Decay rate of recovery phase of Geomagnetic Storms And dissipation of associated Ring Currents

Abstract

Decay rates of the recovery phases of some 130 geomagnetic storms recorded at Alibag during the period 1924 to 1959, classified as great, are investigated with a view to ascertain (1) the precise relation between amplitude of depression of the main phase and the rate of decay of the recovery, phase of storm, (2) the possibility of more than one ring current being associated with each storm and (3) the solar-cycle variation in the decay rate of the recovery phase.

Decay times are determined for the first 36 hours of the recovery phase by fitting an exponential curve and finding the time constant for the amplitude of the fitted curve to decay to l/e of the initial amplitude. As a second method, free-hand smooth curves are drawn best fitting the 36-hourly depressions and picking up the time for the initial amplitudes, given by the fitted curve, to decay to 1/2 its initial value. The recovery phase appears to have two distinct portions, an initial portion of less than 12 hours with a rapid decay rate and a later portion with a slower decay rate, suggesting the possibility of two ring currents being associated with each storm. Decay rates for the two portions of the recovery phase are separately investigated by the method of free-hand smooth curves.

The generally known characteristic that decay rates are faster for the larger intensity storms is seen, but considerable scatter in decay rates is observed for any storm-intensity group. The scatter is less for decay rates of the initial portion 01 the recovery phase. There is a clear tendency, especially for storms of more than 200y (main-phase depression, for decay rates to be faster for the solar minimum epoch than for the maximum epoch. But the relative rates are far from the factor 3 expected by Dessler et al. This factor is, however, approached if extreme values of recovery rates of the initial portion of the recovery phase only are considered. For the later portions of the recovery phase no significant differences in decay rates are observed for the two solar epochs. Currents of the order of 101 amperes are associated with the inner ring current, which is considered large enough to produce shielding effects on the outer ring current field. The main phase depression appears, therefore, to be largely the effect of the inner ring current, especially for the large intensity storms.