Relation Between Magnetic Storms and Solar Activity

This investigation relates to the effect of individual solar phenomena on terrestrial magnetism. Magnetic disturbances have been segregated into various groups on the basis of the recurrence tendency, the storm intensity, and sudden commencements. The magnetic data have been correlated with the central meridian passage of large spot groups, with chromospheric eruptions, and ionospheric fade-outs. The method of correlation has been to plot the number of disturbances of each type occurring from five days before to fifteen days after a solar event, and to compare the counts with a random distribution. Graphs are plotted and the following rather tentative conclusions obtained:— ( a ) There is some evidence supporting the view that “great” magnetic storms are caused by chromospheric eruptions. The time interval between the eruption and the maximum of the great storm is about $$1\frac12$$ days. Eruptions also cause a few smaller storms with a $$2\frac12$$ -day interval. ( b ) A mean period of about 3 days is required for M-region particles to travel from Sun to Earth. ( c ) M-regions tend to avoid the area within about 40° of sunspot groups, but are more frequent than normal just outside this area. ( d ) It is not the eruptions within spot groups that influence M-regions. ( e ) Sunspots do not influence the commencement of an M-region, but sometimes interfere with existing ones and bring them to an end. ( f ) M-regions are identified with coronal streamers, thus explaining some characteristics of M-regions and their maximum of development two years before sunspot minimum. ( g ) Eclipse drawings show that equatorial coronal plumes are generally focussed near a sunspot, and they repel extended coronal streamers. This is thought to account for the negative sunspot–M-region correlation. ( h ) Sudden commencements are associated generally with sunspots, but are not due to eruptions except in the case of “great” storms.