Observational evidence of the excitation of magnetosonic waves by an
He++ ion ring distribution
Abstract
We report plasma wave observations of equatorial magnetosonic waves at
integer harmonics of the local gyrofrequency of doubly-ionized helium
(He++). The waves were observed by Van Allen Probe A on 08 Feb 2014 when
the spacecraft was in the afternoon magnetic local time sector near
L ≈ 4 inside of the plasmasphere. Analysis of the complementary
in-situ energetic ion measurements (1-300 keV) reveals the presence of a
helium ion ring distribution centered near 30 keV. Theoretical linear
growth rate calculations suggest that the local plasma and field
conditions can support the excitation of the magnetosonic waves from the
unstable ring distribution. This represents the first report of the
generation of magnetosonic equatorial noise via a ring distribution in
energetic He++ ions in the near-Earth space plasma environment.