Statistical Analysis of the Horizontal Phase Velocity Distribution of Atmospheric Gravity Waves and Medium‐Scale Traveling Ionospheric Disturbances in Airglow Images Over Darwin (12.4°S, 131.0°E).

Atmospheric gravity waves (AGWs) and medium‐scale traveling ionospheric disturbances (MSTIDs) in the upper atmosphere can be observed in nocturnal airglow images. Spectral analysis of airglow images provides propagation direction and intensity of these waves. However, the spectral analysis of airglow images has not been done yet for stations in the southern hemisphere except for Antarctica. In this study, we calculated the horizontal phase velocity spectra of AGWs in the mesosphere‐lower‐thermosphere (MLT) region near the mesopause and MSTIDs in the F‐region ionosphere from airglow images at wavelengths of 557.7 and 630.0 nm, respectively, by applying the 3‐dimensional Fourier spectral analysis to the airglow images obtained at Darwin (12.4°S, 131.0°E) in Australia for 13.75 years from 2001 to 2007 and from 2011 to 2019. The spectra of AGWs in the MLT region show clear characteristics that the southward power spectral density (PSD) is stronger in summer and weaker in winter. Tropospheric convection was located at north of Darwin in summer and above Darwin in winter, suggesting that the tropospheric convection is a possible source of AGWs in the mesopause region. The spectra of MSTIDs in the F‐region ionosphere show that the dominant northwestward PSD is stronger in winter and during solar quiet periods. These features can be explained if the observed MSTIDs are caused by the ionospheric instabilities. A weak positive correlation was observed between PSDs of AGWs in the mesosphere and MSTIDs. We propose that the MSTIDs propagating other than northwestward may be generated by AGWs. Plain Language Summary: Atmospheric gravity waves (AGWs) and medium‐scale traveling ionospheric disturbances (MSTIDs) in the upper atmosphere affect the atmospheric circulation and radio‐wave transmissions, including satellite positioning. These waves can be observed in nocturnal airglow images. Thus, spectral analysis of airglow images provides propagation direction and intensity of these waves. However, such spectral analysis of airglow images has not been done yet for stations in the southern hemisphere except for Antarctica. In this study, we calculated the horizontal phase velocity spectra of AGWs and MSTIDs from airglow images at wavelengths of 557.7 and 630.0 nm, respectively, by applying the 3‐dimensional Fourier spectral analysis to the airglow images obtained at Darwin in Australia for 13.75 years from 2001 to 2007 and from 2011 to 2019. The southward propagating AGWs are stronger in summer and weaker in winter. We suggest that the tropospheric convection is a possible source of these AGWs. The northwestward MSTIDs are stronger in winter and during solar quiet periods. These features can be explained if the observed dominant northwestward MSTIDs are caused by the ionospheric instabilities, although other directions may be caused by AGWs. Key Points: We obtained the horizontal phase velocity distribution of airglow waves seen in 557.7 and 630.0‐nm images at Darwin over 13.75 yearsTropospheric convection is a possible source of atmospheric gravity waves (AGWs) seen in 557.7‐nm airglow images in the mesopause regionMedium‐scale traveling ionospheric disturbances in 630.0‐nm images are caused not only by the ionospheric instabilities, but also by AGWs [ABSTRACT FROM AUTHOR]