Particle-In-Cell Simulations of Energized Oxygen in the Magnetotail

George, Don E.
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Oxygen ions are a major constituent of magnetospheric plasma. Despite this, the role of oxygen in the magnetosphere remains largely unknown, especially its role in magnetic reconnection. Current research shows that thermal oxygen has a minor effect on magnetic reconnection. The study of three-species systems employs particle-in-cell (PIC) simulations. These studies limit this background to thermal O+. Excluding those involving ions accelerated by magnetic reconnection itself, there are no studies involving energized O+. Research still concentrates on the role of low energy thermal O+. Observations show that significant amounts of energized O+ can be present in the magnetotail current sheet. Observations also show that O+ bifurcated current sheets exist. Researchers postulate that a statistical population of Speiser-orbiting O+ should form a bifurcated current sheet (BCS). This had not been verified using numerical simulation. Research presented here examines a thinning current sheet with energized O+ present. The thinning leads to the onset of magnetic reconnection. This research resulted in two scholarly works, presented herein; one is published and one is in peer review for publication. I use three-species, 2.5D kinetic particle-in-cell simulations using a thermal O+ background. PIC simulations of magnetospheric plasma are well established in the field. I then energize the thermal O+ based on published in situ measurements. The first paper demonstrates that a statistical population of Speiser-orbiting energized heavy ions will produce a bifurcated current sheet. A single population of ions produces the bifurcation, i.e. two spatially separated peaks in the current distribution. Moreover, magnetic reconnection is not required to produce the bifurcated current sheet. The second paper demonstrates that energized O+ has a major impact on magnetic reconnection. In the presence of energized O+, the current sheet has a two-regime onset response. At lower energization, O+ increases time-to-onset and suppresses the rate of evolution. At higher energization, O+ decreases time-to-onset and enhances evolution via a plasmoid instability. The principal findings of this research are given in one statement: Energized, not thermal, oxygen has a major impact on magnetic reconnection.

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Magnetism, Oxygen, Ions, Energized oxygen, Kinetic plasma simulations, Magnetic reconnection, Magnetosphere, Magnetotail, Particle-in-cell
Physics and Astronomy