Geomagnetic activity effect on the global ionosphere during the 2007–2009 deep solar minimum

In this paper the significant effect of weaker geomagnetic activity during the 2007–2009 deep solar minimum on ionospheric variability on the shorter‐term time scales of several days was highlighted via investigating the response of daily mean global electron content (GEC, the global area integral of total electron content derived from ground‐based GPS measurements) to geomagnetic activity index Ap. Based on a case during the deep solar minimum, the effect of the recurrent weaker geomagnetic disturbances on the ionosphere was evident. Statistical analyses indicate that the effect of weaker geomagnetic activity on GEC variations on shorter‐term time scales was significant during 2007–2009 even under relatively quiet geomagnetic activity condition; daily mean GEC was positively correlated with geomagnetic activity. However, GEC variations on shorter‐term time scales were poorly correlated with geomagnetic activity during the solar cycle descending phase of 2003–2005 except under strong geomagnetic disturbance condition. Statistically, the effects of solar EUV irradiance, geomagnetic activity, and other factors (e.g., meteorological sources) on GEC variations on shorter‐term time scales were basically equivalent during the 2007–2009 solar minimum.