‘Sudden Commencements’ in Geomagnetism

Abstract

‘SUDDEN commencements’ which appear in magnetogram traces have been recognized in the past more or less by their similarity to accepted typical movements, and the ‘normal’ s.c. has been subdivided into two types according as the main movement is preceded by a smaller movement in the opposite direction or not. In the paper on ‘sudden commencements’ by H. W. Newton¹ in 1948, the second type was regarded as one showing the preliminary movement in all three traces, though in the later analysis it was shown that the initial movement or ‘tail’ might take place in one element only, such as H or D. The frequency in which the tail movement was observable in D and not in H was slightly greater than vice versa.