The best established
example of this yet obtained is afforded in the case of the sun
and the earth.
The fact that the frequency of magnetic storms goes through a
period of about eleven years, and is proportional to the frequency
of sun-spots, has been well established. The recent work of
Professor Bigelow shows the coincidence to be of remarkable
exactness, the curves of the two phenomena being practically
coincident so far as their general features are concerned. The
conclusion is that spots on the sun and magnetic storms are due to
the same cause. This cause cannot be any change in the ordinary
radiation of the sun, because the best records of temperature show
that, to whatever variations the sun's radiation may be subjected,
they do not change in the period of the sun-spots. To appreciate
the relation, we must recall that the researches of Hale with the
spectro-heliograph show that spots are not the primary phenomenon
of solar activity, but are simply the outcome of processes going
on constantly in the sun which result in spots only in special
regions and on special occasions. It does not, therefore,
necessarily follow that a spot does cause a magnetic storm. What
we should conclude is that the solar activity which produces a
spot also produces the magnetic storm.
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