Gravity waves in the thermosphere: Solar activity dependence

A statistical study of the thermospheric gravity waves has been carried out using multiwavelength daytime oxygen airglow emission intensity and equatorial electrojet (EEJ) strength data, which originate from four different altitude regions of the thermosphere. The thermospheric daytime oxygen airglow emission intensities at wavelengths 557.7, 630.0, and 777.4 nm, obtained during the January to March period in the three years 2011-2013, have been used. The percentage number of days in which waves with spectral periods in the gravity wave range have occurred are found to be greater for the relatively higher solar activity duration (in 2013) compared to that of low solar activity (in 2011). This observation is explained to be due to the altering background atmospheric density and temperature (that vary with solar activity), which, in turn, influences the propagation and dissipation of waves. Moreover, the higher frequency gravity waves (of periods Brunt-Väisälä to 30 min) have been found to be present in greater numbers in the thermosphere compared to that of low-and-moderate frequency gravity waves (of periods 30-60 min). This behavior in the frequency selection by ambient conditions at thermospheric altitude is in accordance with earlier theoretical and simulation works. The ratios of high- to low-frequency occurrences have also been found to be greater in higher solar activity period of 2013 compared to that of the relatively low solar activity period of 2011. These results thus provide experimental evidence to the earlier simulation works suggesting similar behavior, as found here, for thermospheric gravity waves.