Magnetic storms during the space age: Occurrence and relation to varying solar activity

We review here the occurrence of magnetic storms during the space age (1957 - 2021), as observed by two storm indices, the Dst index and the Dxt index. We study the solar sources of storms, describe the dramatic changes in the different types of storms during the space age, and explain these changes in terms of the long-term change of solar activity and solar magnetic fields during the decline of the Modern Grand Maximum.

We find 2526/2743 magnetic storms in the Dxt/Dst index, out of which 45% are weak (-50 nT <  Dxt/Dst ≤ -30 nT), 40% moderate (-100 nT < Dxt/Dst ≤ -50 nT), 12% intense (200 nT < Dxt/Dst ≤ -100 nT) and 3% major (Dxt/Dst ≤ -200 nT) 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 - 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 HSS/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. Moreover, because of the 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 to occur in the early declining phase in recent cycles, while in the more active cycles 20-22 they occurred in the late declining phase.