Extreme Space-Weather Events and the Solar Cycle

Abstract Space weather has long been known to approximately follow the solar cycle, with geomagnetic storms occurring more frequently at maximum than minimum. There is much debate, however, about whether most hazardous events same pattern. Extreme – by definition occur infrequently, and thus establishing their occurrence behaviour difficult even very space-weather records. Here we use 150-year $aa_{H}$ a H record of global activity a number probabilistic models geomagnetic-storm test range hypotheses. We find that all magnitudes during an active phase, centred on maximum, quiet phase around also show available observations are consistent extreme large cycles small cycles. Finally, report difference in extreme-event odd- even-numbered cycles, clustering earlier later odd Despite relatively few for study, demonstrate this inconsistent random occurrence. interpret finding terms overlying coronal magnetic field enhanced magnetic-field strengths heliosphere, which act increase geoeffectiveness sheath regions ahead mass ejections. Putting three “rules” together allows probability event Solar Cycle 25 be estimated, if magnitude length coming cycle can predicted. This highlights both feasibility importance solar-cycle prediction planning scheduling activities systems affected space weather.