Semiannual Variation in Pc5 waves and relativistic electrons: correlations with profiles and phases of the classical hypotheses in two Solar Cycles of observations.

POBLET FACUNDO; AZPILICUETA FRANCISCO; LAM, HING-LAN

Trieste

Workshop; International Space Weather Initiative workshop; 2019

ICTP

Bodeau, 2010; Lam et al., 2012]. As they present a real threat, many efforts have been made in trying to identify the key elements that produce the enhancements. One of these elements are VLF and ULF plasma waves propagating inside the magnetosphere. They can modify the distribution of radiation belt electrons due to the electrons interaction with electric and magnetic field fluctuations of the waves. Relativistic electron enhancements exhibit a considerable temporal variability in multiple timescales. They present an ~11-year variation because they are more often seen in the late-declining phase of the solar cycle. In addition, they present a strong 27-day variation due to the solar rotation. In this work we revisit a particular time variation on relativistic electrons that is the Semiannual Variation (SAV). We used >2 MeV electron fluence data from GOESs satellites to build a daily sequence of 23 years length. We then calculated a superposed annual curve which shows a clear semiannual amplitude modulation. The same calculation was repeated to a daily sequence of Pc5 powers (used to represent ULF waves) built with geomagnetic measurements fromthe Canadian Magnetic Observatory System (CANMOS). The superposed annual curve of Pc5 powers also shows a semiannual amplitude modulation. The profiles and phases of the superposed curves were compared with the profiles and phases of the theoretically predicted semiannual curves. The theoretical curves were constructed considering the main hypotheses to explain the Semiannual Variation that are the Equinoctial hypothesis, the Russell & McPherron effect and the Axial hypothesis. We find that the Equinoctial and Russell & McPherron hypotheses give the best predictions with slightly better correlation values for the Equinoctial hypothesis.