Characterization of Gravity Waves in Three Dimensions in the Daytime Thermosphere Using Combined Optical and Radio Measurements and Estimation of Horizontal Neutral Winds.

Gravity waves, which are considered omnipresent in the Earth's upper atmosphere, are generally investigated by monitoring the fluctuations in different atmospheric parameters. Here, we report the propagation characteristics of thermospheric gravity waves both in horizontal and vertical directions obtained using collocated optical and radio measurements from Ahmedabad, India for February 2021. The measurements of OI 630.0 nm dayglow emission rates over zenith are used to derive time periods of gravity waves. Wave number analyses of variations in the emission over a large field‐of‐view have been performed to derive gravity wave scale sizes and propagation characteristics in the horizontal direction. Time periods, horizontal scale sizes, and propagation directions are found to be in the range of 31–125 min, 78–243 km, and 203°–248° from east, respectively. Vertical wavelengths of the gravity waves are obtained from radio measurements and are in the range of 26–247 km. As the gravity wave characteristics are influenced by the ambient neutral winds, the measured gravity wave propagation characteristics in three dimensions have been used as inputs into the gravity wave dispersion relation to estimate the magnitudes of thermospheric horizontal neutral winds. These estimated daytime winds in the direction of wave propagation are found to be in the range of 1–105 ms−1, and they compare well with those measured independently from MIGHTI added HWM14 model‐derived winds. The daytime winds estimated by this approach are possibly the first of their kind as obtained from ground‐based measurements. Plain Language Summary: Atmospheric gravity waves are mostly generated in the lower atmosphere due to different processes, and they carry energy away from their source regions. Therefore, these waves play a crucial role in the coupling and energetics of the atmosphere. As the neutral density decreases exponentially with height, amplitudes of these waves grow in order to conserve energy. At high altitudes, when the amplitudes become very large, these waves break due to non‐linear wave‐wave interactions and deposit their energy. Such energy deposition can further give rise to secondary and tertiary gravity waves. In favorable background conditions, these waves can propagate deep into the upper atmosphere and alter the prevailing dynamics. Therefore, the characterization of their spatial and temporal scales in the upper atmosphere is extremely important for a comprehensive understanding of atmospheric dynamics. As neutrals and plasma share the same space in the upper atmosphere, we have used simultaneous observations of both the neutrals and plasma, using ground‐based optical and radio measurements, to derive the propagation characteristics of the atmospheric gravity waves in all three dimensions. Further, we have used these parameters to estimate the magnitudes of daytime thermospheric horizontal winds, which are otherwise difficult to obtain from ground‐based platforms in daytime. Key Points: Three‐dimensional characterization of daytime thermospheric gravity waves has been made using collocated optical and radio measurementsEstimated vertical wavelengths of gravity waves from their horizontal characteristics reasonably match with those measured from digisondeThermospheric neutral winds estimated using measured 3‐D gravity wave information show a good match with those measured by MIGHTI [ABSTRACT FROM AUTHOR]