Response of the American equatorial and low‐latitude ionosphere to the X1.5 solar flare on 13 September 2005

Abstract

Based on the coordinated observations by the incoherent scatter radar (ISR), ionosonde, magnetometers, and GPS receivers, the electrodynamic effects on the equatorial and low‐latitude ionosphere have been investigated during the intense solar flare (X1.5/2B) on 13 September 2005. In the initial stage of the flare, the ISR and ionosonde measurements at Jicamarca show the decreases of 10.14 m/s and 20 km in the upward vertical E × B drift velocity and the F2 region peak height, respectively, while equatorial electrojet (EEJ) strength over American sector indicates a sudden increase of 53.7 nT. The decrease of the upward vertical E × B drift velocity reveals the weakening of eastward electric field during the flare, which is firstly and directly observed by instrument. It is well known that the variation of equatorial electric field is mainly attributed to the ionospheric dynamo electric field and partially affected by the penetration of interplanetary electric field. The observations during this flare suggest that the flare‐induced increase of Cowling conductivity changes the ionospheric dynamo electric field and further results in the weakening of eastward electric field and the decrease of the upward vertical E × B drift velocity. Meanwhile, the upward vertical E × B drift velocity and the EEJ strength during the flare are negatively correlated, which is contrary to the knowledge established by Anderson et al. (2002) based on 10 days of observations in the Peruvian longitude sector. The difference may be caused by the flare‐induced enhancement of Cowling conductivity. In addition, GPS total electron content (TEC) observations from six stations in the American equator and low latitudes show an enhancement of 1.47–3.09 TEC units. The measurements of GPS and ISR indicate that the contribution of the enhanced photoionization to the increase of TEC is more than that of electrodynamic effect during the initial stage of the intense flare.