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Radiation belts during geomagnetic storms observed by EPT

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The Energetic Particle Telescope (EPT), launched on the satellite PROBA-V in May 2013, provides flux measurements at 820 km for more than 7 years. They allow us to determine the strong flux variations associated to the geomagnetic storms observed since 2013. Storms are characterized by dropouts followed by flux enhancements, with injections in the slot region, located between the inner and the outer belt, for the strongest events. Altitudes of impenetrable barriers for the dropouts and the injections have been statistically determined for different energy ranges of electrons.
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The Energetic Particle Telescope

Electron flux variations for E>500 keV during geomagnetic storms are investigated using the Energetic Particle Telescope (EPT). This detector launched in May 2013 on board the satellite PROBA‐V at an altitude of 820 km was designed by BIRA-IASB, UCLouvain and QinetiQ Space to provide uncontaminated spectra of:

  • electrons
  • protons
  • alpha particles

Geomagnetic storms

Electron flux dropout events are observed during the main phase of each storm and even during substorms: a rapid reduction of the electron flux is noted throughout the outer electron radiation belt at all energies above about 0,5 MeV on timescales of a few hours.

The electron spectrograms measured by EPT between 2013 and 2019 show that after each geomagnetic storm, dropout events are followed by a flux enhancement starting first at low L values, and reaching the slot or even the inner belt for the strongest storms. L is the equatorial distance parameter of the magnetic shells, expressed in Earth radii. The outer radiation belt is typically located at L > 4, the inner belt at L < 2 Earth radii, and the region between corresponds to the slot region.

We can see that dropouts appear at all energies measured by EPT and penetrate down to L3.5 Earth radii for the strongest events. Dropouts are observed each time Dst has an inverted peak < ‐40 nT.

Flux enhancements appear at lower L only for big storm events with Dst < ‐50 nT. They penetrate down to an impenetrable barrier with a minimum L‐shell related to Dst and to the energy. For E > 2 MeV, this limit is always higher than the inner belt and it is linked to the plasmapause position corresponding to the sharp limit of cold plasma. We showed also that the plasmapause is related to auroral spots.

Human activity also influences electron fluxes of the radiation belts: we demonstrated that the ground-based transmitter in northwest Australia scatters electrons and make them lost from the radiation belts.

 

Further reading

  • Pierrard, V., Botek, E., Ripoll, J.-F., and Cunningham, G.S. (2020). Electron dropout events and flux enhancements associated with geomagnetic storms observed by PROBA-V/EPT from 2013 to 2019. Journal of Geophysical Research: Space Physics, 125(12), e2020JA028487. https://doi.org/10.1029/2020JA028487 Open Access Logo
  • Pierrard, V., Lopez Rosson, G., and Botek, E. (2019). Dynamics of Megaelectron Volt Electrons Observed in the Inner Belt by PROBA-V/EPT. Journal of Geophysical Research: Space Physics, 124(3), 1651-1659. https://doi.org/10.1029/2018JA026289
  • Cunningham, G.S., Botek, E., Pierrard, V., Cully, C., and Ripoll, J.-F. (2020). Observation of High-Energy Electrons Precipitated by NWC Transmitter from PROBA-V Low-Earth Orbit Satellite. Geophysical Research Letters, 47(16), e2020GL089077. https://doi.org/10.1029/2020GL089077 Open Access Logo

 

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Fig. 1 Electron flux observed by EPT from 18 June 2015 to 28 June 2015. The superposed black line corresponds to a linear relation based on the observed Disturbed Storm Time Dst index (Figure from Pierrard et al., 2020).
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Fig. 2 Electron fluxes observed by EPT from the first measurements on 21 May 2013 up to 31 December 2019, for Channel 1 (500-600 keV, upper panel) and Channel 5 (1-2,4 MeV, middle panel). The bottom panel shows the observed Disturbed Storm Time Dst index from 21 May 2013. (Figure from Pierrard et al., 2020)
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Fig. 3 Electron flux measured by EPT in Channel 1 (500-600 keV, upper panel), Channel 4 (0,8-1 MeV, second panel) and Channel 5 (1-2,4 MeV, third panel) from 1 March to 31 December 2015. In the third panel, the black line corresponds to the plasmapause position measured by two instruments onboard the Van Allen Probes. Bottom panel: Dst index measured during the same period. (Figure from Pierrard et al., 2020)