Diagnosing the Causes of Extremely Fast Loss from the Radiation Belts: High Cadence Swarm and GPS Satellite Monitoring

Abstract:

Recent Van Allen Probe observations of rapid dropouts of ultra-relativistic (>2 MeV) electron flux have shown that these can occur on hour timescales, and at times effectively deplete the entire outer zone in radiation belt extinction events (e.g. Ozeke et al., 2017). Given the fast timescales of these losses, the definitive cause for this very fast loss remains unknown. One of the challenges in using Van Allen Probes observations to analyse such events is that the timescales are much shorter than the orbital period of the satellites, with the losses often being revealed from one orbit to the next, but without revealing details of the spatio-temporal evolution of the flux profiles which accompany the loss. This makes is challenging to make a causative distinction between competing hypothesised loss processes such as might arise from electromagnetic ion cyclotron wave scattering into the atmosphere, or from ULF wave enhanced magnetopause shadowing. Here we use the fast orbital period of the Swarm satellites to examine the electrodynamics associated with such fast loss processes from the vantage point of four radiation belt crossings in a single 90 minute orbit. We also use data from the GPS satellite constellation to examine the high time and L-shell resolution evolution of the fluxes during such fast loss events, obtaining resolution of around 0.5L in space and 30 minutes in time. In particular, we present evidence of increased wave power in the Pc1 band in the frame of Swarm in correspondence with these dropouts. Using techniques combining the electric and magnetic fields on Swarm, we can identify intervals of Alfven wave activity and attempt to distinguish it from the Earth penetration of storm-time field aligned currents. Initial results also suggest that during intense dropout events the wave power penetrates down to unusually low L-shells right across the outer radiation belt and that Swarm represents a potentially powerful tool for diagnosing Pc1 wave activity. We present comparisons of the Swarm electrodynamics and the characteristics of fast losses characterised by GPS data for a series of intense storm during the Van Allen Probes era, and examine a series of hypotheses which may be able to explain the fast timescales of the losses revealed by GPS.