Martian Topside Ionosphere Coupling With the Thermosphere Revealed From Ionospheric and Thermospheric Longitudinal Structures Observed by MAVEN

Abstract Martian ionosphere‐thermosphere (I‐T) coupling plays important roles in ionospheric variability. Although it has been examined in some literatures, coupling characteristics and mechanisms at different altitudes and the competition between the thermosphere and crustal magnetic fields for dominating ionospheric variations remain to be fully understood. In this study, these topics were discussed by investigating concurrent ionospheric and thermospheric longitudinal variations using MAVEN's measurements. The thermosphere can dominate ionospheric longitudinal variations directly or indirectly in the altitudinal range up to 300 km under weak crustal field conditions. Neutral density is the decisive factor for longitudinal variations of the topside electron density, whereas neutral scale height is decisive for those of the peak electron density. Longitudinal structures in electron and neutral densities remain in‐phase in the topside ionosphere, while they are anti‐phase below the electron density peak. These characteristics are consistent with in situ photochemical I‐T coupling. The region where the in situ coupling dominates can extend to altitudes above 200 km. Ionospheric longitudinal structures generated in this region can further extend to higher altitudes and amplify with increasing altitudes through upward plasma diffusion. The photochemical I‐T coupling can still dominate ionospheric longitudinal variations below ∼210 km under strong crustal field conditions. Crustal field effect gradually dominates over thermospheric effect as altitude increases. It causes a steady ionospheric structure relative to crustal field distribution, and that structure enhances with increasing altitudes. This study enhances our understanding of Martian I‐T coupling at different altitudes and its role in ionospheric variability.