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. Despite this, published simulation studies have 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+ are used to validate our setup. We energize a thermal background of 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. Our principle findings indicate that 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.