MESSENGER observations of large dayside flux transfer events: Do they drive Mercury's substorm cycle?

The large-scale dynamic behavior of Mercury’s highly compressed magnetosphere is predominantly powered by magnetic reconnection, which transfers energy and momentum from the solar wind to the magnetosphere. The contribution of flux transfer events (FTEs) at the dayside magnetopause to the redistribution of magnetic flux in Mercury’s magnetosphere is assessed with magnetic field data acquired in orbit about Mercury by the Magnetometer on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. FTEs with core fields greater than the planetary field just inside the magnetopause are prevalent at Mercury. Fifty-eight such large-amplitude FTEs were identified during February and May 2012, when MESSENGER sampled the subsolar magnetosheath. The orientation of each FTE was determined by minimum variance analysis, and the magnetic flux content of each was estimated using a force-free flux rope model. The average flux content of the FTEs was 0.06 MWb, and their durations imply a transient increase in the cross-polar cap potential of ~25 kV. For a substorm timescale of 2–3 min, as indicated by magnetotail flux loading and unloading, the FTE repetition rate (10 s) and average flux content (assumed to be 0.03 MWb) imply that FTEs contribute at least ~30% of the flux transport required to drive the Mercury substorm cycle. At Earth, in contrast, FTEs are estimated to contribute less than 2% of the substorm flux transport. This result implies that whereas at Earth, at which steady-state dayside reconnection is prevalent, multiple X-line dayside reconnection and associated FTEs at Mercury are a dominant forcing for magnetospheric dynamics.