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Radiation belts observed by PROBA-V/EPT

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Using the Energetic Particle Telescope, a detector developed at BIRA-IASB with UCLouvain and QinetiQ Space and launched on the satellite PROBA-V in 2013, we have been able to measure energetic electron and proton fluxes at 820 km for already more than 9 years. This instrument, which is presently still active, delivers exceptional observations simultaneous to the NASA Van Allen Probes that have allowed us to determine the dynamics of the radiation belts during geomagnetic storms, and compare their boundaries to the plasmapause and the auroral oval.
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Radiation belts

The Solar wind research group of BIRA-IASB’s Space physics department conducts research on the near-Earth Space Radiation Environment. This region beyond our atmosphere contains very energetic particles, mainly electrons and ions, trapped in the Earth magnetic field into the Van Allen belts.

BIRA-IASB has contributed to the development of a detector, the Energetic Particle Telescope (EPT) that was launched in May 2013 on board the ESA satellite PROBA-V on a polar Low Earth Orbit at 820 km of altitude. This instrument, still active presently, gives exceptional observations that allowed us to determine the dynamics of the electron fluxes during geomagnetic storms, characterized by dropouts, flux enhancements and slower decays after storm events.

We determined the links of the radiation belts boundaries with other regions of the magnetosphere, and especially with the auroral oval as illustrated in Figure 1, the plasmapause position and the ionospheric trough [1]. We found that the inward motion of the outer radiation belt associated with sudden flux enhancements of energetic electrons can be directly related to the plasmapause erosion during geomagnetic storms generated by solar events (Figure 2).

Moreover, the position of the plasmapause projected in the ionosphere corresponds to the ionospheric convection boundary. The equatorward edge motion of the auroral oval generated by the precipitation of energetic particles in the terrestrial atmosphere goes to lower latitudes during geomagnetic storms, like the low altitude plasmapause projected in the ionosphere and the outer radiation belt.

Simultaneous Van Allen Probes observations

The EPT observations were compared to the simultaneous NASA Van Allen Probes measurements taken on a completely different orbit closer to the equatorial plane [2].  We observe a quite isotropic structure of the outer belt during quiet times, contrary to the inner belt. We find a very good overlap of the outer belt for the two spacecraft at 0.5 MeV.

We developed test-particle simulations of the energetic electrons trapped in the terrestrial magnetic field to determine the physical mechanisms leading to the losses of the electrons during dropouts and slower decays after storm injections (Figure 3).

Magnetopause shadowing and radial diffusion can explain the main characteristics of outer radiation belt electron dropouts. Using measurements of Van Allen Probes in the plasmasphere and our 3D dynamic plasmasphere model, we were able to calculate the diffusion coefficients contributing to the loss of energetic electrons. Such simulations reproduce the slot formation and the gradual loss in the outer belt.



Radiation Belt Physics From Top To Bottom: Combining Multipoint Satellite Observations And Data Assimilative Models To Determine The Interplay Between Sources And Losses - ISSI Team

[1] Pierrard V., E. Botek, J.-F. Ripoll, S. A. Thaller, M. B. Moldwin, M. Ruohoniemi, G. Reeves (2021), Links of the plasmapause with other boundary layers of the magnetosphere: ionospheric convection, radiation belts boundaries, auroral oval, Frontiers in Astronomy and Space Sciences, 08 , doiI: 10.3389/fspas.2021.728531

[2] Pierrard V., J.-F. Ripoll, G. Cunningham, E. Botek, O. Santolik, S. Thaller, W. Kurth, M. Cosmides (2021), Observations and simulations of dropout events and flux enhancements in October 2013: Comparing MEO equatorial with LEO polar orbit, J. Geophys. Res.: Space Physics, 126(6), e2020JA028850, doi: 10.1029/2020JA028850.

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Figure 1: Top panels: EPT observations at 820 km of electron fluxes 500-600 keV in electrons/(cm2 s sr MeV) from 14 to 20 March 2014 in the Northern hemisphere (left) and Southern hemisphere with the South Atlantic Anomaly SAA (right). The black circles with increasing radius correspond to constant latitudes of 80°, 60° and 30°.
Bottom panels: (Left) The auroral oval as obtained with the OVATION model in the Northern hemisphere for 14 March 2014 at 15h30. The color scale indicates the energy flux producing the aurora. The dotted circles correspond to constant latitudes of 80°, 60° and 40°. (Right) The EPT observations of electron fluxes represented on a latitude/longitude map with the SAA at low latitudes and the outer belt at high latitudes. The black dots correspond to L=3.5 (inner equatorward edge) and L=8.5 (outer polar edge). [1]
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Figure 2: Upper panel: Electron flux measured by MagEIS on board the Van Allen Probes at 2.28 MeV. The black line corresponds to the plasmapause computed from spacecraft charging measured on board the Van Allen Probes. Second panel: Electron flux measured by PROBA-V/EPT at 1–2.4 MeV. Here, the black line corresponds to the plasmapause obtained from our BSPM model. Third panel: Observed Dst (Disturbed Storm Time) index. Fourth panel: Comparison of the plasmapause observed by Van Allen Probes (black line) with the plasmapause obtained from the BSPM model (red line). All figures cover the period March 1 to December 31, 2015. [1]
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Figure 3: (Left) Simulation of the trajectory of a 1 MeV proton trapped in the magnetic field of the Earth. The motion can be decomposed in three superposed motions: gyration around the field line, oscillation between two mirror points in each hemisphere, and azimuthal drift. (Right) The trajectory of a particle that is lost when the magnetic field is disturbed during a geomagnetic storm. [2]
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