Semiannual variation of Pc5 ultra-low frequency (ULF) waves and relativistic electrons over two solar cycles of observations: comparison with predictions of the classical hypotheses

POBLET, FACUNDO L.; AZPILICUETA, FRANCISCO; LAM, HING-LAN

Annales Geophysicae

EGU Jounal

Año: 2021 vol. 38 p. 953 - 968

Pc5 ULF (ultra-low frequency) waves can energizeelectrons to relativistic energies of > 2MeV in geostationaryorbits. Enhanced fluxes of such electrons can induceoperational anomalies in geostationary satellites. Thevariations of the two quantities in timescales ranging fromdays to solar cycles are thus of interest in gauging their spaceweather effects over different time frames. In this study, wepresent a statistical analysis of two 11-year solar cycles (cycles22 and 23) of data comprising the daily relativistic electronfluence observed by Geostationary Environment Satellites(GOESs) and daily Pc5 ULF wave power derived fromauroral zone magnetic observatories in Canada. First, an autocorrelationanalysis is carried out, which indicates a 27 dperiodicity in both parameters for all solar phases, and sucha periodicity is most pronounced in the declining and latedeclining phase. Also, a 9 and 13 d periodicity are seen insome years. Then, a superposed epoch analysis is performedto scrutinize semiannual variation (SAV), which shows thatfluence near the equinoxes is 1 order of magnitude higherthan near solstices, and Pc5 ULF wave power is 0.5 ordersof magnitude higher near the equinoxes than near thesolstices. We then evaluate three possible SAV mechanisms(which are based on the axial, equinoctial, and Russell andMcPherron effect) to determine which one can best explainthe observations. Correlation of the profiles of the observationalcurves with those of the angles that control each of theSAV mechanisms suggests that the equinoctial mechanismmay be responsible for the SAV of electron fluence, whileboth the equinoctial and the Russell and McPherron mechanismsare important for the SAV of Pc5 ULF wave power.Comparable results are obtained when using functional dependenciesof the main angles instead of the angles mentionedabove. Lastly, superposed curves of fluence and Pc5ULF wave power were used to calculate least-square fits witha fixed semiannual period. Comparison of the maxima andminima of the fits with those predicted by the three mechanismsshows that the equinoctial effect better estimates themaxima and minima of the SAV in fluence while for theSAV in Pc5 ULF wave power the equinoctial and Russelland McPherron mechanisms predict one maximum and oneminimum each.