Loss mechanisms in the radiation belts: comparing dropouts and flux
decays simulated and observed by PROBA-V/EPT and Van Allen Probes/MagEIS
Test-particle and Fokker-Planck simulations of the energetic electrons
trapped in the terrestrial magnetic field are used to study the outer
radiation belt electron flux losses during and after geomagnetic storms.
We compare the results with ESA PROBA-V observations of electron flux at
LEO and with those from the NASA Van Allen Probes mostly at MEO. We find
that loss mechanisms of trapped electrons can be very different
depending on the geomagnetic activity. Dropouts are visible at all
energy during each storm from both satellites. Test-particle simulations
show that the Dst (Disturbance storm time) effect during the main phase
of a geomagnetic storm results in an outward radial drift and a
deceleration of the electrons. This outward drift motion is energy
independent, pitch angle dependent, and represents a significant
distance of about 1 L-shell at L=5 for moderate storms. At fixed
L-shell, this causes a sharp decay of the LEO precipitating flux. The
Dst effect, associated with magnetopause shadowing and radial diffusion
can explain the main characteristics of outer radiation belt electron
dropouts appearing at the beginning of storms. These instantaneous
dropouts have to be distinguished from the gradual scattering that
depopulates the slot region and the outer belt after storms.
Fokker-Planck simulations with event-driven diffusion coefficients at
high temporal resolution reproduce the slot formation and the gradual
loss in the outer belt. The typical energy-dependence of these losses
leads to the absence of scattering for relativistic and
ultra-relativistic electrons in the outer belt, oppositely to dropouts.
This work has received funding from the European Union’s Horizon 2020
research and innovation programme under grant agreement No 870437 for
the SafeSpace (Radiation Belt Environmental Indicators for the Safety of
Space Assets) project, and for the PITHIA-NRF project (funding from the
European Union’s Horizon 2020 research and innovation programme under
grant agreement No 101007599).