Effects of ion dynamics on kinetic structures of the diffusion region during magnetic reconnection

Abstract:

Based on results from Particle-in-cell (PIC) simulations, we report how ion dynamics influence
the Hall electric field and electron velocity distributions in the diffusion region of magnetic reconnection.
The Hall electric field is due to charge imbalance in the diffusion region. At early times, within a few ion cyclotron oscillations from the peak reconnection,
electron orbit dynamics dominate, and the Hall electric field layer assumes the width of the electron current layer.
As the pre-existing current sheet ions are accelerated and jetted away, inflowing ions form an ion phase space hole structure.
The ion hole structure is self-consistently supported by the Hall electric field. The ion meandering orbit width increases
over the course of about 10 ion cyclotron oscillations from several to approximately 40 electron skin depths (two ion skin depths,
where the skin depth is based on the initial current sheet density), and the
Hall electric field layer widens in the same manner to become much broader than the electron diffusion region.
The electron velocity distributions upstream of the electron diffusion region and within the region
of counter streaming ions become fragmented as the ion hole establishes itself.
The fragmentation is carried into the electron diffusion region, and through the electron outflow jet, leading to a multitude of arcs in the electron distributions at the end of the jet. The broadening of the Hall electric field layer resolves a longstanding discrepancy concerning whether the narrowest width of the layer is of the electron [Chen et al., 2008] or ion [Mozer et al., 2002] scale. The fragmentation of the electron distributions may be due to an electron-ion instability, and is under
investigation.