Mesospheric bores at southern midlatitudes observed by ISS-IMAP/VISI: A first report of an undulating wave front

Large-scale spatial structures of mesospheric bores were observed by the Visible and near-Infrared Spectral Imager (VISI) of the ISS-IMAP mission (Ionosphere, Mesosphere, upper Atmosphere and Plasmasphere mapping mission from the International Space Station) in the mesospheric O₂ airglow at 762 nm wavelength. Two mesospheric bore events in southern midlatitudes are reported in this paper: one event at 48-54° S, 10-20° E on 9 July 2015 and the other event at 35-43° S, 24°W-1° E on 7 May 2013. For the first event, the temporal evolution of the mesospheric bore was investigated from the difference of two observations in consecutive passes. The estimated eastward speed of the bore is 100ms^-1. The number of trailing waves increased with a rate of 3.5 waves h^-1. Anticlockwise rotation with a speed of 20° h^-1 was also recognized. These parameters are similar to those reported by previous studies based on groundbased measurements, and the similarity supports the validity of VISI observation for mesospheric bores. For the second event, VISI captured a mesospheric bore with a large-scale and undulating wave front. The horizontal extent of the wave front was 2200 km. The long wave front undulated with a wavelength of 1000 km. The undulating wave front is a new feature of mesospheric bores revealed by the wide field of view of VISI. We suggest that nonuniform bore propagating speed due to inhomogeneous background ducting structure might be a cause of the undulation of the wave front. Temperature measurements from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) onboard the Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics (TIMED) satellite indicated that bores of both events were ducted in a temperature inversion layer.