THEMIS observations of modified Hall fields in asymmetric magnetic field reconnection

Abstract

Field and plasma measurements from three THEMIS spacecraft are compared during a back-and-forth crossing of an asymmetric, reconnecting, sub-solar magnetopause to explain the apparent absence of the quadrupolar magnetic field and part of the bipolar electric field that are found during symmetric magnetic field reconnection. The differences in the reconnection magnetic fields in the asymptotic regions (a factor of three) and the plasma densities (a factor of 30) combine to make the Hall MHD term and the resulting Hall electric field in the Generalized Ohm's law significant only on the magnetospheric side of the magnetopause. Qualitative analysis indicates that the single particle and fluid trajectories in such fields are different from those in the case of symmetric boundary conditions. Copyright 2008 by the American Geophysical Union.