An explanation of criticisms on Dr. S. Chapman's recent paper, “An outline of a theory of magnetic storms.”

Abstract

Criticism of Dr. Chapman was not the primary object of my paper, and it would seem that I have studied brevity too much in referring to differences which to myself were conspicuous. That fault I shall hope to repair now. But first I should make it clear that there are certain points about which there is, I think, no difference of opinion. Our knowledge of magnetic storm phenomena in high latitudes is very limited, but in low and moderate latitudes the following phenomena in the variations of H (horizontal force) during storms having Sc’s (sudden commencements) have been, I think, generally recognised for many years. The result of the Sc movement or movements is almost invariably to enhance H, and a marked fall does not as a rule set in immediately. This rule is not absolutely without exception. For instance, on the occasion of the very large storm on August 11-12 of the present year, a large reverse movement set in so soon after the Sc that the mean values alike for the first hour, the first half hour, and even the first quarter hour of “storm time” are much below the pre-storm value. Still exceptions are rare, and the mean value of H for the first hour or half hour after the Sc is nearly always in excess of the pre-storm value.