“Pancake” electron distributions in the outer radiation belts

Abstract

Abstract. Electron pitch angle distributions sharply peaked at 90 ø pitch angle were first recorded in the energy range 50 eV < E < 500 eV by the GEOS 1 and GEOS2 spacecraft in 1977/1978, from the plasmapause out to geostationary orbit. At the time they were explained as the remnants of pitch angle diffusion driven solely by electron cyclotron harmonic (ECH) waves. Here we report new observations by the Low Energy Plasma Analyser on board the Combined Release and Radiation Effects Satellite, which measured the complete pitch angle distribution over the energy range 100 eV < E < 30 keV. The pancake distributions are seen to develop from injected distributions that are nearly isotropic in velocity space, on a timescale that is greater than 2 hours. The freshly injected distributions are associated with strong ECH and whistler mode waves suggesting that the pancake distributions are likely to be caused by a combination of both wave types. Outside L = 6.0 the fitting analysis at energies in the range 100 eV < E < 1 keV shows that in the marginally stable state the phase space density contours lie approximately along the characteristic curves for diffusion by whistler mode waves determined independently from the plasma wave data. However, inside L = 6.0, significant departures are observed. Our results suggest that whistler mode waves play a dominant role in the formation of pancake distributions outside L = 6.0, whereas inside L = 6.0 and, in particular, in the vicinity of the plasmapause, the ECH waves also play a significant role. Consequently, both types of waves should be considered in any attempt to explain the diffuse aurora and the variation with L taken into account.

The Morphology of Decimetric Emission from Solar Flares: GMRT Observations

“…A natural explanation for this observation would be that the electrons have large pitch angles and reside on magnetic field lines with a gradient in field strength from loop top to footpoints, so that magnetic mirroring traps the electrons near the loop tops. Such electrons would have a "pancake" pitch angle distribution, peaked at 90 • , similar to those commonly seen near the geomagnetic equator in the Earth's radiation belts (e.g., Meredith et al, 1999, Horne andThorne, 2000). Such pitch angle distributions can arise either by the loss of all particles at smaller pitch angles, as occurs via precipitation into the solar chromosphere in solar flares, or by an instability that drives electrons to larger pitch angles.…”

Oblique Whistler-Mode Waves in the Earth’s Inner Magnetosphere: Energy Distribution, Origins, and Role in Radiation Belt Dynamics

“…Such isolated injections do not contribute significantly to the ring current system and are well seen mainly in variations of the AE index, which is well correlated with electron injections (Meredith et al 1999;Gabrielse et al 2014). A long duration auroral activity generated by corotating interaction streams (Tsurutani et al 2006) represents another example of situation where there is no significant D st or K p variations, but electrons may be strongly scattered or accelerated by whistler waves.…”

Transport and loss of the inner plasma sheet electrons: THEMIS observations

“…By using the wave and particle data obtained from the CRRES satellite, Meredith et al [1999] inferred that both ECH and whistler mode waves play significant roles in scattering the plasma sheet electrons to produce the diffuse aurora (whistler mode waves at L ≥ 6 and ECH waves at L ≤ 6, especially near the plasmapause). Sergienko et al [2008] investigated the fine structure of the diffuse aurora using the high-sensitivity ground-based imager and the FAST electron measurements.…”