Energized Oxygen in the Magnetotail: Onset and Evolution of Magnetic Reconnection.

Oxygen ions are a major constituent of magnetospheric plasma, yet the role of oxygen in processes such as magnetic reconnection continues to be poorly understood. Observations show that significant amounts of energized O + can be present in a magnetotail current sheet (CS). A population of thermal O + only has a relatively 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 CS 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 against published results. We then energize a thermal background O + based on published in situ measurements. A range of energization is applied to the background O + . We discuss the effects of energized O + on CS thinning and the onset and evolution of magnetic reconnection. The presence of energized O + causes a two-regime onset response in a thinning CS. 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-regime, 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 central CS. As the energization increases, the CS thinning begins to slow and eventually reverses.