Geomagnetic activity is measured by a number of indices, some of which have been available for many years. The longest running series of any index is that of the aa index. This means that the index is a global index, representing world-wide magnetic activity rather than local features. The graph shows the variation of the yearly-average aa index since 1844. The vertical lines show the time of solar minimum for each of the solar cycles since the start of Cycle Number 9.
Cycles show very different variations in activity.
All exhibit multiple peaks of activity - mostly with three peaks although some with only two. Activity during Cycle 21 was particularly strange because of the deep trough in the middle of the cycle where the average aa index in fact dropped below its value at the previous minimum. All rights reserved.