The Combined Influence of Lower Band Chorus and ULF waves on Radiation
Belt Electron Fluxes at Individual L-shells
Abstract
We investigate the timing and relative influence of VLF in the chorus
frequency range observed by the DEMETER spacecraft and ULF wave activity
from ground stations on daily changes in electron flux (0.23 to over 2.9
MeV) observed by the HEO-3 spacecraft. At each L shell, we use multiple
regression to investigate the effects of each wave type and each daily
lag independent of the others. We find that reduction and enhancement of
electrons occur at different time scales. Chorus power spectral density
and ULF wave power are associated with immediate electron decreases on
the same day but with flux enhancement 1-2 days later. ULF is nearly
always more influential than chorus on both increases and decreases of
flux, although chorus is often a significant factor. There was virtually
no difference in correlations of ULF Pc3, Pc4, or Pc5 with electron
flux. A synergistic interaction between chorus and ULF waves means that
enhancement is most effective when both waves are present, pointing to a
two-step process where local acceleration by chorus waves first
energizes electrons which are then brought to even higher energies by
inward radial diffusion due to ULF waves. However, decreases in flux due
to these waves act additively. Chorus and ULF waves combined are most
effective at describing changes in electron flux at >1.5
MeV. At lower L (2-3), correlations between ULF and VLF (likely hiss)
with electron flux were low. The most successful models, over L=4-6,
explained up to 47.1% of the variation in the data.