The Disappearance of the Post-Midnight High Energy Ion Plasmasphere

Tuesday, 16 December 2014
Lois K Sarno-Smith1, Michael Warren Liemohn1, Roxanne M Katus1, John R Wygant2, Ruth M Skoug3, Brian Larsen3, Michelle F Thomsen4 and Mark Moldwin1, (1)University of Michigan, Ann Arbor, MI, United States, (2)University of Minnesota Twin Cities, Minneapolis, MN, United States, (3)Los Alamos Natl Lab, Los Alamos, NM, United States, (4)Planetary Science Institute, Los Alamos, NM, United States
The Van Allen Probes Helium Oxygen Proton Electron (HOPE) instrument measures a high energy tail of the thermal plasmasphere that has strong MLT dependence in the near Earth space. In our study, we statistically analyze a 16 month period of HOPE data, looking at quiet times with a Kp index of less than 3. The high energy plasmasphere tail is the upper 5% of plasmasphere energies, consisting of ions between 1 - 10 eV. We calculated plasma densities over this energy range and see that there is strong depletion in O+ and H+ from 1-4 MLT and a similar but less dramatic density decline in He+. Our results are compared with the Van Allen Probes Electric Fields and Waves (EFW) instrument space craft potential to rule out spacecraft charging. We conclude that the post-midnight ion disappearance is due to diurnal ionospheric temperature variation and charge exchange processes.