Energetic electron precipitation during substorm injection events: High‐latitude fluxes and an unexpected midlatitude signature

Abstract

[1] Geosynchronous Los Alamos National Laboratory (LANL-97A) satellite particle data, riometer data, and radio wave data recorded at high geomagnetic latitudes in the region south of Australia and New Zealand are used to perform the first complete modeling study of the effect of substorm electron precipitation fluxes on low-frequency radio wave propagation conditions associated with dispersionless substorm injection events. We find that the precipitated electron energy spectrum is consistent with an e-folding energy of 50 keV for energies <400 keV but also contains higher fluxes of electrons from 400 to 2000 keV. To reproduce the peak subionospheric radio wave absorption signatures seen at Casey (Australian Antarctic Division), and the peak riometer absorption observed at Macquarie Island, requires the precipitation of 50-90% of the peak fluxes observed by LANL-97A. Additionally, there is a concurrent and previously unreported substorm signature at L < 2.8, observed as a substorm-associated phase advance on radio waves propagating between Australia and New Zealand. Two mechanisms are discussed to explain the phase advances. We find that the most likely mechanism is the triggering of wave-induced electron precipitation caused by waves enhanced in the plasmasphere during the substorm and that either plasmaspheric hiss waves or electromagnetic ion cyclotron waves are a potential source capable of precipitating the type of high-energy electron spectrum required. However, the presence of these waves at such low L shells has not been confirmed in this study.Citation: Clilverd, M. A., et al. (2008), Energetic electron precipitation during substorm injection events: High-latitude fluxes and an unexpected midlatitude signature,

Combined THEMIS and ground‐based observations of a pair of substorm‐associated electron precipitation events

“…Only by using multiparameter and multi‐instrument observations of the ionization changes produced by EEP is it possible to accurately characterize the EEP events. The combination of ground‐based and satellite measurements provides the clearest morphology of EEP characteristics, and this work builds on previous studies of this kind [e.g., Clilverd et al , 2008, 2010].…”

“…Typically, EEP from a substorm injection occurs near midnight magnetic local time (MLT), with the precipitation region (in the ionosphere) rapidly expanding eastward with velocities that correspond to electron drift velocities associated with energies of 50–300 keV [ Berkey et al , 1974]. The electron energies involved in substorm injections seen by satellites such as LANL are typically 50–1000 keV, with the highest fluxes occurring at the lowest energies [ Baker et al , 1985; Clilverd et al , 2008]. While the satellite observations provide some information on the energy spectra of the injected electrons and the fluxes in drift orbit, it is very difficult to determine what proportion of the electrons are being precipitated into the atmosphere through onboard satellite measurements.…”

“…Radiation belt processes can generate EEP for long periods (∼10 days), which also contributes to their chemical effect in the atmosphere [ Rodger et al , 2010b; Clilverd et al , 2010]. In contrast, substorm‐driven EEP is short‐lived, but can generate EEP with higher fluxes at <500 keV than some radiation belt processes [ Clilverd et al , 2008]. As such, it is important that the characteristics of substorm‐driven EEPs are understood in detail.…”

Electron source at the outer boundary of the radiation belts: Storm time case

“…As documented previously, the electron integral energy spectrum can be fitted to a power law at high energies [Xiao et al, 2008;Clilverd et al, 2008Clilverd et al, , 2012. For a power law index of Àn, it can be shown that…”

Impact of Energetic Electron Precipitation on the Upper Atmosphere: Nitric Monoxide

“…Although [26] mention electron energies up to 400 keV, we follow [27] with a mean energy of 100 keV. The second one is a mean case with E= 10 keV.…”