Ion composition and pressure changes in storm time and nonstorm substorms in the vicinity of the near‐Earth neutral line

Abstract

Using CLUSTER/CODIF data from close to ∼19 Re in the magnetotail, we have performed a superposed epoch analysis of storm time and nonstorm substorms to determine how the ion composition changes during a substorm. We find that the median O+ density and pressure in the plasma sheet are a factor of 5 higher during storm times than during nonstorm times. However, we do not observe significant changes in the composition during a substorm that would indicate that ionospheric outflow is playing a dynamic role in loading the plasma sheet or triggering the substorm at this location. There are differences between the storm time and nonstorm substorms, and it is intriguing to consider whether the composition differences play a role. The storm time substorms exhibit more loading and faster unloading than the nonstorm substorms. In addition, we observe differences in the H+ and O+ behavior at onset in the storm time substorms that we attribute to the different dynamics of the two ion species at the reconnection site and during the field reconfiguration due to their different gyroradii. The H+ density and pressure decrease over the whole energy range at substorm onset, while the O+ density and pressure decrease less, and the O+ temperature increases. That more O+ is left after substorm onset indicates that either the O+ is more quickly replenished from O+ in the lobes and/or that the more energetic O+, due to its larger gyroradius, is not depleted when the field reconfigures and is accelerated in the thin current sheet.