Solar activity and space weather

Abstract

After providing an overview of solar activity as measured by the sunspot number (SSN) and space weather events during solar cycles (SCs) 21-24, we focus on the weak solar activity in SC 24. The weak solar activity reduces the number of energetic eruptions from the Sun and hence the number of space weather events. The speeds of coronal mass ejections (CMEs), interplanetary (IP) shocks, and the background solar wind all declined in SC 24. One of the main heliospheric consequences of weak solar activity is the reduced total (magnetic + gas) pressure, magnetic field strength, and Alfvén speed. There are three groups of phenomena that decline to different degrees in SC 24 relative to the corresponding ones in SC 23: (i) those that decline more than SSN does, (ii) those that decline like SSN, and (iii) those that decline less than SSN does. The decrease in the number of severe space weather events such as high-energy solar energetic particle (SEP) events and intense geomagnetic storms is deeper than the decline in SSN. The reduction in the number of severe space weather events can be explained by the backreaction of the weak heliosphere on CMEs. CMEs expand anomalously and hence their magnetic content is diluted resulting in weaker geomagnetic storms. The reduction in the number of intense geomagnetic storms caused by corotating interaction regions is also drastic. The diminished heliospheric magnetic field in SC 24 reduces the efficiency of particle acceleration, resulting in fewer high-energy SEP events. The numbers of IP type II radio bursts, IP socks, and high-intensity energetic storm particle events closely follow the number of fast and wide CMEs (and approximately SSN) because all these phenomena are closely related to CME-driven shocks. The number of halo CMEs in SC 24 declines less than SSN does, mainly due to the weak heliospheric state. Phenomena such as IP CMEs and magnetic clouds related to frontside halos also do not decline significantly. The mild space weather is likely to continue in SC 25, whose strength has been predicted to be not too different from that of SC 24.

What Is Unusual About the Third Largest Geomagnetic Storm of Solar Cycle 24?

“…Toward the end of this cycle, an intense storm has been observed on 26 August 2018 with a Dst index of −175 nT. Only two storms in solar cycle 24 are stronger than this event: The 17 March 2015 (−222 nT) and 23 June 2015 (−204 nT) storms (see e.g., Gopalswamy, Akiyama, et al., 2015; Liu et al., 2015; Webb & Nitta, 2017; Wu et al., 2016). The 26 August 2018 event is characterized by weak solar eruption, significant flux rope rotation in the corona and interplanetary (IP) medium, and an intense geomagnetic storm, as first reported by Chen et al.…”

Refined Modeling of Geoeffective Fast Halo CMEs During Solar Cycle 24

“…Most models have been developed and validated on events that occurred during specific phases of a specific solar cycle, with the bulk of the model validation and parameters testing work done during cycle 23. It is well known that substantial differences in the physical properties of the solar wind populating the heliosphere during different solar cycles may exist (see e.g., Gopalswamy, 2006; Gopalswamy et al., 2022; Gopalswamy, Xie, et al., 2015; Gopalswamy, Yashiro, et al., 2015; Michalek et al., 2022, and references therein). Therefore, permanent assessment of the solar cycle variations, regarding in particular the background solar wind, needs to become an integral part of the forecast operations.…”

The Sun and Space Weather

“…Recent investigations have shown that the space weather due to CIRs are milder in SC 24 [114,[143][144][145][146]. Gopalswamy et al [144] reported that the number of intense geomagnetic storms caused by CIRs has dropped by 75% in SC 24. Grandin et al [146] report that stream SIR/HSSs are 20-40% less geoeffective during cycle 24 than during SC23.…”

“…The severest reduction is in the number of GLEs: 16 in SC 23 vs. just 2 in SC 24 (i.e., an 87% drop). This has been attributed to the reduced acceleration efficiency in the weakened ambient magnetic field, so particles did not attain high energies [55,144]. An additional reason could be the presence of fewer seed particles [188].…”