Despite the ambiguous homonym, mesospheric “bores” are anything but boring. Occurring in the mesosphere (50–85 kilometers above Earth’s surface), the bores are a type of gravity wave disturbance that propagates through air, sometimes creating rippling patterns in high-altitude clouds that can be seen with the naked eye. The exact cause of the phenomenon is still under investigation, but previous theoretical and modeling studies have suggested that a “ducting region”—formed by a temperature inversion layer, wind shear, or both—combined with an existing gravity wave can produce the telltale ripples of a mesospheric bore as the wave is channeled through the thin “duct” layer.
In a new study, Hozumi et al. move beyond theoretical considerations and present the first large-scale statistical analysis of mesospheric bores captured in satellite imagery. The data set, recorded by the Visible and Near Infrared Spectral Imager on board the International Space Station, contains 306 bore events captured over 3 years from September 2012 to August 2015. Although the researchers didn’t try to parse in detail how the bores form, the work provides a lot of new information about when and where they form, which may help reveal their underlying mechanics.
Mesospheric bores were most prevalent during the equinox seasons (fall and spring) and tended to form at equatorial latitudes. During winter, they were more likely to form at midlatitudes. Regardless of time of year, the bores typically propagated from the winter toward the summer hemisphere.
The authors note that this geographic and temporal distribution traces regions of the mesosphere that host two atmospheric tides with large temperature variations, a phenomenon that likely lends itself to the thermal ducting theorized as a necessity for bore formation. In addition, the authors posit that because the bores tend to originate in the winter high latitudes, the polar night jet—a strong westerly wind that develops near the edge of the polar night—may be responsible for creating the gravity waves required for the bores’ formation. (Journal of Geophysical Research: Space Physics, https://doi.org/10.1029/2019JA026635, 2019)
—David Shultz, Freelance Writer