Geographical and seasonal variations of gravity wave events in the upper mesosphere were investigated from the nightglow imaging data obtained by the Visible and near-Infrared Spectral Imager (VISI) on the Ionosphere, Mesosphere, upper Atmosphere and Plasmasphere (IMAP) onboard the International Space Station (ISS). The nadir-imaging data of the O2(0-0) atmospheric band (762 nm) with the typical emission peak around 95 km altitude, is used to investigate small-scale waves (horizontal wavelength less than ~100 km) on a global scale. To detect gravity wave events, the variance of hi-pass filtered nightglow images within a local 100 km circle was evaluated with a threshold three times the standard deviation from the average variance of the background level. A data screening algorithm that evaluates a variance of upwelling contamination light emission was also introduced to remove contaminated data. Applying the variance filter and data screening algorithm to a nearly three years data set, from November 2012 to August 2015, occurrence maps of wave events for four seasons were derived. Middle to high latitudes in the summer hemisphere show higher occurrence with three longitudinal maxima. These summer hemisphere enhancements are likely attributed to poleward propagating waves from subtropical regions generated by convective activities. The occurrence map also shows several seasonal hot spots, e.g., over the southern Andes in the local winter, and the Indochina peninsula in boreal spring. Gravity wave hot spots were found near both orographic sources and regions of strong convection.
