Observational Evidence of the Effect of Source Field Geometry on Geomagnetic Induction in Southern India

The uniform and non-uniform source current systems that prevail in the equatorial belt in association with the short-period magnetic events have been used to investigate the effect of source field geometry on the geomagnetic induction in southern India. The study utilized a large number of short-period events isolated from two long sequences of magnetic storms recorded through an array of magnetometers in the equatorial belt of Peninsular India. The effect of source field geometry was highlighted with the help of contour plots of ΔZ / ΔH ratios separately for day (non-uniform source) and night (uniform source) time events. The daytime ratios are found to be uniformly lower than their corresponding nighttime counterparts, with two well defined zones of attenuation located around the periphery and central axis of the equatorial electrojet. The representation of these differences in terms of the first three spatial derivatives have facilitated to infer that the reduction in daytime ΔZ / ΔH ratios at stations close to the periphery of the electrojet is due to the effect of mutual cancellation of external and internal parts in vertical field components. Consistent with the theory, the role of the second and higher order derivatives in the reduction of ΔZ / ΔH near the electro jet axis is interpreted to indicate the weakening of the intensity of induced currents due to the non-uniformly of the source field.