Investigation of Unusual Solar Flare Effect on the Global Ionospheric Current System

In this paper, we present our investigation of geomagnetic crochets observed following the X-class solar flare events on 9 August 2011 (X6.9) and 24 September 2011 (X1.4). Both events occurred during geomagnetically quiet times (Kp <= 3). The analysis of the geomagnetic crochets near the magnetic equator revealed very different responses in the ionospheric currents between these two events. During the 9 August 2011 event, a reduction of the eastward equatorial electrojet was observed in all daytime stations studied, while the daytime eastward equatorial electrojet was observed to enhance during the 24 September 2011 event. A spherical harmonic analysis was applied to the observations from nearly 160 ground stations at middle and low latitudes to derive the equivalent ionospheric current system for the geomagnetic crochets as well as background solar-quiet (Sq) variations for both events. The equivalent current system of the geomagnetic crochets for the 24 September 2011 event showed a similar pattern with the background Sq current system and thus can be attributed to the enhancement of Sq currents. However, the crochet current system for the 9 August 2011 event revealed a different pattern from the background Sq current system in both Northern and Southern Hemispheres. It is suggested that unusual geomagnetic crochets during the 9 August 2011 event may be dominated by the ionospheric currents at D-region altitudes.