Geographical and Seasonal Variability of Mesospheric Bores Observed from the International Space Station

Geographical and seasonal variations of mesospheric bores were derived from mesospheric airglow observations by the Visible and near Infrared Spectral Imager (VISI) of Ionosphere, Mesosphere, upper Atmosphere and Plasmasphere (IMAP) mission onboard the International Space Station. In the three-year data set spanning September 2012 to August 2015, 306 mesospheric bore events were found between 55ºS and 55ºN in the O₂(0-0) airglow whose peak height is around 95 km. The distribution of the bore events showed a high occurrence at equatorial latitudes especially during the equinox seasons and at winter midlatitudes. These latitudes and seasons are also known for being the place and time where the migrating diurnal and semidiurnal tides have a large temperature amplitude at the upper mesosphere altitude. This coincidence suggests that the majority of mesospheric bores occurred in a temperature inversion layer, which is related to the tides. The local time variation of the bore occurrences at midlatitudes showed a minimum around midnight. The local time variation at equatorial latitudes is more widely distributed compared to those at midlatitudes. The dominant propagation direction of mesospheric bores is from the winter hemisphere to the summer hemisphere.