The controlling effect of the cold plasma density over the acceleration and loss of ultra‐relativistic electrons

Novel analysis of phase space densities at multiple energies allows for differentiation between various acceleration mechanisms at ultra‐relativistic energies. This method allows us to trace how particles are being accelerated at different energies and show how long it takes for acceleration to reach particular energy. This method clearly demonstrates the importance of local acceleration and also demonstrates the importance of outward radial diffusion in transporting electrons to GEO.

Acceleration to such high energies occurs only when cold plasma in the trough region is extremely depleted, down to the values typical for the plasma sheet. We perform event and statistical analysis of these depletions and show that the ultra‐relativistic energies are reached for each such depletion that is accompanied by the intensification of ~2MeV. VERB‐2D simulations are then used to explain these observations. There is also a clear difference between the loss mechanisms at MeV and multi‐MeV energies due to EMIC waves that can very efficiently scatter ultra‐relativistic electrons but leave MeV electrons unaffected.

Modelling and observations clearly show that cold plasma has a controlling effect over the ultra‐ relativistic electrons that are 10^6‐10^7 times more energetic. We also present how the new understanding gained from the Van Allen Probes mission can be used to produce the most accurate data assimilative forecast. Under the recently funded EU Horizon 2020 Project Prediction of Adverse effects of Geomagnetic storms and Energetic Radiation (PAGER) we study how ensemble forecasting from the Sun can produce long‐term probabilistic forecasts of the radiation environment in the inner magnetosphere.