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Abstract:
We study here the occurrence of magnetic storms during the space age (1957 - 2021). The occurrence of storms follows the slow, unsteady decline of solar activity after the centennial maximum in 1957, and the related changes in solar magnetic structure. We divide the storms according to their heliospheric driver to CME, HSS/CIR and HCS (heliospheric current sheet) storms and explain how the solar change modifies their occurrence via changes in sunspots, solar polar fields, coronal holes and in the structure of the HCS.Space age started with a record number of storms in 1957 - 1960, with roughly one storm per week. Solar polar fields increased to their maximum in cycle 22, which led to an exceptionally thin HCS, and a space age record of large HSS/CIR storms in the declining phase of cycle 22. We quantify the sunspot - CME storm relation in the five cycles of space age separately. In the minimum of cycle 23, CME storm occurrence reduced below that predicted by sunspots.Weak sunspot activity since cycle 23 has weakened solar polar fields and widened the HCS. Because of the wide HCS, the Earth has spent 50% of its time in slow solar wind since cycle 23, which has notably decreased the occurrence of large and moderate HSS/CIR storms. 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.
