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. Key Points Energized O+ has a major impact on the onset and evolution of magnetic reconnection The presence of energized O+ causes a two‐regime onset response in a thinning current sheet At lower energization, O+ increases time‐to‐onset and suppresses the rate of evolution; at higher energization, the opposite occurs