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

Correlations between solar, interplanetary medium parameters and geomagnetic activity proxies in 27-day averages (a Bartels’ rotation) were analysed for the 2009-2016 time interval. In this analysis, two new proxies were considered: 1) B ZS GSM, calculated as the daily percentage of the IMF southward component along the GSM Z-axis and then averaged every 27 days; 2) four magnetospheric indices (T-indices), calculated from the local North-South (X) contributions of the magnetosphere's cross-tail (TAIL), the symmetric ring current (SRC), the partial ring current (PRC) and the Birkeland current (FAC), derived from Tsyganenko and Sitnov 2005 (TS05) semi-empirical magnetospheric model. Our results suggest that, among the parameters tested in this study, solar facular areas, interplanetary magnetic field intensity and new proxies derived from TS05 model could be taken into account in an empirical model, with a 27-day resolution, to explain geomagnetic activity felt on the Earth's surface in terms of solar surface features and the IMF condition. We further retrieve a clear annual oscillation in series of 27-day-mean values of toward/away asymmetries of geomagnetic activity indices, which can be interpreted in the light of Russell-McPherron hypothesis for the semiannual variation of geomagnetic activity.

Seasonal and local time variations of auroral electrojet: CHAMP observation

<p>The auroral electrojet is an important element of the polar current system and an essential subject in space weather research. Based on the scalar magnetic field data from CHAMP satellite, we studied the influences of solar illumination and the dipole tilt angle (DTA) on the auroral electrojet as well as its seasonal variations. Furthermore, the auroral electrojet measured by satellite was compared with the auroral electrojet indices derived from the ground stations. It is shown that on the dayside, the auroral electrojet is more intense at a smaller solar zenith angle (SZA), whereas it’s more intense on the nightside when the SZA is larger. The daytime current is mainly controlled by the solar illumination, while the nighttime current is affected by the substorm. Compared with the solar illumination, the dipole tilt angle plays a minor role. The auroral electrojet shows an obvious annual and semiannual variation. The eastward electrojet and the dayside westward electrojet are more intense in summer than in winter, while the nightside westward electrojet is more intense in winter than in summer. The daytime westward electrojet is more intense at solstices, whereas the nighttime westward electrojet is more intense at equinoxes. The westward electrojet shows a good correlation with AL and SML indices. The eastward electrojet correlates well with the SMU index, but shows obvious difference with the AU index. The discrepancy can be attributed to the fact that the peak eastward electrojet is located outside the detection range of the auroral electrojet stations.</p>

GEOMETRIC FACTOR IN SEASONAL VARIATIONS OF DAILY AVERAGE VALUES OF THE GEOMAGNETIC INDEX Dst

The work uses data on the geomagnetic index Dst for the period 1966–2015. Under quiet conditions, the occurrence of seasonal variations of the daily average Dst index depends on geometric factors of the interaction between the solar wind and the magnetosphere; and under disturbed conditions, on the development of a partial ring current in the magnetosphere. At large negative values of the Dst index, there is no seasonal variation in it. The imperfection of the network of Dst stations is assumed to lead to the formation of annual variation in Dst. The formation of a semiannual variation is associated with the movement of the plasma sheet relative to the plane of the geomagnetic equator during the annual rotation of Earth around the Sun. Based on the data on semiannual variations in the number of days n(Dst), the critical daily average value of the geomagnetic index Dst is determined, starting from which we can speak of disturbed days: Dst≤–24 nT.

GEOMETRIC FACTOR IN SEASONAL VARIATIONS OF DAILY AVERAGE VALUES OF THE GEOMAGNETIC INDEX Dst

The work uses data on the geomagnetic index Dst for the period 1966–2015. Under quiet conditions, the occurrence of seasonal variations of the daily average Dst index depends on geometric factors of the interaction between the solar wind and the magnetosphere; and under disturbed conditions, on the development of a partial ring current in the magnetosphere. At large negative values of the Dst index, there is no seasonal variation in it. The imperfection of the network of Dst stations is assumed to lead to the formation of annual variation in Dst. The formation of a semiannual variation is associated with the movement of the plasma sheet relative to the plane of the geomagnetic equator during the annual rotation of Earth around the Sun. Based on the data on semiannual variations in the number of days n(Dst), the critical daily average value of the geomagnetic index Dst is determined, starting from which we can speak of disturbed days: Dst≤–24 nT.

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

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.

Semiannual variation of Pc5 ULF waves and relativistic electrons over two solar cycles of observations: comparison with predictions of the classical hypotheses

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 time scales 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 (Cycle 22 and 23) of data comprising the daily relativistic electron fluence observed by GOES geostationary satellites and daily Pc5 power derived from auroral zone magnetic observatories in Canada. Firstly, an autocorrelation analysis is carried out, which indicates 27-day 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-day and 13-day periodicity, though not present in all the years, are seen in some years. Then, a superposed epoch analysis is performed to scrutinize Semiannual Variation (SAV), which shows fluence near the equinoxes is one order of magnitude higher than near solstices and Pc5 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 Russel & 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 & McPherron mechanisms are important for the SAV of Pc5 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 power were used to calculate least-square fits with a fixed semiannual period. Comparison of 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 power the Equinoctial and Russell & McPherron mechanisms predict one maximum and one minimum each.