Seasonal features of geomagnetic activity: evidence for solar activity dependence?

Abstract. Seasonal features of geomagnetic activity and their solar wind-interplanetary drivers are studied using more than 5 solar cycles of geomagnetic activity and solar wind observations. This study involves a total of 1239 geomagnetic storms of varying intensity identified using the Dst index from January 1963 to December 2019, a total of 75863 substorms identified from the SML index from January 1976 to December 2019, 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, geomagnetic storms, including moderate (−50 nT ≥ Dst > −100 nT) and intense (−100 nT ≥ Dst > −250 nT), 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 found to be driven by the annual variation of the solar wind speed Vsw and the semi-annual variation of the coupling function V Bs (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.


The monthly mean intensities of the Dst and ap indices show a semi-annual variation. Both of them exhibit spring peaks during March. While Dst has a fall peak during October, ap exhibits a peak during September. On the other hand, the monthly mean AE index increases gradually from January, attains a peak around April, decreases with a much slower rate till September, after which the decrease rate is faster, and finally attains a minimum during December. Thus the AE index shows an annual 115 variation, different from the Dst and ap indices. It is worth mentioning that the AE index (Davis and Sugiura, 1966) includes an upper envelope (AU) and a lower envelope (AL) related to the largest (positive) and smallest (negative) magnetic deflections, respectively among the magnetometer stations used. The AU and AL components are thought to represent the strengths of the eastward and westward AE, respectively. It is thus interesting to study the seasonal features of these components separately.
This can be done in a future work.

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Among the solar wind-magnetosphere coupling parameters, V B s exhibits a semi-annual variation, with larger average values during February-April months, another sharp peak during October and with a solstice minimum. For the monthly mean IMF B 0 , a clear minimum can be noted during July, and B 0 increases gradually on both sides of July. No clear seasonal features can be inferred from the variations of the monthly mean V sw , D 500 and Akasofu -parameter.

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It should be noted that the seasonal features as described above ( Figure 2) present an average scenario composed by superposition of several solar cycles. This seasonal behaviour may have different behaviour in different solar cycles. In Figure 3 we have performed Lomb-Scargle periodogram analysis (Lomb, 1976;Scargle, 1982) of the above events and parameters.
As expected, the F 10.7 solar flux shows a prominent (at > 95% significance level) ∼ 11-year periodicity ( Figure 3 Table 2 lists significant periodicities which are less than the ∼ 11-year solar cycle period. As clear from Figure 3 and Table 2, substorms, moderate and intense geomagnetic storms exhibit prominent semi-annual (∼ 6-month period) variation. However,    The results shown in Figure 3 and Table 2 are consistent with those in Figure 2. From the above analyses, the coupling function V B s which exhibits a ∼ 6-month periodicity can be inferred as the driver of the semi-annual variations in substorms,   Another noteworthy feature is that the occurrence rates during the even and weak solar cycles are lower than during the odd and strong cycles, respectively. However, the seasonal modulation between the equinoctial maximum and the solstice minimum is comparable between the weak (∼ 55%) and strong (∼ 46%) cycles (Table 3).

Geomagnetic storms
Variations of the moderate and intense geomagnetic storms are shown in Figures 5 and 6, respectively. From the year-month contour plots, the moderate storms are found to peak around the descending phases, while the intense storms peak around the solar maximum. When monthly variations of the storms are considered in each year, there is hardly any seasonal variation.
However, when observations during several solar cycles are grouped together, the semi-annual variations can be noted in the 190 moderate storms. There is not much difference in moderate and intense storm occurrence rates between the odd and even cycles. However, the occurrence rates of the storms are slightly larger in the strong cycles compared to the weak ones, while seasonal modulation between the equinoctial maximum and the solstice minimum between the two is comparable (Table 3).
Another noteworthy feature is the lowest occurrence of intense storms during the solar cycle 24 which is the weakest in space exploration era. between the equinox minimum to the solstice maximum is significantly higher in the strong cycles (∼ 85%) compared to the weak cycles (∼ 67%) (Table 3). During SC24, the overall Dst strength is the weakest and there is no prominent seasonal modulation.
The variation of the monthly mean ap index (Figure 8) is identical to the Dst index variation. However, the seasonal modulation is comparable between the strong (∼ 37%) and weak (∼ 40%) cycles for the ap index (Table 3).

