Energized Oxygen in the Magnetotail: Onset and Evolution of Magnetic
Reconnection
Abstract
Oxygen ions are a major constituent of magnetospheric plasma, yet the
role of oxygen in processes such as magnetic reconnection is poorly
understood. Observations show that significant energized $O^+$ can
be present in a magnetotail current sheet. A population of thermal
$O^+$ only has a minor effect on magnetic reconnection. Despite
this, published studies have so far only concentrated on the role of the
low-energy thermal $O^+$. We present a study of magnetic
reconnection in a thinning current sheet with energized $O^+$
present. Well-established, three-species, 2.5D Particle-In-Cell (PIC)
kinetic simulations are used. Simulations of thermal $H^+$ and
thermal $O^+$ validate our setup. We energize a thermal background
$O^+$ based on published measurements. We apply a range of
energization to the background $O^+$. We discuss the effects of
energized $O^+$ on current sheet thinning and the onset and
evolution of magnetic reconnection. Energized $O^+$ has a major
impact on the onset and evolution of magnetic reconnection. Energized
$O^+$ causes a two-regime onset response in a thinning current
sheet. As energization increases in the lower-regime, reconnection
develops at a single primary {X}-line, increases time-to-onset, and
suppresses the rate of evolution. As energization continues to increase
in the higher-regimes, reconnection develops at multiple {X}-lines,
forming a stochastic plasmoid chain; decreases time-to-onset; and
enhances evolution via a plasmoid instability. Energized $O^+$
drives a depletion of the background $H^+$ around the current
sheet. As energization increases, the thinning begins to slow and
eventually reverses, leading to disruption of the current sheet via a
plasmoid instability.