A linearized coupled model of acoustic-gravity waves and the lower ionosphere at Mars

Context . Highly variable ionospheric structures were recently detected on Mars using spacecraft measurements. Acoustic-gravity waves (AGWs) could be the underlying mechanism. Studying the response of the Martian ionosphere to AGWs could provide us with an important understanding of the neutral wave-ionospheric coupling processes. Aims . We developed a linearized wave model to explore the plasma-neutral coupling driven by AGWs in the lower ionosphere of Mars. This model can describe the propagation and dissipation of AGWs in a realistic atmosphere and is the first of its kind to incorporate plasma behaviors associated with photochemistry and electromagnetic fields. Methods . 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 can also be used to investigate the wave-driven variations in the plasma velocity and density, electromagnetic field, and thermal structures. Results . 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 the 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 + , O 2 + , and CO 2 + , have higher density amplitudes when interacting with waves of larger periods. The presence of photochemistry leads to a decrease in the plasma density amplitude, and there exists a moderate correlation between the density variations of plasma and those of 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, values that 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