Different Behavior of Density Perturbations Between Dayside and Nightside in the Martian Thermosphere and the Ionosphere Associated With Atmospheric Gravity Waves.

To investigate the excitation mechanism of ionospheric perturbations on Mars by the Neutral Gas and Ion Mass Spectrometer (NGIMS) onboard Mars Atmosphere and Volatile EvolutioN (MAVEN), we categorize ionospheric perturbations into three cases: (a) the ion-neutral coupling cases where ion and neutral perturbations are well coupled, (b) the ion-specific cases where ion perturbations move independently from neutrals, and (c) the coronal mass ejection cases associated with solar wind extreme events. A representative number of cases from total profiles are compared with a numerical model to determine the fraction that can be explained by an atmospheric gravity waves (GW). The neutral perturbations on the dayside at 170-190 km altitudes are in excellent agreement with the GW. Whereas, contrary to previous thoughts, neutral perturbations are not necessarily explained by the GW especially on the nightside at 190-210 km. Ion perturbations on the dayside at 170-190 km also show a good agreement with the GW. The agreement becomes extremely low on the nightside at 190-210 km, reaching the limit of strong ion-neutral coupling around 190 km. Further investigation found that the behavior of the ion perturbations explicitly depends on the dayside and nightside. Its dominant driver potentially differs clearly between dayside and nightside. Statistics of relative perturbations demonstrate a clear effect associated with species scale height in neutrals. Whereas, the correlation between ions and neutrals breaks down at high solar zenith angle near southern dusk. We see currently unexplained behavior that cannot be fully interpreted by GW both at night and near southern dusk.