It is well known that the number of geomagnetic disturbances varies
during the solar cycle. Near solar minimum, disturbances are quite
rare. But as the cycle rises to a peak of sunspot number, disturbances become
more numerous.
This picture of geomagnetic activity is basically correct; but the
more detailed picture is much more complicated. During most solar cycles,
geomagnetic activity varies considerably, rising to peaks and then falling
to troughs.
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 index is based on observations
at two nearly antipodal stations (Canberra in Australia and Hartland in
the United Kingdom). 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. The numbers at the base
of the graph is the Cycle Number and those in brackets is the amplitude
(of the smoothed sunspot number) for each cycle.
Cycles show very different variations in activity. All exhibit
multiple peaks of activity - mostly with three peaks although some with
only two. Some cycles have the peak activity concentrated early in the
solar cycle (e.g. Cycles 11, 13, and 19). Other cycles have activity
concentrated late in the Cycle (e.g. Cycles 10, 14, and 17), whilst
others are more evenly distributed.
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. Cycles 9, 12 and 18 also have deep
troughs but not as pronounced as that for Cycle 21.
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