospheres is controlled by magnetic reconnection, a process that determines the degree of connectivity between the interplanetary magnetic field (IMF) and a planet's magnetic field. During MESSENGER's second flyby of Mercury, a steady southward IMF was observed and the magnetopause was threaded by a strong magnetic field, indicating a reconnection rate ~10 times that typical at Earth. Moreover, a large flux transfer event was observed in the magnetosheath, and a plasmoid and multiple traveling compression regions were observed in Mercury's magnetotail, all products of reconnection. These observations indicate that Mercury's magnetosphere is much more responsive to IMF direction and dominated by the effects of reconnection than that of Earth or the other magnetized planets.

The Near-Saturn Magnetic Field Environment

“…This is a condition widely accepted for the terrestrial magnetopause. Further studies corroborate this importance on reconnection onset at other planets such as at Mercury [Slavin et al, 2009] and more recently DiBraccio et al [2013] showed that reconnection occurs for a wide range of magnetic shear angles, the rotation of the magnetic field from the magnetosheath into the magnetosphere, likely because of the low β conditions. Gershman et al (2013) also reported a large-scale PDL often forming just exterior of Mercury's subsolar magnetopause as a result of piled-up draped magnetic flux around the magnetosphere.…”

MESSENGER observations of large flux transfer events at Mercury

“…We present data in L-M-N coordinates, where B N is directed radially outward normal (using the Slavin et al [2009a] model) to the closest point on the magnetopause, B L is perpendicular to B N and anti-parallel to the planetary magnetic dipole, and B M completes the right-handed system.…”

Comparison of ultra‐low‐frequency waves at Mercury under northward and southward IMF

“…The spread in frequency about the ion-ion resonance frequency could be due to Doppler shifts. From Slavin et al [2009], the cross-tail electric field during M2 is estimated to be 2 mV/m, which yields an E Â B drift speed of 20 km/s in a 100-nT magnetic field, so the maximum range in predicted Doppler shifts would be 0.03 to 0.2 Hz. There is a need for theoretical studies of wave generation by loss-cone instabilities and of the propagation of these waves in hot and cold plasmas in Mercury's magnetosphere to determine whether a local generation mechanism is the best candidate for explaining the observed frequencies and polarization properties of these waves.…”

“…The outbound diamagnetic cavities, also termed a boundary layer (BL) by Slavin et al [2008Slavin et al [ , 2009, were observed during M1 from 19:10:37 to 19:14:14.8 UTC starting at 1.44 R M and a subsolar angle of 109°and during M2 from 08:44:26 to 08:49:11 starting at 1.23 R M and a sub-solar angle of 109°. Within the BL, emission peaks due to ULF waves are clearly visible just above the He ++ cyclotron frequency (f cHe++ ) during both flybys.…”