Rise times versus magnitudes of sudden commencements of geomagnetic storms

Abstract

A study of the rise times versus the magnitudes of the positive sudden commencements of 41 randomly selected geomagnetic storms recorded at Alibag (Φ = 9.5°, A = 143.6°, geomagnetic coordinates) for the period 1949-1960 is presented. The study shows rise times of 1 to 6 minutes and shows that the rise times have an inverse relationship with the magnitudes of the SC's-the rise time being smaller as the magnitude is larger, and vice versa. The Chapman and Ferraro theory of the SC predicts rise times of less than about two minutes with an inverse relationship between rise time and amplitude. On the other hand, numerical computations by Dessler and co-workers from a hydromagnetic model have shown this inverse relationship and have shown rise times of the order of 1 to 6 minutes, which appear to fit better with the data of Alibag presented in this paper.