The effects of solar activity on the Earth and its magnetic field. The largest effects originate from magnetic disturbances such as flares and coronal mass ejections which travel out from the Sun and produce geomagnetic storms as they interact with the Earth’s magnetosphere. A sharp increase in the strength of the Earth’s magnetic field at the start of a storm results from a compression of the field on the sunward side. Disturbances occur world-wide and may continue for a day or so. Magnetic substorms are more localized, occurring near the geomagnetic poles, and are associated with auroral displays. Links between solar activity and the Earth’s weather have been proposed, notably at the time of the Maunder minimum.