The response of African equatorial GPS-TEC to intense geomagnetic storms during the ascending phase of solar cycle 24

This study presents the response of African equatorial GPS-TEC to intense geomagnetic storms that occurred during the ascending phase (2011-2012) of solar cycle 24. Specifically, four intense geomagnetic storms were considered: September 26-27, 2011 storm (Dst: -103 nT), October 25, 2011 (Dst: -137 nT), March 9, 2012 storm (Dst: -133 nT), and July 15, 2012 storm (Dst: -126 nT). Furthermore, the responses of GPS-TEC data from three African equatorial stations: Addis Ababa (9.04 degrees N, 38.77 degrees E, 0.18 degrees N magnetic latitude) [Ethiopia]; Lagos (6.52 degrees N, 3.4 degrees E, 3.04 degrees S magnetic latitude) [Nigeria]; and Malindi (3.00 degrees S, 40.20 degrees E, 10.98 degrees S magnetic latitude) [Kenya] to the geomagnetic storms under investigation were also studied. We also examine the causative roles of interplanetary (IP) structures in the formation of the intense geomagnetic storms. All the intense storms were found to be associated with CME-induced transients, and their drivers were sheath fields behind the shocks. At the African equatorial zone, TEC exhibits positive response to intense geomagnetic storms, with enhancements in the order of 6-25 TECU around 1300-1500 UT. (C) 2013 Elsevier Ltd. All rights reserved.