[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,

[1] The Antarctic-Arctic Radiation-belt (Dynamic) Deposition-VLF Atmospheric Research Konsortium (AARDDVARK) provides a network of continuous long-range observations of the lower ionosphere in the polar regions. Our ultimate aim is to develop the network of sensors to detect changes in ionization levels from $30--90 km altitude, globally, continuously, and with high time resolution, with the goal of increasing the understanding of energy coupling between the Earth's atmosphere, the Sun, and space. This science area impacts our knowledge of space weather processes, global atmospheric change, communications, and navigation. The joint New Zealand-United Kingdom AARDDVARK is a new extension of a well-established experimental technique, allowing long-range probing of ionization changes at comparatively low altitudes. Most other instruments which can probe the same altitudes are limited to essentially overhead measurements. At this stage AARDDVARK is essentially unique, as similar systems are only deployed at a regional level. The AARDDVARK network has contributed to the scientific understanding of a growing list of space weather science topics including solar proton events, the descent of NO x into the middle atmosphere, substorms, precipitation of energetic electrons by plasmaspheric hiss and electromagnetic ion cyclotron waves, the impact of coronal mass ejections upon the radiation belts, and relativistic electron microbursts. Future additions to the receiver network will increase the science potential and provide global coverage of space weather event signatures.

A detailed comparison is undertaken of the energetic electron spectra and fluxes of two precipitation events that were observed in 18/19 January 2013. A novel but powerful technique of combining simultaneous ground‐based subionospheric radio wave data and riometer absorption measurements with X‐ray fluxes from a Balloon Array for Relativistic Radiation‐belt Electron Losses (BARREL) balloon is used for the first time as an example of the analysis procedure. The two precipitation events are observed by all three instruments, and the relative timing is used to provide information/insight into the spatial extent and evolution of the precipitation regions. The two regions were found to be moving westward with drift periods of 5–11 h and with longitudinal dimensions of ~20° and ~70° (1.5–3.5 h of magnetic local time). The electron precipitation spectra during the events can be best represented by a peaked energy spectrum, with the peak in flux occurring at ~1–1.2 MeV. This suggests that the radiation belt loss mechanism occurring is an energy‐selective process, rather than one that precipitates the ambient trapped population. The motion, size, and energy spectra of the patches are consistent with electromagnetic ion cyclotron‐induced electron precipitation driven by injected 10–100 keV protons. Radio wave modeling calculations applying the balloon‐based fluxes were used for the first time and successfully reproduced the ground‐based subionospheric radio wave and riometer observations, thus finding strong agreement between the observations and the BARREL measurements.

Precipitation of energetic electrons to the atmosphere is both a loss mechanism for radiation belt particles and a means by which the geospace environment influences the Earth's atmosphere; thus, it is important to fully understand the extent of this precipitation. A set of polar orbiting satellites have been used to identify periods when energetic charged particles fill the slot region between the inner and outer radiation belts. These suggest that electrons with energies >30 keV penetrate this region, even under levels of modest geomagnetic activity. Those events with sufficient fluxes of particles produce enough ionization to be detected by a ground‐based radar in Antarctica; this precipitation lasts for ~10 days on average. Analysis of these data reveals that the average precipitation penetrates to the stratopause (~55‐km altitude). For some (if not all) of these events, the likely cause of the most energetic precipitation is an interaction between (relativistic) electrons and plasmaspheric hiss leading to little, if no, local time variation in precipitation. This does not preclude a longitudinal effect given that all radar measurements are fixed in longitude. During winter months the radar is under the stable southern polar atmospheric vortex. This transports atmospheric species to lower altitudes including the ozone destroying chemicals that are produced by energetic precipitation. Thus, the precipitation from the slot region in the Southern Hemisphere will likely contribute to the destruction of ozone and changes to atmospheric heat balance and chemistry; more work is required to determine the true impact of these events.

.[1] Using ground-based subionospheric radio wave propagation data from two very low frequency (VLF) receiver sites, riometer absorption data, and THEMIS satellite observations, we examine in detail energetic electron precipitation (EEP) characteristics associated with two substorm precipitation events that occurred on 28 May 2010. In an advance on the analysis undertaken by Clilverd et al. (2008), we use phase observations of VLF radio wave signals to describe substorm-driven EEP characteristics more accurately than before. Using a >30 keV electron precipitation flux of 5.6 Â 10 7 el. cm À2 sr À1 s À1and a spectral gradient consistent with that observed by THEMIS, it was possible to accurately reproduce the peak observed riometer absorption at Macquarie Island (L = 5.4) and the associated NWC radio wave phase change observed at Casey, Antarctica, during the second, larger substorm. The flux levels were near to 80% of the peak fluxes observed in a similar substorm as studied by Clilverd et al. (2008). During the initial stages of the second substorm, a latitude region of 5 < L < 9 was affected by electron precipitation. Both substorms showed expansion of the precipitation region to 4 < L < 12 more than 30 min after the injection. While both substorms occurred at similar local times, with electron precipitation injections into approximately the same geographical region, the second expanded in an eastward longitude more slowly, suggesting the involvement of lower-energy electron precipitation. Each substorm region expanded westward at a rate slower than that exhibited eastward. This study shows that it is possible to successfully combine these multi-instrument observations to investigate the characteristics of substorms.