Linked from
The 20 pages that link to Geomagnetic storm, each with the reason it gives.
Solar windBroader topic: Fast streams and interplanetary disturbances can trigger these storms.
Space weatherBroader topic: It is the principal magnetic response to sustained solar forcing.
Solar coronaBroader topic: A geoeffective coronal mass ejection can trigger a geomagnetic storm.
Space physicsBroader topic: It is a prominent near-Earth outcome of solar–magnetosphere coupling.
HeliophysicsBroader topic: Storms can disrupt power grids, navigation, and satellite operations.