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. Large-scale spatial structures of mesospheric bores were observed by theVisible and near-Infrared Spectral Imager (VISI) of the ISS-IMAP mission(Ionosphere, Mesosphere, upper Atmosphere and Plasmasphere mapping missionfrom the International Space Station) in the mesospheric O2 airglowat 762 nm wavelength. Two mesospheric bore events in southern midlatitudesare 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, thetemporal evolution of the mesospheric bore was investigated from thedifference of two observations in consecutive passes. The estimated eastwardspeed of the bore is 100 m s −1 . The number of trailing waves increasedwith a rate of 3.5 waves h −1 . Anticlockwise rotation with a speed of20 ∘  h −1 was also recognized. These parameters are similar tothose reported by previous studies based on ground-based measurements, andthe similarity supports the validity of VISI observation for mesosphericbores. For the second event, VISI captured a mesospheric bore with alarge-scale and undulating wave front. The horizontal extent of the wavefront was 2200 km. The long wave front undulated with a wavelength of1000 km. The undulating wave front is a new feature of mesospheric boresrevealed by the wide field of view of VISI. We suggest that nonuniform borepropagating speed due to inhomogeneous background ducting structure might bea cause of the undulation of the wave front. Temperature measurements fromthe 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 atemperature inversion layer.

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