In this work, we have studied the characteristics of equatorial plasma bubbles (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 (8.7°N, 77.8°E geographic, 1.7°N dip latitude) and Kolhapur (16.7°N, 74.3°E geographic, 11.5°N dip latitude) are utilized here. On 17 March 2015, EPBs were observed to drift eastward during 14:30–16:30 UT between 3°S and 15°N dip latitudes. A westward drift presumably under the influence of the disturbance dynamo electric field initially appeared at higher dip latitudes almost 10 h 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 premidnight hours on 17 March 2015. In this study, the possible mechanisms that can explain these observations are discussed in light of the current understanding of the equatorial electrodynamics and EPBs.
We analyze an equatorial plasma bubble (EPB) event observed in optical 630 nm image data simultaneously from Gadanki (13.5°N, 79.2°E), Kolhapur (16.8°N, 74.2°E), India. The total electron content data from Gadanki together with the ionosonde data from an equatorial Indian station, Tirunelveli (8.7°N, 77.8°E) confirmed the association of observed EPB event with equatorial spread F (ESF). The optical 630 nm images from a farther low‐latitude Indian station Ranchi (23.3°N, 85.3°E) show clear signatures of tilted east‐west wave structures propagating toward equator. Further, the upward wave energy noted in mesospheric airglow data was found to be negligible. These data suggest that possibly the off‐equatorial tilted east‐west structures triggered the observed EPB/ESF event.
Well-de ned coherent wave sources associated with the passage of short-period gravity waves were observed in all-sky images of OH emission on a total 21 occasions during January to May 2008 at Allahabad (25.45 • N, 81.85• E, dip lat ∼16.49• N) in India. The wave motions exhibited similar spatial and temporal properties during each month, but the north-east ward motions were distinctly dominant in April and May 2008. It is a wellknown theory that the upward propagating gravity waves may be blocked or absorbed at a critical layer. We have computed the magnitude and direction of atmospheric gravity waves subject to blocking by horizontal winds, i.e., critical layer directional ltering. The HWM-93 model (Hedin et al., 1996) was used to compute the two components of neutral wind velocity at Allahabad for the period of observation of gravity waves during March and April 2008. Data from two components of wind velocity were then used to construct the blocking diagrams, which show the directions and apparent phase velocities of wave propagation blocked at a given altitude. The blocking diagrams were then compared with experimental observations of gravity waves in OH airglow to determine the accuracy of the wind model and explain the critical layer theory.
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