As might be expected, geomagnetic disturbances are not equally likely
at all phases of the solar cycle and for all different solar cycles. The Figure
shows the solar cycles since 1932 in terms of sunspot number (yellow line
and left hand scale) and the occurrence of geomagnetic disturbances (filled
hatched area and right hand scale). For the purposes of this graph,
a disturbance has been defined as a day on which the Ap index
equals, or exceeds, a value of 25 and the number of such days in each
calendar year has been counted.
The occurrence of geomagnetic
disturbances shows a trend generally similar to the solar cycle with periods
of reduced frequency of disturbances near solar minimum. However, there
appears to be two components which determine the occurrence of disturbances
during the main part of the cycle. Firstly, there is a component
which varies in phase with the solar cycle and seems to be related
to events such as solar flares. Secondly, there is a
component which results in large peaks of disturbance in the declining phase
of some, but not all, solar cycles. This component is
related to coronal holes and produced large peaks
of disturbance in 1950-53 (the declining phase of Cycle 18), in 1974-76 (Cycle
20) and in 1982-84 (Cycle 21) but no corresponding large peak in the declining
phase of Cycle 19. The geomagnetic disturbances which make up this
activity is distinctive in that they are recurrent
disturbances (i.e. spaced at intervals of 27 days) reflecting the properties
of the coronal holes from which they arise.
The recurrent disturbances in the declining phase have given rise to
a successful technique for predicting the amplitude of the
next solar cycle. It turns out that the size and strength of the declining
phase burst of disturbances is related to the amplitude of the next
solar cycle. Hence, a large solar cycle such as Cycle 19 was preceded by
a burst of recurrent disturbances prior to the onset of the cycle (in
this case in 1950-53). On the other hand, a relatively weak cycle such
as Cycle 20 was preceded by a small burst of disturbances (around 1963
and hardly visible in the Figure).
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