Correlations between chorus properties and electron velocity distributions: Van Allen Probes

Abstract:

Magnetospheric chorus consists of whistler waves driven unstable by anisotropic
electron velocity distributions. A gap in the power spectrum of chorus at a fre-
quency close to half the electron cyclotron frequency (Ω_e/2) are often observed
so that chorus can be categorized into four types accordingly: banded chorus
(with two bands in the spectrum separated by Ω_e/2), lower band only (with
only one band below Ω_e/2), upper band only (with only one band above Ω_e/2)
and broadband (with only one band including Ω_e/2). Here we present a study
to correlate chorus properties with electron velocity distributions based on the
hypothesis that each band of chorus is excited by an anisotropic electron com-
ponent. On Jan 14 2013, Van Allen Probes satellite A measured strong chorus
activity, and all four types were observed. We analyze HOPE and EMFISIS data
and show that there is a good correlation between the observed wave frequency
and propagation direction and the predictions of kinetic linear dispersion theory
using electron component densities and temperatures obtained by fitting HOPE
data to a multi-component bi-Maxwellian distribution function. However, the
temperature anisotropies observed by HOPE are usually close to the instabil-
ity threshold and therefore it is not possible at this point to predict whether a
certain band can be excited based on measured electron velocity distributions.
LA-UR-14-26177.