Electron and ion edges and the associated magnetic topology of the reconnecting magnetopause

Using high-resolution burst mode THEMIS data, we have examined in detail the electron and ion edges of the reconnecting magnetopause and the associated magnetic topologies of 23 high shear reconnecting magnetopause crossings. The electron edge is identified as the most earthward detection of entering magnetosheath electrons and the accompanying first loss of magnetospheric electrons. The electron edge thus marks the most earthward measurable open (reconnected) field line. The ion edge, identified as the most earthward detection of entering magnetosheath ions, was always detected either sunward of the electron edge or simultaneous with it, indicating that the entire low-latitude boundary layer (LLBL) was on open field lines. The radial separation of the electron and ion edges is due to a time-of-flight effect associated with the fact that the entering magnetosheath electrons have considerably higher speeds than the entering magnetosheath ions. Importantly, our study reveals that an examination of three-dimensional particle distributions covering the entire range of energies of the various magnetosheath and magnetospheric populations present is essential for a correct determination of the magnetic topology of the LLBL. Deducing the topology from electron pitch angle distributions covering a limited energy range can lead to incorrect deduction of the topology.