Acceleration and thermalization of cold ions during magnetic reconnection

Abstract:

Depending on the activity level, cold ions of ionospheric origin can be abundant in the Earth's magnetosphere. They are commonly observed in regions of ongoing magnetic reconnection where they can introduce additional length scales, lead to mass loading, and reduce the reconnection rate in a complex fashion. Cold ions have been observed deep within the exhaust, behind dipolarization fronts. Close to the equatorial plane, the cold ion population was found to consist of two counter-streaming beams parallel the magnetic field. However, how exactly they couple into the dynamics of the dipolarization front, including how they get captured by the moving magnetic trap and energized, and how their presence impacts local kinetic scale lengths, has not been firmly established. In this study, we employ 2.5D particle-in-cell simulations to investigate the acceleration of cold ions in the reconnection exhaust, how and where the cold ion beams become thermalized, as well as the effect of the cold ions on the dipolarization front dynamics. We find that the counter-streaming beams persist in the equatorial plane throughout the exhaust between the X line and the dipolarization front, but that the details of their acceleration mechanism vary. The temperature of the inflowing plasma has high impact on the density and temperature structure within the exhaust. Compared to a simulation with hot inflow plasma, the density behind the dipolarization front can be larger by a factor of two.