Too high for aircraft to sample but too low for satellites to orbit, the mesosphere is a strange and hard-to-study middle layer of Earth’s atmosphere. In addition to lightning-like phenomena known as elves and sprites, the 60- to 100-kilometer-high region occasionally displays bizarre temperature inversions, in which the air gets hotter, not colder, as the altitude increases.
Scientists have long hypothesized that these flipped temperature profiles, called mesospheric temperature inversions (MTIs), are caused by atmospheric ripples called gravity waves. But a new study suggests a different culprit: heat released by chemical reactions that occur when solar radiation hits the atmosphere.
To determine what drives MTI formation, Singh and Pallamraju analyzed more than 60,000 spectral images of nightglow—faint luminescence in the mesosphere that occurs when air molecules absorb solar radiation. The images were captured by a near-infrared imaging spectrograph at the optical aeronomy observatory located near the Mount Abu Infrared Observatory maintained by the Physical Research Laboratory in Ahmedabad, India.
Past studies have shown that the intensity ratios of hydroxide and oxygen nightglow bands correspond to temperature in the mesosphere. Here the authors were able to measure the temperature gradient by capturing these two emissions originating at different altitudes: 94 and 87 kilometers. They used the measurements to track MTIs over 745 nights and to measure how often gravity waves occurred on those nights.
Gravity waves occurred during only 7% of MTI events, suggesting that they do not play as important a role in MTIs as previously thought, the team reports. The findings suggest that on certain nights, chemical reactions that occur after solar radiation hits atmospheric elements (such as oxygen and hydrogen) and releases heat play a larger role in producing MTIs. (Journal of Geophysical Research: Space Physics, https://doi.org/10.1029/2018JA025703, 2018)
—Emily Underwood, Freelance Writer