‘Sudden Commencements’ in Geomagnetic Field Variations

Abstract

A STUDY of the phenomenon of ‘sudden commencements’ in geomagnetic field variations has been made by Newton¹ and also recently by Ferraro and Parkinson², but the observations analysed are insufficient to ascribe definite cause and characteristics to this phenomenon. The records of Alibag Observatory (ϕ = 18.6, λ = 72.9) over the period 1905–44 have been analysed to provide further data. The first type of s.c., characterized by a rise in H and a fall in V, is very prominent in the Alibag records and can be picked up without any difficulty. The second type described by Ferraro and Parkinson, where the increase in H is preceded by a small movement in the opposite direction, is absent from the Alibag records. The third type, Called the ‘inverted s.c.'s’, does occur at Alibag, but their frequency is small. It is quite possible that they are not really s.c.'s but form part of normal fluctuations. Altogether, about 800 s.c.'s were recorded during the period under review, of which only 28 can be classified as ‘inverted s.c.'s’. In order to fit the curve suggested by Ferraro and Parkinson, the number of s.c.'s of the second type at Alibag (geomagnetic longitude 143.6°) during the period under discussion should have been about 200 instead of the actual figure, 0. This difference is certainly beyond all limits of statistical error and must be a real one. It shows that the suggestion made that the second type of s.c.'s depends upon geomagnetic longitudes is not true, at least in the equatorial regions.