Ion distributions near the reconnection sites: Comparison between simulations and THEMIS observations

We study the evolution of ion distribution functions in a magnetotail thin current sheet using as a case study the 26 February 2008 substorm event observed by the THEMIS mission. The observed mushroom-shaped velocity-space ion distributions, whereby warmer ions are mostly moving duskward, are shown to appear when the current sheet thickness becomes comparable with thermal ion gyroradius. As magnetic reconnection starts Earthward of the THEMIS satellite (probe) P1, these duskward-moving warmer ions also start to stream tailward, gradually separating in velocity space from the colder ions that dominate the density moment. Test-particle simulations demonstrate that the observed evolution of ion distributions is consistent with cucumber-type trajectories in a current sheet with an X-type configuration.