Radial diffusion simulations of the 20 September 2007 radiation belt dropout

J Albert

doi:10.5194/angeo-32-925-2014

Abstract

Abstract. This is a study of a dropout of radiation belt electrons, associated with an isolated solar wind density pulse on 20 September 2007, as seen by the solid-state telescopes (SST) detectors on THEMIS (Time History of Events and Macroscale Interactions during Substorms). Omnidirectional fluxes were converted to phase space density at constant invariants M = 700 MeV G−1 and K = 0.014 RE G1/2, with the assumption of local pitch angle α &amp;approx; 80° and using the T04 magnetic field model. The last closed drift shell, which was calculated throughout the time interval, never came within the simulation outer boundary of L* = 6. It is found, using several different models for diffusion rates, that radial diffusion alone only allows the data-driven, time-dependent boundary values at Lmax = 6 and Lmin = 3.7 to propagate a few tenths of an RE during the simulation; far too slow to account for the dropout observed over the broad range of L* = 4–5.5. Pitch angle diffusion via resonant interactions with several types of waves (chorus, electromagnetic ion cyclotron waves, and plasmaspheric and plume hiss) also seems problematic, for several reasons which are discussed.

Highlights

Rapid, global dropouts in the radiation belts are currently of much interest, as it has become evident that they are not well understood
This study of the 20 September 2007 dropout is based on measurements by the solid-state telescopes (SST) detectors on the five THEMIS (Time History of Events and Macroscale Interactions during Substorms) spacecraft (Angelopoulos, 2008)
As mentioned, only spin-averaged data are available for this period, so to proceed the flux measurements are assumed to be dominated by locally mirroring particles

Summary

Introduction

Global dropouts in the radiation belts are currently of much interest, as it has become evident that they are not well understood. Somewhat parallel to the study of acceleration events, there are two dominant paradigms, both plausible: local precipitation due to pitch angle scattering by wave–particle interactions, and outward radial transport (diffusion) potentially combined with magnetopause shadowing. The former was invoked by earlier studies of microbursts, but more recent work, based on global observations as well as unsuccessful attempts to detect precipitation, emphasizes the latter. This study of the 20 September 2007 dropout is based on measurements by the solid-state telescopes (SST) detectors on the five THEMIS (Time History of Events and Macroscale Interactions during Substorms) spacecraft (Angelopoulos, 2008). Personal communication, 2013), this investigation is restricted to the radial diffusion mechanism, both in its pure form and as often augmented with a simple, semi-empirical local loss term (e.g., Shprits et al, 2005)

Observations

Global setting

THEMIS observations

Radial diffusion simulation

Local loss processes

Findings

Conclusions