Magnetic Storms During the Space Age: Occurrence and Relation to Varying Solar Activity

Abstract

We study the occurrence of magnetic storms in space age (1957–2021) using Dst and Dxt indices. We find 2,526/2,743 magnetic storms in the Dxt/Dst index, out of which 45% are weak, 40% moderate, 12% intense and 3% major storms. Occurrence of storms in space age follows the slow decrease of sunspot activity and the related change in solar magnetic structure. We quantify the sunspot—coronal mass ejection (CME) storm relation in the five cycles of space age. We explain how the varying solar activity changes the structure of the heliospheric current sheet (HCS), and how this affects the high‐speed solar wind stream (HSS)/corotating interaction region (CIR) storms. Space age started with a record number of storms in 1957–1960, with roughly one storm per week. Solar polar fields attained their maximum in cycle 22, which led to an exceptionally thin HCS, and a space age record of large HSS/CIR storms in 1990s. In the minimum of cycle 23, for the only time in space age, CME storm occurrence reduced below that predicted by sunspots. Weak sunspot activity since cycle 23 has weakened solar polar fields and widened the HCS, which has decreased the occurrence of large and moderate HSS/CIR storms. Because of a wide HCS, the Earth has spent 50% of its time in slow solar wind since cycle 23. The wide HCS has also made large and moderate HSS/CIR storms occur in the early declining phase in recent cycles, while in the more active cycles 20–22 they occurred in the late declining phase.

Space Weather Disturbances in Non‐Stormy Times: Occurrence of dB/dt Spikes During Three Solar Cycles

“…The days of fast solar wind had local maxima at the declining phases of solar cycles 23-24, and local minima near the beginning of the inclining phases of cycles 23-25. This is consistent with previous observations showing that the highest solar wind speeds are observed during the declining solar cycle phase when coronal holes extend to lower solar latitudes (Gosling et al, 1976;Mursula et al, 2017Mursula et al, , 2022.…”

“…This is consistent with the highest solar wind speeds occurring at the declining solar cycle phase (Gosling et al, 1976;Mursula et al, 2017Mursula et al, , 2022, as well as the formation of corotating interaction regions (CIRs), which together with coronal mass ejections (CMEs), are known to cause increased geomagnetic activity (Richardson et al, 2006;Zhang et al, 2007). Note that the peak in the occurrence of the high-speed solar wind has been observed to shift from the late to the earlier declining phase in solar cycles 23-24 (Mursula et al, 2022). This is consistent with our observations of the peak in the number of SUBs occurring somewhat earlier in the declining phase of cycle 23-24 as compared to cycle 22 (Figure 6d).…”

“…Moreover, the number of SUB intervals varies within each solar cycle, with local maxima at the declining phase (∼1994 for cycle 22, 2003 for cycle 23, and 2015-2017 for cycle 24), and local minima near the solar cycle minima (1986-1987, 1997, 2009, and 2020). This is consistent with the highest solar wind speeds occurring at the declining solar cycle phase (Gosling et al, 1976;Mursula et al, 2017Mursula et al, , 2022, as well as the formation of corotating interaction regions (CIRs), which together with coronal mass ejections (CMEs), are known to cause increased geomagnetic activity (Richardson et al, 2006;Zhang et al, 2007). Note that the peak in the occurrence of the high-speed solar wind has been observed to shift from the late to the earlier declining phase in solar cycles 23-24 (Mursula et al, 2022).…”

Extreme Geomagnetic Disturbances (GMDs) Observed in Eastern Arctic Canada: Occurrence Characteristics and Solar Cycle Dependence

“…Borovsky and Denton (2006) also noted that both CIRs and the high-speed solar wind streams that typically follow them can be drivers of storms, and that when recurring CIR-driven storms were ongoing (which tends to be in the declining phase of the solar cycle), the durations of high-speed streams were longer. Mursula et al (2022) showed the importance of variations in the width of the heliospheric current sheet (HCS) for the relative occurrence of geomagnetic storms related to CMEs, HSS/CIRs, and slow solar wind, respectively, such that a wide HCS made large and moderate HSS/CIR storms occur in the early declining phase in recent cycles 23 and 24 (1996-2019), while in the more active cycles 20-22 (1964-1996) they occurred in the late declining phase (their Figure 5). Tsurutani et al (2006) reviewed the causes, characteristics, and consequences of CIRs and high-speed streams, in particular their association with auroral substorms.…”

“…Distribution of ≥ 20 nT/s GMD events as a function of geomagnetic storm phase.Definitions for the four storm categories are taken fromMursula et al (2022).…”