Rotation, bifurcation and merging of medium-scale traveling ionospheric disturbances over a low-mid latitude transition region

This paper reports the observation of propagating wave structures in OI 630.0 nm airglow images captured by a CCD imager at Kashmir University (KU), Srinagar, India (34.1 degrees N, 74.8 degrees E, and 25.9 degrees N magnetic latitude). These structures were detected on the night of August 17th, 2017, with an average horizontal wavelength around 330 km and average phase velocities around 92 m/s traveling in southwest (SW) direction. The SW moving wave patterns had north-west (NW) to south-east (SE) aligned phase fronts that lasted for ~150 min and had a time period of about 1 h. Based on observed parameters such as wavelength, time span, and propagating speed, these structures are categorized as nighttime medium-scale traveling ionospheric disturbances (MSTIDs). The observed southwestward movement, phase speed, horizontal wavelength and period of these nighttime MSTIDs are consistent with the existing literature. The appearance of gravity waves in the lower atmosphere during this night means that gravity waves from below, in combination with the Perkins' instability, might be responsible candidates for generation of these MSTIDs. Near midnight the MSTID bands are seen to rotate, bifurcate and some even merging with other bands giving rise to some unique Y-shaped MSTID structures-the first such results observed from this location. These structures align along North-South direction and move westward unlike the MSTID bands before midnight which move towards southwest. In addition, we also report the shrinking phenomena of the post midnight MSTID dark bands.