Resonant generation of an Alfven wave by a substorm injected electron
cloud: a Van Allen probe case study
Abstract
In the paper a unique phenomenon - the resonant generation of ultra-low
frequency (ULF) wave by energetic electrons in the magnetosphere - is
presented. On 27 October 2012 in the morning side of the magnetosphere
Van Allen Probe A registered an ULF wave with period of 100 s and an
amplitude of 0.7 nT. A cloud of energetic electrons was observed
simultaneously with the wave. It is established that the electron cloud
was injected into the magnetosphere as a result of a substorm. Electron
fluxes in several energy channels were modulated with the frequency of
the observed wave. It is shown that these flux modulations are caused by
the drift resonance of 38 keV electrons and the wave being a fundamental
harmonic of an Alfven mode with an azimuthal wavenumber
m~ 110-115 propagating to the east, and generated
through the gradient instability due to steep earthward density gradient
of the resonant electrons.