Variations in the Earth’s magnetic field were first observed in the nineteenth century as a sudden increase (by about 10-3 G) followed by a slow decrease. These geomagnetic storms were sometimes found to occur one or two days after large solar flares, and further evidence for a link between the two phenomena came with the discovery of an 11-yr periodicity in both flare and geomagnetic activity. However, since then we have learnt that the interaction is an exceedingly complex one.