What is a geomagnetic storm?

Geomagnetic storms occur when a coronal mass ejection or fast solar wind stream slams into Earth's magnetosphere, transferring huge amounts of energy. They can push the aurora far towards the equator, creating spectacular northern and southern lights. Strong storms can also disrupt satellites, GPS, radio communications and power grids. In March 1989 a severe storm caused the collapse of the Hydro-Québec power grid, leaving millions of people in Canada without electricity for about nine hours. The most powerful recorded storm, the Carrington Event of 1859, set telegraph wires sparking.