The complex structure of the reconnecting magnetopause

Abstract

Electric and magnetic fields observed in a one-of-a-kind example of a Polar satellite magnetopause crossing are consistent with static guide magnetic and electric fields, Hall magnetohydrodynamic (MHD) electric and magnetic fields, and a Z-component of the magnetic field that varied from −80 nT to +80 nT across the magnetopause [F. S. Mozer, S. D. Bale, and T. D. Phan, Phys. Rev. Lett. 89, 015002 (2002)]. In spite of this excellent agreement with simulations, other features of the data were unanticipated. An empirical model, based on these measured fields and the assumption that the parallel electric field was zero, is developed to explain such features by showing that (1) Postreconnection E×B/B2 flows, carrying electrons, magnetic field lines, and Poynting flux towards the X-line rather than away from it, occur at some locations. (2) The model and measured tangential electric fields varied significantly through the magnetopause. If the magnetopause was a time stationary structure, Faraday’s law requires that it be three-dimensional on a spatial scale in the Y-direction of a few ion skin depths. This three-dimensionality may explain why only one example having fields that agree with Hall MHD simulations has been found. (3) There were regions within the magnetopause where electromagnetic energy may have been generated (in the normal incidence frame tied to the magnetopause). (4) Significant conversion of electromagnetic energy can occur inside the magnetopause in the absence of an electron diffusion region, parallel electric fields, or the electrons being decoupled from the magnetic field. It is emphasized that these properties are consequences of the Hall MHD and guide electric and magnetic fields in the absence of any additional non-MHD processes.