Modeling the effects of local time variation of plasma sheet properties on proton ring current energy and peak location

Abstract

We have run kinetic ring current simulations with a large range of outer boundary plasma properties during a test time interval conducive to a storm. We varied the density, temperature and local time location of the source plasma sheet to investigate their influence on the proton ring current total energy and peak location. We found that: (1) the density is the main factor controlling the total ring current energy for the given field model; (2) the source local time location is also a major factor, with substantially larger total energies obtained for post‐midnight source locations; (3) for any given density, enhanced source temperature leads to an enhanced total energy, but only if the source is post‐midnight; (4) the density has essentially no effect on the ring current peak location; (5) a lower source temperature leads to a peak ring current closer to Earth, but it has little effect on the local time of the ring current peak. However, results (4) and (5) would differ if using an electro‐magnetically self‐consistent model. (6) The source location has a substantial impact on both the radial location and local time of the peak; in particular, the peak ring current is displaced post‐midnight for post‐midnight source locations when considering energy densities in the energy range [1, 40] keV. We also investigated the dependence of the ring current peak location on storm epoch, i.e., during the main phase. We found that the ring current peak moves inward during the first few hours of the storm, as expected from inward transport. For all storm epochs, the peak is found at a local time close to that of the source population for the energy range [1, 40] keV, but is found at dusk throughout the storm for the full energy range ([0.5, 400] keV). Finally, we show how the source location influences the energy spectrum of ring current protons. As a result of larger proton energization, by electrically drifting inward for a longer time, a post‐midnight source location leads to substantially larger (lower) ring current energy content for energies above (below) 40 keV than for a source centered at midnight. Our main conclusions are that (1) a post‐midnight source location leads to a substantially stronger total ring current energy; (2) it may only lead to a post‐midnight peak at middle‐to‐low energies; but (3) in such cases the energies at which such displaced peaks occur only constitute a small fraction of the total ring current energy.