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.

Electron acceleration from contracting magnetic islands during reconnection

A long-standing problem in the study of space and astrophysical plasmas is to explain the production of energetic electrons as magnetic fields 'reconnect' and release energy. In the Earth's magnetosphere, electron energies reach hundreds of thousands of electron volts (refs 1-3), whereas the typical electron energies associated with large-scale reconnection-driven flows are just a few electron volts. Recent observations further suggest that these energetic particles are produced in the region where the magnetic field reconnects. In solar flares, upwards of 50 per cent of the energy released can appear as energetic electrons. Here we show that electrons gain kinetic energy by reflecting from the ends of the contracting 'magnetic islands' that form as reconnection proceeds. The mechanism is analogous to the increase of energy of a ball reflecting between two converging walls--the ball gains energy with each bounce. The repetitive interaction of electrons with many islands allows large numbers to be efficiently accelerated to high energy. The back pressure of the energetic electrons throttles reconnection so that the electron energy gain is a large fraction of the released magnetic energy. The resultant energy spectra of electrons take the form of power laws with spectral indices that match the magnetospheric observations.