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

长期以来，人们一直知道太空天气大致遵循太阳周期，地磁风暴​​在太阳最大时比太阳最小时更频繁地发生。但是，关于最危险的事件是否遵循相同的模式，存在很多争议。根据定义，极端事件很少发生，因此即使具有很长的空间天气记录，也很难确定其发生行为。在这里，我们使用150年\（aa_ {H} \）记录了全球地磁活动，并使用了许多地磁风暴发生概率模型来检验一系列假设。我们发现，在以太阳最大值为中心的活跃阶段，各种级别的风暴比在围绕太阳最小值的安静阶段更为频繁。我们还表明，可用的观测结果与在大太阳周期中比小周期中更频繁发生的最极端事件一致。最后，我们报告了奇数和偶数太阳周期中极端事件发生的差异，事件在偶数周期中聚类较早，而在奇数周期中聚类较晚。尽管可以研究的事件相对较少，但我们证明这与随机发生是不一致的。我们用上覆的日冕磁场和日光层中增强的磁场强度来解释这一发现，这可以在极端日冕物质抛射之前起到增加鞘管区域的地球效应的作用。如果可以预测即将到来的周期的大小和长度，将这三个“规则”放在一起可以估算出太阳周期25发生极端事件的可能性。这突出了太阳周期预测在计划和安排受极端空间天气影响的活动和系统的计划中的可行性和重要性。如果可以预测即将到来的周期的大小和长度。这突出了太阳周期预测在计划和安排受极端空间天气影响的活动和系统的计划中的可行性和重要性。如果可以预测即将到来的周期的大小和长度。这突出了太阳周期预测在计划和安排受极端空间天气影响的活动和系统的计划中的可行性和重要性。