Semiannual variation in relativistic electron fluxes of the outer radiation belt: phases comparison with classical hypotheses predictions

Abstract Relativistic electron fluxes of the outer Van Allen belt present a clear semiannual signature, with maxima fluxes near the equinoxes and minima near the solstices. We investigate the semiannual variation (SAV) of 3.75 MeV and 3.4 MeV electron fluxes from Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) and Van Allen probe A (RBSP-A) data, respectively. The fluxes of both missions are binned in McIlwain’s L-shell values and in day of year (DOY). In order to obtain the SAV from the data, a superposed epoch analysis and then a 30-day smoothing is carried out for the entire L-shell range utilized. The results show clear semiannual patterns with maximum differences of ∼ 1 and ∼ 2 orders of magnitude between values near the equinoxes and solstices, for SAMPEX and RBSP-A fluxes, respectively. The comparison of the maxima-minima dates of the observed SAVs with corresponding dates predicted by the Axial, Equinoctial and Russell & McPherron (RM) theory shows that the latter gives the best estimations for both data sets. However, considerable delays from the theories’ predictions are found. The most important delay occurs in the Mar/Apr maximum and is found to be ∼ 26.8 days on average for L ≳ 3.5 RBSP-A data. In the case of SAMPEX, this delay is found for L 3.8 with a maximum of 22.4 days. Accurately determining the Mar/Apr maximum and the Sep/Oct maximum has implications in space weather since it indicates when the SAV will affect the most particular flux enhancements capable of inducing satellite operational anomalies.