The solar rotation period (27 days as seen from the earth) is an important
time scale in determining the occurrence of geomagnetic
disturbances. This comes about in several different ways. Firstly, a
large sunspot region may live for several rotations and produce
energetic flares as it crosses the visible disk of the sun. Geomagnetic
disturbances associated with these flares then occur preferentially
during one half of the solar rotation but not in the other. Secondly, the
streams of high speed solar wind produced by coronal
holes are sometimes very long-lived and can be associated with a disturbance
each time the stream is suitably located with respect to the
sun-earth direction. The effect is similar
to a lighthouse in which the light is seen once per rotation. Such sequences
of recurrent disturbances
can sometimes be observed for as long as a year although most patterns
are shorter lived, especially near the peak of the cycle. Recurrent
patterns tend to show variation
from rotation-to-rotation in the timing of the disturbance
and in its strength. In particular, the strength is affected by the seasonal
trend and there is also an intrinsic
variability related to the sun and to the response of the
earth's magnetic field to the solar wind.
Recurrent patterns are particularly useful for forecasting
purposes as they allow predictions of disturbances and quiet
periods some time in advance. Naturally, the most reliable
forecasts of this type are a single solar rotation (27 days) in
advance but it is sometimes possible to forecast for
several rotations. The major problem for forecasters
is to pick the start and the end of sequences although there are sometimes
difficulties when disturbances are weak, or are delayed. The patterns are
most stable in the declining phase
and least reliable in the rising phase and
maximum of the cycle. Such longer term forecasts can be quite useful but
there is always the risk that some unexpected event can produce
a disturbance.
The following figure shows an interval of recurrent disturbances. In the
figure, sequences of 27 days, the approximate solar rotation period as seen
from the earth, have been stacked under each other. Days have been
colour coded so that the more disturbed periods are coloured red, brown or green.
Recurrent disturbances are seen as patches having
vertical extent. A particularly good example appears on the right side
of the figure as a recurrent sequence lasting many rotations.
Educational