Dipolarization Impacts on the Ion Composition of the Near-Earth Plasma Sheet during magnetic storms

Abstract:

The near-earth nightside plasma sheet is the bridge between the inner and outer magnetosphere. During storm times, hot plasma from this region is adiabatically convected inwards leading to the buildup of the storm-time ring current. Thus, understanding this near-earth hot plasma sheet population is one of the keys to predicting the strength of the ring current. However, the composition of this plasma is highly variable. During storm times, the plasma sheet composition in the ~6 – 12 Re tail region changes due to O+ entry from the lobes (from the cusp) and the direct feeding from the night side auroral region. In addition, at substorm onset the plasma sheet O+ ions can be preferentially accelerated. We use dipolarization events observed by MMS from the near-earth tail region during magnetic storms, to monitor the composition changes and energization in the ~6 – 12 Re plasma sheet region. We use data from both the HPCA ion composition instrument (1 ev/e-40 keV/e) to identify the source ions and the higher energies to determine how the composition changes at the higher energy end. For this we also use energetic particle observations from the EIS instrument. This way we determine how often the substorms are associated with the low energy beams, and how the composition changes at the higher energies due to the dipolarization energization.