A Linearized Coupled Model of Acoustic-gravity Waves and the Lower Ionosphere at Mars

Highly variable ionospheric structures on Mars have been recently observed via spacecraft measurements. Acoustic-gravity waves (AGWs) could be an underlying mechanism. Studying the response of the Martian ionosphere to AGWs could provide us with an important understanding of the neutral wave-ionospheric coupling process. To explore the plasma-neutral coupling driven by AGWs in the lower ionosphere of Mars, a linearized wave model has been developed. This model can describe the propagation and dissipation of AGWs in a realistic atmosphere and first incorporates plasma behaviors associated with photochemistry and electromagnetic fields. We adopted a full-wave model as the first part of our coupled model to delineate wave propagation in a realistic atmosphere. The second part of our model consists of the governing equations describing the plasma behaviors. Therefore, our model not only replicates the result of the full-wave model but also investigates the wave-driven variations in the plasma velocity and density, electromagnetic field, and thermal structures. Our model results reveal that ions are mainly dragged by neutrals and oscillate along the wave phase line below ~200 km altitude. Electrons are primarily subject to gyro-motion along magnetic field lines. The wave-driven distinct motions among charged particles can generate the perturbed electric current and electric field, further contributing to localized magnetic field fluctuations. Major charged constituents, including electrons, O⁺, O₂⁺, and CO₂⁺, have higher density amplitudes when interacting with larger-periodic waves. The presence of photochemistry leads to a decrease in the plasma density amplitude, and there exists a moderate correlation between plasma density variations and those in the neutrals. Our numerical results indicate that the wave-driven variations range from several percent to ~ 80% in the plasma density and from ~ 0.2% to 17% in the magnetic field, which are consistent with the spacecraft observations. Further calculations reveal that the wave-induced plasma-neutral coupling can heat the neutrals yet cool the plasmas. Electrons are cooler than ions in the coupling process. The wave-driven heating by neutral-ion collisions exceeds that by neutral-electron collisions but tends to be lower than the wave dissipative heating and photochemical heating. Our model has potential applications in studying the AGWs-driven variable ionospheric structures and can be used for other planets.