MESSENGER Observations of Mercury's Nightside Magnetosphere Under Extreme Solar Wind Conditions: Reconnection‐Generated Structures and Steady Convection

Abstract

AbstractMercury's nightside magnetosphere is investigated under the impact of a coronal mass ejection (CME) and a high‐speed stream (HSS) with MErcury Surface, Space ENviroment, GEochemistry, and Ranging (MESSENGER) observations. The CME was shown to produce a low plasma (ratio of thermal pressure to magnetic pressure) magnetosheath, while the HSS creates a higher magnetosheath. Reconnection at the dayside magnetopause was found to be stronger during the CME than the HSS, but both were stronger than the average condition (Slavin et al., 2014, https://doi.org/10.1002/2014JA020319). Here we show that the CME and HSS events produced large numbers of flux ropes and dipolarization fronts in the plasma sheet. The occurrence rates for the structures were approximately 2 orders of magnitude higher than under average conditions with the rates during CME's being twice that of HSS's. The flux ropes appeared as quasiperiodic flux rope groups. Each group lasted approximately 1 min and had a few large flux ropes followed by several smaller flux ropes. The lobe magnetic flux accounted for around half of the Mercury's available magnetic flux with the flux during CME's being larger than that of HSS's. The CME produced a more dynamic nightside magnetosphere than the HSS. Further, for the CME event, the tail magnetic reconnection produced a distorted Hall magnetic field pattern and the X‐line had a dawn‐dusk extent of 20% of the tail width. No magnetic flux loading and unloading events were observed suggesting that, during these intense driving conditions, Mercury's magnetosphere responded with a type of quasi‐steady convection as opposed to the tail flux loading‐unloading events seen at Earth.