Postmidnight depletion of the high‐energy tail of the quiet plasmasphere

Abstract

The Van Allen Probes Helium Oxygen Proton Electron (HOPE) instrument measures the high‐energy tail of the thermal plasmasphere allowing study of topside ionosphere and inner magnetosphere coupling. We statistically analyze a 22 month period of HOPE data, looking at quiet times with a Kp index of less than 3. We investigate the high‐energy range of the plasmasphere, which consists of ions at energies between 1 and 10 eV and contains approximately 5% of total plasmaspheric density. Both the fluxes and partial plasma densities over this energy range show H+ is depleted the most in the postmidnight sector (1–4 magnetic local time), followed by O+ and then He+. The relative depletion of each species across the postmidnight sector is not ordered by mass, which reveals ionospheric influence. We compare our results with keV energy electron data from HOPE and the Van Allen Probes Electric Fields and Waves instrument spacecraft potential to rule out spacecraft charging. Our conclusion is that the postmidnight ion disappearance is due to diurnal ionospheric temperature variation and charge exchange processes.

Local time variations of high‐energy plasmaspheric ion pitch angle distributions

“…Figure (right column) shows the relative variability or the standard deviation ( σ D ) divided by the mean ( μ ). We have used fraction instead of percent to quantify the uncertainty and variability of the measurements to be consistent with our earlier studies on this subject [ Sarno‐Smith et al , ]. For 1.5 eV and 3.0 eV, the relative variability is higher in the postmidnight sector, especially at pitch angles near 0° and 180°.…”

“…Outside the postmidnight sector, variability is higher at all MLTs compared to the 1.5 and 3.0 eV energy channels. Sarno‐Smith et al [] also found similar variability differences at different MLTs but showed in Figure 7 that although the postmidnight sector had higher relative variability, the bulk of the postmidnight sector 1–10 eV fluxes were still significantly below (greater than an order of magnitude) the 1–10 eV fluxes outside the postmidnight sector.…”

“…It should be noted that spacecraft potential is a function of total plasma density and temperature; however, this study solely focuses on the 1–10 eV fluxes, so changes in the 1–10 eV fluxes may be independent to spacecraft potential changes. Previous studies found that Van Allen Probes spacecraft potential is about 0.75 V in the 2 < L < 3 region and there are not exceptionally large positive potentials in the postmidnight sector [ Sarno‐Smith et al , , ]. Nonetheless, it is important to remember that this positive potential does add some uncertainty to the aforementioned pitch angle distributions at all MLTs even with spacecraft potential corrected fluxes.…”

“…However, without full density or temperature resolution, we are unable to conclude if the depletion is from temperature, density, or combination of both. A previous study suggested that the 1–10 eV plasmasphere depletion might be linked to ionospheric outflow [ Sarno‐Smith et al , ]. Here ionospheric outflow refers to the heating of the topside ionosphere and subsequent transport of plasma to the plasmasphere along flux tubes.…”

Temperature Dependence of Plasmaspheric Ion Composition

“…Using this Van Allen Probes-based model the estimated total plasmaspheric mass inside L = 5 is between 600 t ( Figure A1c) and 790 t ( Figure A1d). However, there is a very shallow postmidnight dip in T H + (or T He + ) that is not inconsistent with the postmidnight depletion in superthermal H + ions observed by Van Allen Probes (Sarno-Smith et al, 2015;, Sarno-Smith, Liemohn, Skoug, Santolik, et al,2016. Nonetheless, even in the lower-limit case the O + distribution contributes tens of metric tons, about 13% of the total plasmaspheric mass.…”