Model Evolutions of Primary Acoustic-Gravity Waves in the Thermosphere: Effects of the Ambient State

Acoustic Gravity Waves (AGWs) are an atmospheric phenomenon responsible for the transport of energy and momentum from the lower atmosphere to the upper atmosphere. Waves are known to grow in amplitude as they propagate upwards in a decreasingly dense atmosphere until they reach the lower thermosphere, where wave energy is dissipated due to increased molecular viscosity in the rarefied gas (Hines, 1960; Heale et al., 2014). Previous studies have shown that mean temperatures, densities, and zonal winds in the thermosphere and exosphere tend to be higher during solar maxima than solar minima, leading to farther propagation of AGWs into the atmosphere (Fritts and Vadas, 2008). Solar maxima have also been linked to brighter airglows in the upper atmosphere (Li et al., 2022), which can also be effected by the change in local pressure and temperature due to AGWs.

We use a cartesian or cylindrical axisymmetric, two-dimensional numerical model to study the effects of varied initial states on the propagation of AGWs into the thermosphere. This model, based on the Model for Acoustic-Gravity wave Interactions and Coupling (e.g., as used by Heale et al., 2014, and references therein) is used in its “MAGIC Forest” form (Snively and Calhoun, AGU FM, 2021). Using NRLMSIS 2.1 (Emmert et al., 2022) directly with MAGIC Forest to create an initialized atmosphere, we observe wave propagation into the thermosphere during low and high solar activity and under varied regional, seasonal, and local time conditions. We then test the influence of winds generated with HWM14 to assess typical effects of variability found in empirical models. The role of the source type and amplitude is also investigated. In each case, we examine the resulting dominant wave speeds, horizontal and vertical wavelengths, propagation depths, and thermal and wind fluctuations in the thermosphere up to the base of the exosphere. We also consider how these ambient conditions affect common observations of acoustic-gravity waves by their effects on atmospheric densities or airglow intensities.