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

Abstract

The South Atlantic Anomaly (SAA) is an area where Earth's magnetic field is particularly low. This comes from the fact that the Earth's magnetic axis is tilted with respect to the Earth's rotation axis by an angle of approximately 11° and that the axis of the magnetic idealized dipole is located some 400 km away from the Earth's center (Brekke, 2013). As a consequence, the SAA is also a region where inner radiation belt particles can mirror at lower altitudes increasing the local particle flux.Unlike the geographic poles, Earth's magnetic poles are not fixed and tend to wander over time, and in parallel, the intensity of Earth's magnetic field is varying over time, for example, presently decreasing (Brekke, 2013). Hence the SAA has been drifting and growing in recent years (Amit et al., 2021;Anderson et al., 2018;Stassinopoulos et al., 2015), and those secular trends have an effect on the inner belt, especially as observed at Low Earth Orbit (LEO) (Girgis et al., 2021).In addition to secular drifts, the inner belt particles are affected by solar cycle variation. In fact, the low-altitude geomagnetically trapped population is known to be coupled to the atmospheric density through changes induced by solar activity (Anderson et al., 2018;Bruno et al., 2021b). Protons with energies above a few tens of MeV mostly originate through the Cosmic-Ray Albedo Neutron Decay (CRAND) mechanism. Because the CRAND source comes mostly from low geomagnetic latitudes due to high cutoff rigidities, the solar modulation of the cosmic rays has a relatively small effect on the variability of the trapped protons (Bruno et al., 2021b). The major variations are due to the atmospheric loss processes, including ionization and scattering of neutral and ionized atoms, induced by solar extreme ultraviolet (EUV) radiation heating the Earth's upper atmosphere. In fact, increased EUV during solar maxima leads to higher neutral and ionospheric densities, causing a decrease of proton fluxes concentrated at low altitudes and L shells.One of the particularities of the particle fluxes at low altitudes is that the high-energy trapped proton fluxes are strongly anisotropic, that is, the proton fluxes depend on their arrival direction in the plane perpendicular to the local magnetic field vector as well as on their pitch angle (angle between their velocity vector and

South Atlantic Anomaly Evolution Seen by the Proton Flux

“…Next, we observe a phase-lag anti-correlation of the SAA surface with the solar activity. This anti-correlation is well known (Casadio & Arino, 2011;Dachev et al, 1999;Huston et al, 1998;Kuznetsov et al, 2010;Li et al, 2001;Pierrard et al, 2023;Qin et al, 2014). This can be explained in two ways.…”

Geomagnetic Storm Effects on the LEO Proton Flux During Solar Energetic Particle Events

“…More recently, the large SEP event of September 2017 was investigated by Pierrard et al. (2023), who confirmed that solar protons could reach the inner belt but were immediately lost due to the following magnetic storm.…”

“…For instance, Pierrard et al (2014) observed the remaining trapped solar protons around the whole month of June 2013 between L = 4-5 after the intense SEP event in May 2013. More recently, the large SEP event of September 2017 was investigated by Pierrard et al (2023), who confirmed that solar protons could reach the inner belt but were immediately lost due to the following magnetic storm.…”