Weakest Solar Cycle of the Space Age: A Study on Solar Wind–Magnetosphere Energy Coupling and Geomagnetic Activity

Abstract

Solar Cycle 24, from December 2008 to December 2019, is recorded to be the weakest in magnitude in the space age (after 1957). A comparative study of this cycle with Solar Cycles 20 through 23 is presented. It is found that Solar Cycle 24 is not only the weakest in solar activity, but also in average solar wind parameters and solar wind–magnetosphere energy coupling. This resulted in lower geomagnetic activity, lower numbers of high-intensity long-duration continuous auroral electrojet ( $AE$ ) activity (HILDCAA) events and geomagnetic storms. The Solar Cycle 24 exhibited a $\approx 54$ – $61\%$ reduction in HILDCAA occurrence rate (per year), $\approx 15$ – $34\%$ reduction in moderate storms ( $-50~\text{nT} \geq Dst > -100~\text{nT}$ ), $\approx 49$ – $75\%$ reduction in intense storms ( $-100~\text{nT} \geq Dst > -250~\text{nT}$ ) compared to previous cycles, and no superstorms ( $Dst \leq -250~\text{nT}$ ). Implications of the solar and geomagnetic weakening to space weather science and operations are discussed.