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

Abstract

In this work, we have studied the characteristics of equatorial plasma bubbles (EPBs), such astheir zonal drift and tilt, from the low-latitude and dip equatorial region in the Indian longitude sectorduring the main phase of the 17 March 2015 storm. All-sky airglow imaging observations from Tirunelveli(8.7°N, 77.8°E geographic, 1.7°N dip latitude) and Kolhapur (16.7°N, 74.3°E geographic, 11.5°N dip latitude) areutilized here. On 17 March 2015, EPBs were observed to drift eastward during 14:30–16:30 UT between 3°Sand 15°N dip latitudes. A westward drift presumably under the influence of the disturbance dynamo electricfield initially appeared at higher dip latitudes almost 10 h after the storm onset, and subsequently, the samewas observed at lower dip latitudes. The EPBs attained a peak westward drift at ~17:00 UT followed by agradual decrease in their speed till ~18:30 UT. After regaining their westward speed, the EPBs continued todrift westward till 22:00 UT. Moreover, a latitudinal gradient in the drift motion of the EPBs was also observedon this night. Another interesting observation made from the images obtained from Tirunelveli was thepresence of a large westward tilt of the EPBs. The most intriguing finding of this study, however, was theasymmetry in the tilt of the EPBs at conjugate points during the premidnight hours on 17 March 2015. In thisstudy, the possible mechanisms that can explain these observations are discussed in light of the currentunderstanding of the equatorial electrodynamics and EPBs.