Mercury’s Tail Current Sheet from MESSENGER Magnetic Field Measurements
Tuesday, 16 December 2014
We have estimated the spatial variations in the position and average thickness of Mercury’s magnetospheric tail current sheet from orbital magnetic field data collected by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. We have investigated the changes in these average properties with varying solar wind conditions and magnetospheric activity. The time-averaged thickness of the current sheet was obtained from superposed epoch analysis (SEA) of the 1-s-averaged vector magnetic field data within ± 10 min of the identified magnetic equator position at different down-tail distances. The average thickness was then estimated from a given SEA by identifying the time interval during which the field completed a rotation from the sunward to the anti-sunward direction, or vice versa, accompanied by a depression in the field magnitude. We have found that the current sheet has a thickness of ~0.8 RM (where RM is Mercury’s radius, or 2440 km) close to the planet (~ 1.1 RM) and thins to ~0.2 RM in the far tail region (~2.8 RM). We examined individual orbits to catalogue the existence and number of current sheet crossings encountered on each orbit. These data allow us to (1) determine whether the thickness obtained from the SEA is an actual thickness or an apparent thickness controlled by rapid motions of the current sheet, and (2) estimate the statistical likelihood of observing the current sheet as a function of down-tail distance. For example, some magnetically quiet orbits that cross the magnetic equator at down-tail distances greater than 2 RM do not record a current sheet crossing and appear to cross closed field lines in the vicinity of the magnetic equator, even though they lie in a region in which the tail current sheet is usually observed. This result suggests that the inner (near-planet) edge of the current sheet is not stationary but may move anti-sunward (or sunward) under quiet (or active) magnetospheric conditions.