Electron demagnetization and heating in quasi-perpendicular shocks

摘要

Field and plasma measurements made during a quasi-perpendicular terrestrial bow shock crossing by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) E spacecraft on 21 August 2010 show that the measured electric field, being as great as 300 mV/m in the ramp, produced potential fluctuations over an electron gyrodiameter, which were as much as 15 times greater than the electron temperature. This fact requires that the electrons were unmagnetized in the ramp as a result of being scattered many times in a single gyroperiod by the fluctuating electric field. Supporting observations include the facts that the perpendicular electron heating and the parallel electron heating were equal in magnitude and simultaneous in time within experimental uncertainties and a factor of about five smaller than expected for perpendicular adiabatic heating of magnetized electrons. The conclusion that the electrons were unmagnetized is generalized through observations of 70 bow shock crossings on THEMIS and 140 shock crossings on Cluster to conclude that electrons are at least frequently unmagnetized in shock ramps such that their trajectories are more random than adiabatic and that they are energized during their chaotic traversal of the shock by the cross-shock quasi-DC electric field in the Normal Incidence Frame. Key PointsElectric field fluctuations cause electrons in the bow shock to be unmagnetizedUnmagnetized electrons are heated by the cross-shock electric fieldThe data is not consistent with magnetized electrons crossing the shock