On the semi-annual variation of relativistic electrons in the outer radiation belt
<p>The nature of the semi-annual variation in the relativistic electron fluxes in the Earth&#8217;s outer radiation belt is investigated using Van Allen Probes (MagEIS and REPT) and GOES (EPS) data during solar cycle 24. We perform wavelet and cross-wavelet analysis in a broad energy and spatial range of electron fluxes and examine their phase relationship with the axial, equinoctial and Russell-McPherron mechanisms. It is found that the semi-annual variation in the relativistic electron fluxes exhibits pronounced power in the 0.3 &#8211; 4.2 MeV energy range at L-shells higher than 3.5 and, moreover, it exhibits an in-phase relationship with the Russell-McPherron effect indicating the former is primarily driven by the latter. Furthermore, the analysis of the past 3 solar cycles with GOES/EPS indicates that the semi-annual variation at geosynchronous orbit is evident during the descending phases and coincides with periods of a higher (lower) HSS (ICME) occurrence.</p><p>This work has received funding from the European Union's Horizon 2020 research and innovation programme &#8220;SafeSpace&#8221; under grant agreement No 870437 and from the European Space Agency under the &#8220;European Contribution to International Radiation Environment Near Earth (IRENE) Modelling System&#8221; activity under ESA Contract No 4000127282/19/NL/IB/gg.</p>