Cold ion crescent echoes in the exhaust of symmetric magnetic reconnection

A low-energy (cold) ion population of ionospheric origin is often present in the Earth's magnetic tail alongside with a thermal (hot) ion population. The presence of cold ions introduces small-scale effects in the dynamics of magnetic reconnection owing to their small gyroradius. Hall electric fields at the reconnection separatrices demagnetize and accelerate inflowing cold ions in the direction perpendicular to the local magnetic field. Once cold ions have passed through the accelerating layer, they enter the exhaust and continue to follow regular E × B drift motion. The overlap of cold ion trajectories produces a partial ring distribution function. A simple analytical model is proposed, pointing to the regular formation of these crescent-looking distribution functions, which we denote “crescent echoes.” We performed two-dimensional particle-in-cell simulations to investigate these cold ion velocity space signatures in the reconnection exhaust. We carried out the simulations with various cold ion proportions relative to the hot ion component. In the reference run with very small, but non-negligible cold ion density, the distribution functions are very close to that from the analytical model. The reference run results are contrasted with the dynamics in the cold ion-dominated plasma in which ion-ion or lower-hybrid modes might develop. We conclude that an increased density of cold ions leads to blurring of the crescent echoes into a more thermal distribution due to wave activity development.