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

Abstract

Abstract. Pc5 ULF (ultra-low frequency) waves can energize electrons to relativistic energies of >2 MeV in geostationary orbits. Enhanced fluxes of such electrons can induce operational anomalies in geostationary satellites. The variations of the two quantities in timescales ranging from days to solar cycles are thus of interest in gauging their space weather effects over different time frames. In this study, we present a statistical analysis of two 11-year solar cycles (cycles 22 and 23) of data comprising the daily relativistic electron fluence observed by Geostationary Environment Satellites (GOESs) and daily Pc5 ULF wave power derived from auroral zone magnetic observatories in Canada. First, an autocorrelation analysis is carried out, which indicates a 27 d periodicity in both parameters for all solar phases, and such a periodicity is most pronounced in the declining and late declining phase. Also, a 9 and 13 d periodicity are seen in some years. Then, a superposed epoch analysis is performed to scrutinize semiannual variation (SAV), which shows that fluence near the equinoxes is 1 order of magnitude higher than near solstices, and Pc5 ULF wave power is 0.5 orders of magnitude higher near the equinoxes than near the solstices. We then evaluate three possible SAV mechanisms (which are based on the axial, equinoctial, and Russell and McPherron effect) to determine which one can best explain the observations. Correlation of the profiles of the observational curves with those of the angles that control each of the SAV mechanisms suggests that the equinoctial mechanism may be responsible for the SAV of electron fluence, while both the equinoctial and the Russell and McPherron mechanisms are important for the SAV of Pc5 ULF wave power. Comparable results are obtained when using functional dependencies of the main angles instead of the angles mentioned above. Lastly, superposed curves of fluence and Pc5 ULF wave power were used to calculate least-square fits with a fixed semiannual period. Comparison of the maxima and minima of the fits with those predicted by the three mechanisms shows that the equinoctial effect better estimates the maxima and minima of the SAV in fluence while for the SAV in Pc5 ULF wave power the equinoctial and Russell and McPherron mechanisms predict one maximum and one minimum each.

Relating 27-day Averages of Solar, Interplanetary Medium Parameters and Geomagnetic Activity Proxies in Solar Cycle 24

“…There are, nonetheless, two other mechanisms proposed to explain SAV (e.g. Poblet, Azpilicueta, and Lam, 2020;Lockwood et al, 2020): the axial hypothesis, that states the main cause is the position of the Earth with respect to the solar equator (the heliographic latitude), with maximum near the equinoxes Figure 8. The θ RM during the spring equinox, with IMF toward the Sun.…”

“…There are, nonetheless, two other mechanisms proposed to explain SAV (e.g. Poblet, Azpilicueta, and Lam, 2020;Lockwood et al, 2020): the axial hypothesis, that states the main cause is the position of the Earth with respect to the solar equator (the heliographic latitude), with maximum near the equinoxes (±7.5 o ) and minimum near solstices (∼ 0 o ); the equinoctial hypothesis, that identifies as the most relevant parameter the angle between the Earth-Sun line and the geomagnetic dipole axis of the Earth (the tilt angle, or its complement). Zhao and Zong (2012) noticed that, if R-M is the dominant mechanism behind SAV, a toward/away asymmetry should be seen in geomagnetic activity.…”

Relating 27-Day Averages of Solar, Interplanetary MediumParameters, and Geomagnetic Activity Proxies in SolarCycle 24

“…There are, nonetheless, two other mechanisms proposed to explain SAV (e.g. Poblet, Azpilicueta, and Lam, 2020;Lockwood et al, 2020): the axial hypothesis, that states the main cause is the position of the Earth with respect to the solar equator (the heliographic latitude), with maximum near the equinoxes (±7.5 o ) and minimum near solstices (∼ 0 o ); the equinoctial hypothesis, that identifies as the most relevant parameter the angle between the Earth-Sun line and the geomagnetic dipole axis of the Earth (the tilt angle, or its complement).…”

On the semi-annual variation of relativistic electrons in the outer radiation belt

“…Furthermore, Emery et al (2011) used > 2 MeV electron fluxes from Geostationary Operational Environmental Satellite (GOES) and argued that the SAV, which was relatively strong in the 1995-1997 solar minimum, was a combination of the Russell-McPherron effect and the appearance of equinoctial peaks in the amplitudes of solar rotation periods of 13.5 and 27 d. Finally, Poblet et al (2020) used GOES > 2 MeV electron fluxes during the 1987-2008 time period and concluded that the equinoctial mechanism seems to be the dominant driver of the SAV of electron fluence at GEO.…”