Background gravity wave activity and its variability as determined from Dynasonde data

Abstract:

The importance of gravity waves for the dynamics of the thermosphere-ionosphere system is a well-known fact. Numerical models of the thermosphere-ionosphere must account for gravity wave effects by using various parameterization schemes with currently unknown levels of accuracy. There are several existing methods allowing for the detection of individual instances of gravity wave manifestations. However, little information exists regarding the background wave spectrum and its variability in the thermosphere. We employ Travelling Ionospheric Disturbances (TID’s) as tracers for gravity wave propagation, using results of Dynasonde data analysis as the starting point in our calculations. Data from the Wallops Island (VA), San Juan (Puerto Rico), Boulder (CO), Tromso and Svalbard (Norway) instruments are used. The interval between the sounding sessions is 2 minutes and the NeXtYZ inversion procedure (a part of the Dynasonde software package) allows us to obtain a vertical resolution for the parameters of ionospheric plasma in real height better than 1 km. The full output of the inversion procedure consists of height profiles of electron density, X (East-West) and Y (North-South) tilts of the constant plasma density contours and Doppler velocities, all of which showing persistent signs of gravity wave induced TID’s. We demonstrate using a few examples that the characteristics of the observed TID’s are in good agreement with the dispersion relation describing gravity waves. The ionospheric tilt measurements allow for determination of the direction of propagation for every wave mode. For each calendar month of 2013, the tilt spectra is obtained as a function of altitude using a Lomb-Scargle Welch implementation. The analysis is applied to all 6 stations proving unprecedented geographical coverage. The result is a complete picture of the altitude, temporal and location variability of gravity wave activity in the accessible thermospheric altitude interval with included differentiation between North-South and East-West propagating waves.