Characterization of relativistic electron flux rise times during the recovery phase of geomagnetic storms as measured by the NS41 GPS satellite

Intense relativistic electron enhancements in the Earth's radiation belts are observed during periods of enhanced geomagnetic activity. Different physical processes—identified as being responsible for these enhancements—would lead to different characteristic rise times of the electron fluxes. Here we present for the first time MeV electron flux rise times near the equator as estimated from 512 years of data from the NS41 Global Positioning System (GPS) satellite, in an effort to relate measured electron flux rise timescales with those predicted by theory. The GPS orbit crosses the heart of the radiation belts, covering the L>4 region and measuring equatorial fluxes at L∼4.2. We have calculated L* values using the Tsyganenko 2001 storm magnetic field model and have limited our study to the equator by imposing L*=4–4.5. Forty events, for which fluxes rise by a factor of 5 or more after the storm main phase, were selected from the analysis of the >1.22 MeV electron channel. The main results of our study are as follows: (1) the electron flux rise time distribution is very large indicating that there are a large variety of events observed at GPS orbit, similar to that observed at GEO, (2) fluxes rise in two phases, an initial fast rise is observed where most of the flux increase takes place, followed by a slower increase to the maximum flux, (3) fluxes gain 1–2 orders of magnitude on a timescale of 1–2 days, on average, in good agreement with timescales predicted by electron-chorus resonant interaction in quasi-linear theory using average wave characteristics for AE>500 nT, and (4) the direction of the IMF Bz could be an important parameter in determining the behavior of the flux of relativistic electrons during the recovery phase.