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5 years of EPT observations

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The Energetic Particle Telescope (EPT) was launched on the ESA satellite PROBA-V on 7 May 2013 to a LEO polar orbit at an altitude of 820 km. The instrument provides what are now 5 years of observations; these show strong flux variations, especially for the electron fluxes during geomagnetic storms, which are also associated with injections in the inner belt. EPT is a detector designed to discriminate the electrons, protons, and helium ions so that it can make direct unambiguous high-quality measurements in the radiation belts—including in the inner zone, despite the penetrating proton environment.
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Geomagnetic storms

The EPT detector shows that the fluxes of electrons  are very variable in the outer belt (L>4 Re). The outer belt is separated from the inner belt (L<2) by the slot region. During the strongest geomagnetic storms, clear injections in the slot and inner belt are visible. The lower panel of Figure 1 shows the Disturbed Storm Time (Dst) index during the same 5 years period. Inverted peaks of Dst correspond to geomagnetic storms.

Some MeV electrons are observed in low quantities in the inner belt, even during periods when they are not observed by Van Allen Probes. They are not due to proton contamination but to injections of particles from the outer belt during  the strongest geomagnetic storms of March and June 2015, and September 2017. Electrons with lower energy are injected also during less strong storms.

With the new high resolution EPT instrument, we can study the dynamics of relativistic electrons, including MeV electrons in the inner radiation belt, revealing how and when such electrons are injected and how long they reside before being scattered into the Earth’s atmosphere or lost by other mechanisms.

Flux maps

Plotted on a map at 820 km of altitude, intense  electron fluxes are observed at high latitudes corresponding to the penetration of the outer belt at low altitudes, and in the South Atlantic Anomaly (SAA) corresponding to electrons trapped in the inner belt. In the SAA, two populations with different origins are observed in the North and South parts.


  • Katsiyannis, A.C., Dominique, M., Pierrard, V., Rosson, G.L., Keyser, J.D., Berghmans, D., Kruglanski, M., Dammasch, I.E., Donder, E.D. (2018). The detection of ultra-relativistic electrons in low Earth orbit. Journal of Space Weather and Space Climate, 8, A01. Open Access
  • López Rosson, G. (2018). Van Allen radiation belts dynamics revealed from PROBA V/EPT observations (doctoral dissertation). Université catholique de Louvain, Louvain-la-Neuve, Belgium. Retrieved from  Open Access
  • López Rosson, G., Pierrard, V. (2017). Analysis of proton and electron spectra observed by EPT/PROBA-V in the South Atlantic Anomaly. Advances in Space Research, 60(4), 796–805.
  • Pierrard, V., Lopez Rosson, G., 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.
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Figure 1. Electron flux measured by EPT in Channel 1 (500-600 keV) from 21 May 2013 up to 1 January 2018 (Credit: Pierrard et al., 2019).
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Figure 2. Electron flux measured by EPT in Channel 5 (1-2.4 MeV) (upper panel) and Channel 6 (2.4-8 MeV) (bottom panel) from 21 May 2013 up to 1 January 2018 (Credit: Pierrard et al., 2019). Injections are still observed for the strongest storms for electrons with energy E<2.4 MeV but not for higher energies.
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Figure 3. Map of the electron fluxes observed by EPT in channel 5 (1.0-2.4 MeV) during October 2013. 5 bins used to determine electron flux variations are also illustrated, as well as constant L (black) lines corresponding to L=3.5 (closer to equator) and L=6.5 (higher latitudes) (Credit: Pierrard et al., 2019).