Nearly all geomagnetic perturbations that are of origin external to the earth are directly or indirectly caused by the sun. Therefore one would expect that, in general, geomagnetic activity would be high when the sun becomes active. While this may be true so far as the overall impressions from visual inspection of magnetograms are concerned, the relation between solar activity and geomagnetic activity is by no means simple. This is in part because the geomagnetic field undergoes many different types of variations which have vastly different magnitudes, time scales, and scale sizes. For instance, global effects of a severe magnetic storm may last for several days, or substorms that mainly disturb polar regions typically last from one to a few hours. At another extreme, magnetic pulsations that have periods from a fraction of 1 second to a few tens of minutes may sustain their activity more or less continuously or intermittently for several hours, or even days, or they may have isolated occurrences. Magnetic perturbations that are associated with large-scale disturbances have magnitudes from several tens to a few thousand nanoteslas (1 nT = 1γ), while magnetic pulsations have amplitudes from the order of picotesla (1 pT = 1 mγ) to tens of nanotesla. Thus one can define several different kinds of geomagnetic activity that represent diversely different disturbance phenomena. However, by the term ‘geomagnetic activity’ we usually mean large-scale disturbances that are mainly caused by magnetic storms and substorms, and we follow this convention here.