Dependence of time derivative of horizontal geomagnetic field on sunspot number and aa index

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