Changes in the daytime thermospheric gravity wave propagation characteristics over low-latitudes in response to the variation in solar flux

Gravity waves play an important role in the propagation of energy and momentum from the lower atmospheric regions into the upper atmosphere. Depending on the varying dissipation conditions, these waves generated in the lower atmosphere can propagate higher into the atmosphere. As the neutrals and plasma share the same volume in the upper atmosphere, these waves affect both these densities as they propagate. In our analysis, using the information on the variations in phase offsets at different heights corresponding to isoelectron density contours at several defined transmission frequencies of digisonde, vertical phase propagation speeds, and vertical scale sizes of gravity waves in the daytime thermosphere have been estimated. Investigation of gravity waves by radio measurements, unlike the conventional optical techniques, which are limited to clear sky conditions alone, enable an unhindered study of gravity wave dynamics in all weather conditions. We have found a very good semblance between the variations in derived vertical phase speeds of gravity waves in the thermosphere and Solar F<inf>10.7cm</inf> flux. Further, gravity wave activity, i.e., the number of gravity waves present in the thermosphere in daytime increases with increasing Solar flux. We quantify these inter-relationships and present linear relations of gravity wave activity and their propagation speeds as a function of variation in Solar flux.