Simultaneous acceleration and pitch angle scattering of field‐aligned electrons observed by the LEPA on CRRES

Abstract

[1] Using data from the Low Energy Plasma Analyzer we present detailed pitch angle versus energy electron distributions at the time of field-aligned electron events. We present three case studies, as well as the findings of a larger survey of 158 events. These distributions show that electrons are apparently scattered in pitch angle out of field-aligned electron beams observed around 0.1-1 keV. Generally, this pitch angle scattering is accompanied by simultaneous acceleration. As electrons are scattered from the fieldaligned beam to pitch angles of $60°, they typically increase to energies of around 10 keV. The electrons are then seen to scatter to 90°pitch angle with no further change in energy. The observations suggest that this scattering and acceleration takes place near the geomagnetic equator and over a limited range of latitudes.

Trapping and acceleration of upflowing ionospheric electrons in the magnetosphere by electrostatic electron cyclotron harmonic waves

“…However, by using the inverse of the diffusion rate, one may estimate a timescale of 5–15 min for wave amplitudes of 1 mV m −1 or so. This corresponds well to the evolution field‐aligned distributions observed by Abel et al [].…”

“…Strong diffusion is usually associated with pitch angle scattering into the loss cone, but in the case we are considering here, the loss cone would be filled by upflowing electrons from the ionosphere. Observations show that between 50 and 100 eV the electron distribution inside the loss cone is indeed higher than that at larger pitch angles at the same energy [ Abel et al , ]. Since pitch angle diffusion extends to 60° or more electrons will be diffused out of the loss cone to larger pitch angles very effectively by ECH waves.…”

“…A necessary condition for beam‐plasma wave excitation is that there should be a positive gradient in the velocity distribution function ∂ f ( v )/ ∂ v ∥ >0. While a positive gradient has been found in low‐altitude observations [ Lin et al , ] the beams referred to in the equatorial observations are not actually beams but are in fact a peak in the differential energy flux (DEF) [ Abel et al , ]. Since f ( v ) is proportional to DEF/ E 2 a peak in the DEF does not mean that ∂ f ( v )/ ∂ v ∥ >0 or that the plasma is unstable to beam‐generated waves.…”

Medium energy pitch angle distribution during substorm injected electron clouds

“…Of the earlier observations of electron distributions around 1 keV many show strongly perpendicular (pancake) distributions [Meredith et al, 2000, and references therein]. Abel et al [2002] presents events of initially strongly field-aligned electron distributions that later were observed to spread in pitch angle and also move to higher energy. Observations as mentioned above were performed by instruments on GEOS-1, GEOS-2 and the LEPA instrument on CRRES.…”

Quasi-parallel electron beams and their possible application in inferring the auroral arc's root in the magnetosphere

“…In many strong upgoing electron beam events investigated, their mean energies typically range from ∼ 100 eV up to a few keV. When the electron beams are accelerated out from both hemispherical ionospheres and enter the magnetosphere, they undergo beam-plasma interactions, which are conducive to a number of plasma waves acting to scatter the electron beam and broaden its pitch-angular distribution (e.g., Zhang et al, 1993;Abel et al, 2002a). Such wave-induced diffusion processes compete with the adiabatic shrinking of pitch angle due to the reduction of the magnetic field inside the magnetosphere.…”