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The variation of the AE index ( Figure 9) is significantly different than the variations of the Dst and ap indices. In a solar cycle, AE peaks around the descending phase. On the yearly basis, the average AE values are enhanced from March/April to September/October. The summer solstice values are significantly higher compared to the winter solstice values. This indicates an annual variation, in agreement with the Lomb-Scargle periodogram (Figure 3). There is no semi-annual variation. The average values during the strong and odd solar cycles are higher compared to the weak and even solar cycles, respectively.

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SC24 exhibited the lowest values of AE compared to other solar cycles.
higher occurrence rate of the HSSs during this phase. This is also confirmed by the variations of D 500 (not shown). Interestingly, during the descending phase of SC20, the V sw peak can be noted around March-April; during the SC21 descending phase, two equinoctial peaks are almost symmetric; during the SC22 descending phase peaks are recorded during the first half of the year; they shift to the second half of the year during the SC23 descending phase; and during the SC24 descending phase, no prominent feature can be inferred. Thus, overall, a shift of the seasonal peak of V sw from the first half to the second half of the 220 year can be observed between the even and the odd cycles. In addition, during the first half of the year, the average values are significantly high during the odd and strong cycles than during the even and weak cycles, respectively. Firstly, the semi-annual variation is not a "universal" feature of the geomagnetic activity. While substorms, moderate and 235 intense magnetic storms exhibit the semi-annual variation with two equinoctial maxima and a summer solstice minimum, super storms (with very low occurrence rate) and HILDCAA events do not exhibit any clear seasonal dependence. For geomagnetic indices, the ring current index Dst and the global geomagnetic activity index ap exhibit the semi-annual variation, while the auroral ionospheric electrojet current index AE exhibits an annual variation with a summer solstice maximum and a winter minimum. These results clearly demonstrate varying solar, interplanetary, magnetospheric and ionospheric processes behind 240 the geomagnetic events and indices. While the magnetic reconnection (Dungey, 1961) between the southward IMF and the northward dayside geomagnetic field is the key for any geomagnetic effect, variation in the reconnection process and modulation by other processes may result in different geomagnetic effects (e.g., Gonzalez et al., 1994;Hajra, 2021a;Hajra et al., 2021, and references therein). In general, major magnetic storms are associated with strong magnetic reconnection continuing for a few hours, weaker reconnection for an hour or less can cause substorms. On the other hand, discrete and weaker magnetic wind speed V sw does not have any semi-annual component, only annual and longer-scale components. As the main focus of the present work is the seasonal features, for a discussion on the longer-scale variations in V sw , we refer the reader to previous 250 works (e.g., Valdés-Galicia et al., 1996;El-Borie, 2002;El-Borie et al., 2020;Hajra, 2021a;Hajra et al., 2021, and references therein). However, this result is very interesting. This clearly implies that the solar wind does not have any intrinsic semiannual variation, and that the semi-annual variation in V B s is due to magnetic configuration (B s ) as suggested previously (e.g., Cortie, 1912;McIntosh, 1959;Boller and Stolov, 1970;Russell and McPherron, 1973). This has a large contribution in the semi-annual variations of the substorms, moderate and intense storms, and geomagnetic Dst and ap indices. On the other 255 hand, absence of any clear seasonal features in super storms and HILDCAAs indicates more complex solar wind-magnetic coupling process during these events, which needs further study. As previously established, HILDCAAs are associated with HSSs emanated from the solar coronal holes (e.g., Tsurutani and Gonzalez, 1987;Hajra et al., 2013). Dominating longer-scale variations in V sw (as revealed in the present work) may be a plausible reason for lack of any seasonal feature in HILDCAAs (Hajra et al., 2014a;Hajra, 2021c). Annual variation in the auroral ionospheric AE index may be attributed to the ionospheric 260 conductivity variations (see, e.g., Wang and Lühr, 2007;Tanskanen et al., 2011).
In addition to the above, we found a clear solar activity dependence of the above-mentioned seasonal features. The spring-fall asymmetry in substorms and average V sw variation between odd and even solar cycles are consistent with results reported by Mursula et al. (2011). While semi-annual variability (seasonal modulation between the equinoctial maximum and the solstice minimum) was comparable between the strong and weak solar cycles, the overall occurrence rate of the geomagnetic events 265 and the average values of the parameters were significantly stronger during the odd and strong cycles compared to the even and weak cycles, respectively. Further study is required for a better understanding of the solar cycle dependencies of the geomagnetic activity seasonal features. In conclusion, this study, along with several previous works (e.g., Mursula et al., 2011;Hajra et al., 2013Hajra et al., , 2016Hajra, 2021a), calls for the careful re-analyses of the solar, interplanetary, magnetospheric and ionospheric observations before applying theoretical semi-annual models.