Simultaneous observations of mesospheric bore and front over the Himalaya

Interesting observational evidence of interaction between a mesospheric bore and a mesospheric front is found in O(¹S) 557.7 nm airglow images over the western Himalayan region on 25 April 2022. The event is unique as it is the first report of a mesospheric bore interacting with a typical mesospheric front. The vertical profiles of temperature and Brunt Vaisala frequency indicate presence of a strong mesospheric inversion layer (MIL) which acts as a stable thermal duct for the propagation of the mesospheric bore. Analysis suggests that local chemical heating plays a significant role in sustaining a strong MIL/thermal duct. The bore front shows an anti-clockwise rotation, which is attributed to the differential phase speed of different regions of the bore due to variations in duct depth. The bore propagation above is observed to push the underlying OH layer downward, resulting in a maximum horizontal slope of the peak height of the OH volume emission rate (VER) on 25 April 2022. The results highlight the bore-front interaction, mesospheric background condition for bore propagation, and its effect on the altitudinal shift of adjacent airglow emission layers.