Oxygen Ions in Magnetotail Reconnection

Spacecraft have observed a significant fraction of oxygen ions (O+) in Earth's magnetotail X-line during the periods of enhanced geomagnetic activity. It is important to understand how such O+ influences the reconnection process and how the O+ ions are heated due to reconnection. To this end we have used a 2.5D implicit Particle-in-Cell simulation (iPic3D) in a 2D Harris current sheet in the presence of H+ and O+. By comparing the simulation runs for oxygen concentrations of 50%, 5% and 0% (i.e. latter run only H+ ions), we found that (1) the dipolarization front (DF) propagation is encumbered by the current sheet O+ inertia, which reduces the DF speed and delays the fast reconnection phase; (2) the reconnection rate in the 50% O+ Run is much less than the 0% O+ Run, which can be attributed to the O+ drag on the convective magnetic flux via an ambipolar electric field in the O+ diffusion region; (3) without entering the exhaust, the lobe O+ can be accelerated near the separatrices away from the X-point by the Hall electric field and form the hot population downstream of the DFs; (4) the pre-existing current sheet O+ ions are reflected by the DFs and form a hook-shaped distribution in phase space, from which the DF speed history can be deduced; (5) the DF thickness is proportional to the O+ concentration in the pre-existing current sheet. These results illustrate the differences between storm-time and non-storm substorms due to a significant concentration of oxygen ions. The oxygen heating results are expected to be observable by the Magnetospheric Multiscale (MMS) mission in the magnetotail.