A statistical approach to determining energetic outer radiation belt electron precipitation fluxes

Abstract

Subionospheric radio wave data from an Antarctic-Arctic Radiation-Belt (Dynamic) Deposition VLF Atmospheric Research Konsortia (AARDDVARK) receiver located in Churchill, Canada, is analyzed to determine the characteristics of electron precipitation into the atmosphere over the range 3 < L < 7. The study advances previous work by combining signals from two U.S. transmitters from 20 July to 20 August 2010, allowing error estimates of derived electron precipitation fluxes to be calculated, including the application of time-varying electron energy spectral gradients. Electron precipitation observations from the NOAA POES satellites and a ground-based riometer provide intercomparison and context for the AARDDVARK measurements. AARDDVARK radiowave propagation data showed responses suggesting energetic electron precipitation from the outer radiation belt starting 27 July 2010 and lasting~20 days. The uncertainty in >30 keV precipitation flux determined by the AARDDVARK technique was found to be ±10%. Peak >30 keV precipitation fluxes of AARDDVARK-derived precipitation flux during the main and recovery phase of the largest geomagnetic storm, which started on 4 August 2010, were >10 5 el cm À2 s À1 sr À1. The largest fluxes observed by AARDDVARK occurred on the dayside and were delayed by several days from the start of the geomagnetic disturbance. During the main phase of the disturbances, nightside fluxes were dominant. Significant differences in flux estimates between POES, AARDDVARK, and the riometer were found after the main phase of the largest disturbance, with evidence provided to suggest that >700 keV electron precipitation was occurring. Currently the presence of such relativistic electron precipitation introduces some uncertainty in the analysis of AARDDVARK data, given the assumption of a power law electron precipitation spectrum.

Long‐term determination of energetic electron precipitation into the atmosphere from AARDDVARK subionospheric VLF observations

“…However, Simon Wedlund et al . 's [] study found good agreement between the POES and AARDDVARK‐determined gradients, giving us additional confidence in the use of the POES‐fitted energy gradients as we describe in the following section.…”

“…The received amplitudes of fixed frequency VLF transmissions vary in a constant manner during undisturbed conditions. Energetic electron precipitation (EEP) events can be detected as deviations from the subionospheric quiet day curve as a change in amplitude of the received signal relative to the QDC [ Rodger et al ., ; Simon Wedlund et al ., ]. This is equivalent to the QDC approach used for riometers, which has become standard practice in that community.…”

D‐Region High‐Latitude Forcing Factors

“…The electron fluxes measured by Geostationary Operational Environment Satellites indicate the intensity of the outer electron radiation belt at geostationary orbit. These trapped fluxes are used as a proxy to describe the high‐latitude, high‐energy, electron precipitation (Clilverd et al, ; Simon Wedlund et al, ) into the D‐region along the NAA‐SOD path. The EEP flux data were treated as follows.…”

Investigating energetic electron precipitation through combining ground‐based and balloon observations

“…In this section the BARREL 1C mono‐energetic electron precipitation spectra and fluxes are used to calculate the magnitude of the perturbations seen by the Halley ground‐based instruments, i.e., the AARDDVARK receiver and the riometer. The methods of calculating the perturbation magnitude for these instruments are given in detail in Rodger et al [] and Simon Wedlund et al []. The vertical charge density profile is given by the BARREL analysis, with horizontally homogeneous patch structure assumed.…”