Seasonal Variations in Magnetic Storms

A new list of magnetic storms for the period 1839 to 1930 has been prepared. This list represents a uniform scale of world wide disturbances for this entire period. The storms of this list show a seasonal variation in frequency with equal maxima at equinox and equal minima at solstice which follows approximately the cosine curve y=F(3+cos2θ) where θ is the longitude of the sun measured on the earth's ecliptic. The ultraviolet light theory of magnetic storms regards the storms as being due to three effects, gravitational ion drift currents, dynamo currents due to vertical convection and dynamo currents due to lateral winds. The first two are world wide effects and when calculated from the theory account for a little less than one third of the change in magnetic activity. The third effect is local and is estimated to account for a little over a third of the total change in activity. Accidental variations in the occurrence of non-solar magnetic storms with semiannual periods account for the remaining third of the change in magnetic storm frequency. It is to be noted that the difference between winter and summer or spring and fall activity which has previously been observed is not shown by the storms of the present list. It is concluded that annual variations in magnetic storm activity can be explained as the result of annual variations in the declination of the sun.