MESSENGER observations of magnetospheric substorm activity in Mercury's near magnetotail

Abstract

MErcury Surface, Space ENviroment, GEochemistry, and Ranging (MESSENGER) magnetic field and plasma measurements taken during crossings of Mercury's magnetotail from 2011 to 2014 have been examined for evidence of substorms. A total of 26 events were found during which an Earth-like growth phase was followed by clear near-tail expansion phase signatures. During the growth phase, just as at Earth, the thinning of the plasma sheet and the increase of the magnetic field intensity in the lobe are observed, but the fractional increase in field intensity could be ∼3 to 5 times that at Earth. The average timescale of the growth phase is ∼1 min. The dipolarization that marks the initiation of the substorm expansion phase is only a few seconds in duration. During the expansion phase, lasting ∼1 min, the plasma sheet is observed to thicken and engulf the spacecraft. The duration of the substorm observed in this paper is consistent with previous observations of Mercury's Dungey cycle. The reconfiguration of the magnetotail during Mercury's substorm is very similar to that at Earth despite its very compressed timescale.