Solar activity and space weather

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