Investigation on mesospheric gravity waves over Indian low latitude stations using sodium airglow observations and a few case studies based on thermal and wind structures

Systematic measurements using sodium (Na) airglow photometers were carried out from Mt. Abu (24.6 degrees N, 72.7 degrees E) and Gadanki (13.5 degrees N, 79.2 degrees E) during cloudless and moonless nights of winter and spring equinoctial months during 2006-2009. Na airglow intensity fluctuations with periodicities ranging from 5 min to 3 h are analyzed to determine the systematic pattern, if any, in the gravity wave modes that affect Na airglow emission. The power spectra derived from Na airglow intensity variation from Mt. Abu and Gadanki reveal common occurrence of 15-30 min periodicities although spectral powers maximize at longer periodicities over these stations. Further, a few case studies were performed using mesospheric temperature and horizontal wind obtained from the SABER and TIDI onboard TIMED satellite nearly over both places. This investigation provides an experimental indication on the possible relationship between the gravity wave periodicities and the occurrence of neutral instabilities in mesosphere. Based on the case studies, it is shown that whenever the neutral instabilities occur within the Na airglow emission layer, the periodicities of gravity waves tend to become shorter in comparison with the periodicities that are observed when the neutral instabilities occur beyond Na airglow emission altitude or do not occur. (c) 2012 Elsevier Ltd. All rights reserved.