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

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 x 10(7) el. cm(-2) sr(-1) s(-1) and 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.