First observation of interhemispheric asymmetry in the EPBs during the St. Patrick's Day geomagnetic storm of 2015

In this work, we have studied the characteristics of EPBs, such as their zonal drift and tilt, from the low latitude and dip equatorial region in the Indian longitude sector during the main phase of the 17 March 2015 storm. All-sky airglow imaging observations from Tirunelveli (8.7oN, 77.8oE geographic, 1.7oN dip latitude) and Kolhapur (16.7oN, 74.3oE geographic, 11.5oN dip latitude) are utilized here. On 17 March 2015, EPBs were observed to drift eastward during 14:30-16:30 UT between 3oS and 15oN dip latitudes. A westward drift presumably under the influence of the disturbance dynamo electric field (DDEF) initially appeared at higher dip latitudes almost 10 hours after the storm onset, and subsequently the same was observed at lower dip latitudes. The EPBs attained a peak westward drift at ~17:00 UT followed by a gradual decrease in their speed till ~18:30 UT. After regaining their westward speed, the EPBs continued to drift westward till 22:00 UT. Moreover, a latitudinal gradient in the drift motion of the EPBs was also observed on this night. Another interesting observation made from the images obtained from Tirunelveli was the presence of a large westward tilt of the EPBs. The most intriguing finding of this study, however, was the asymmetry in the tilt of the EPBs at conjugate points during the pre-midnight hours on 17 March 2015. In this study, the possible mechanisms that can explain these observations are discussed in the light of the current understanding of the equatorial electrodynamics and EPBs.