Seasonal features of geomagnetic activity: a study on the solar activity dependence

Adriane Marques de Souza Franco,Mauricio José Alves Bolzan,Rajkumar Hajra,Ezequiel Echer

doi:10.5194/angeo-39-929-2021

Adriane Marques de Souza Franco, Mauricio José Alves Bolzan + Show 2 more

Open Access

https://doi.org/10.5194/angeo-39-929-2021

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Abstract

Abstract. Seasonal features of geomagnetic activity and their solar-wind–interplanetary drivers are studied using more than five solar cycles of geomagnetic activity and solar wind observations. This study involves a total of 1296 geomagnetic storms of varying intensity identified using the Dst index from January 1963 to December 2019, a total of 75 863 substorms identified from the SuperMAG AL/SML index from January 1976 to December 2019 and a total of 145 high-intensity long-duration continuous auroral electrojet (AE) activity (HILDCAA) events identified using the AE index from January 1975 to December 2017. The occurrence rates of the substorms and geomagnetic storms, including moderate (-50nT≥Dst>-100nT) and intense (-100nT≥Dst>-250nT) storms, exhibit a significant semi-annual variation (periodicity ∼6 months), while the super storms (Dst≤-250 nT) and HILDCAAs do not exhibit any clear seasonal feature. The geomagnetic activity indices Dst and ap exhibit a semi-annual variation, while AE exhibits an annual variation (periodicity ∼1 year). The annual and semi-annual variations are attributed to the annual variation of the solar wind speed Vsw and the semi-annual variation of the coupling function VBs (where V = Vsw, and Bs is the southward component of the interplanetary magnetic field), respectively. We present a detailed analysis of the annual and semi-annual variations and their dependencies on the solar activity cycles separated as the odd, even, weak and strong solar cycles.

Highlights

Solar-wind–magnetosphere energy coupling causes disturbances in the magnetosphere of the Earth (e.g., Dungey, 1961; Axford and Hines, 1961; Tsurutani et al, 1992; Gonzalez et al, 1994; Tsurutani et al, 2020)
The F10.7 solar flux variation shows a clear ∼ 11-year solar activity cycle, with the minimum flux during the solar minimum, followed by flux increases during the ascending phase leading to the peak flux during the solar maximum, and flux decreases during the descending phase of the solar cycle (Fig. 1a)
Embedded in the large-scale ∼ 11-year variations, there are several short-term fluctuations in the data; some of the latter may be associated with the annual or semi-annual variations, which will be explored in detail

Summary

Introduction

Solar-wind–magnetosphere energy coupling causes disturbances in the magnetosphere of the Earth (e.g., Dungey, 1961; Axford and Hines, 1961; Tsurutani et al, 1992; Gonzalez et al, 1994; Tsurutani et al, 2020). Duration and efficiency of the coupling, resultant geomagnetic disturbances (von Humboldt, 1808) can be classified as magnetic storms, substorms and high-intensity longduration continuous auroral electrojet (AE) activities (HILDCAAs) (see Gonzalez et al, 1994; Hajra et al, 2020; Hajra, 2021a). Intense auroral substorms continuing for a few days without occurrence of any major magnetic storms have been called HILDCAAs (Tsurutani and Gonzalez, 1987; Hajra et al, 2013) to distinguish them from nominal substorms and major magnetic storms (Tsurutani et al, 2004; Guarnieri, 2006)

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