Dependence of Time Derivative of Horizontal Geomagnetic Field on Sunspot Number and aa Index

This work investigated the interrelationship between the monthly means of time derivatives of horizontal geomagnetic field (dH/dt), sunspot number (Rz) and aa index for period of substorms (-90nT-1800nT) during the years 1990-2009. 232 substorms were identified during the period of study. Time derivative of horizontal geomagnetic field (dH/dt) used as proxy for geomagnetic induced current (GIC), exhibited high positive correlation with sunspot number (0.86) and aa index (0.8998). 92.665% of the obtained geomagnetic activity is explicable by the combined effect of sunspot number aa index. The distribution of substorms as a function of years, gives a strong support for the existence of geomagnetic activity increases which implies that as the sunspot number increases the base level of geomagnetic activity increases.