Proton Flux Variations During Solar Energetic Particle Events, Minimum and Maximum Solar Activity, and Splitting of the Proton Belt in the South Atlantic Anomaly

The analysis of the proton flux variations observed by the Energetic Particle Telescope (EPT) at energies >9.5 MeV from the launch of PROBA-V satellite on 7 May 2013 up to October 2022 shows an anti-correlation between the proton fluxes and the solar phase. At solar minimum, the fluxes are higher at low L corresponding to the northern border of the South Atlantic Anomaly (SAA). This solar cycle modulation of the inner belt is mainly due to losses by increased atmospheric interactions during solar maximum. Strong Solar Energetic Particle (SEP) events, like in January 2014, June 2015, and September 2017, inject energetic protons at high latitudes, but not in the inner belt where protons are trapped at long term at low L. Nevertheless, big geomagnetic storms, including those following SEP a few days after, can cause losses of protons at the outer border of the proton belt, due to magnetic field perturbations. A double peak in the proton belt is observed during long period of measurements only for the EPT channel of 9.5-13 MeV. The narrow gap between the two peaks in the inner belt is located around L = 2. This resembles to a splitting of the proton belt, separating the SAA into two different parts, North and South. The high-resolution measurements of PROBA-V/EPT allow the observation of small-scale structures that brings new elements to the understanding of the different source and loss mechanisms acting on the proton radiation belt at LEO.

Automated summary

The analysis of proton flux variations observed by the Energetic Particle Telescope (EPT) on the PROBA-V satellite from May 7, 2013 to October 2022 reveals an inverse correlation between proton fluxes and solar phase. The fluxes are higher at low latitudes during solar minimum, particularly at the northern border of the South Atlantic Anomaly (SAA). This modulation of the inner belt is primarily influenced by increased atmospheric interactions during solar maximum. While Solar Energetic Particle (SEP) events inject energetic protons at high latitudes, the inner belt at low L remains unaffected due to long-term proton trapping. However, geomagnetic storms, including those following SEP events, can cause proton losses at the outer border of the proton belt due to magnetic field disturbances. A double peak in the proton belt is observed only in the EPT channel of 9.5-13 MeV during long periods of measurement. The narrow gap between the two peaks, located around L = 2, resembles a splitting of the proton belt, separating the SAA into North and South regions. The high-resolution measurements of PROBA-V/EPT provide new insights into the source and loss mechanisms of the proton radiation belt at Low Earth Orbit (LEO).