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

TL;DR

This study analyzes magnetic storm occurrences during the space age (1957-2021), revealing how solar activity variations influence storm frequency, intensity, and their relation to solar magnetic structures and heliospheric current sheet changes.

Contribution

It provides a comprehensive analysis of magnetic storm occurrence patterns over six decades, linking them to solar activity cycles and heliospheric current sheet dynamics, which is a novel long-term perspective.

Findings

Approximately 45% weak, 40% moderate, 12% intense, 3% major storms.

Storm occurrence decreased with declining solar activity and changing solar magnetic structure.

Large HSS/CIR storms were most frequent during cycle 22, with a shift in timing in recent cycles.

Abstract

We study the occurrence of magnetic storms in space age (1957-2021) using Dst and Dxt indices. We find 2526/2743 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 - 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. 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.