Science http://doi.org/ngs (2013)

Electrons trapped in the Van Allen radiation belt surrounding the Earth can have several megaelectronvolts of energy. Scientists believe that either these charged particles originate from the outer magnetosphere where the magnetic fields are weaker (so-called radial acceleration), or low-energy electrons within the belt itself are accelerated to ultrarelativistic velocities — by resonating with electromagnetic waves, for example. NASA's Van Allen Radiation Belt Storm Probes (RBSP) have now been able to separate the signatures of these two different processes, indicating that local acceleration dominates.

Geoff Reeves and co-workers analysed data collected during an intense period of electron acceleration on 9 October 2012 in which the flux of megaelectronvolt electrons increased by three orders of magnitude. Measuring this flux in the conventional terms of energy, pitch and position does not distinguish between local and a radial acceleration. The RBSP instead sees the radiation belt in terms of 'magnetic coordinates'. For example, the radial location of an electron measured from the centre of the Earth changes during radial acceleration, but is almost constant for local acceleration. It was this latter signature that the RBSP spacecraft observed on 9 October.