Martian upper atmosphere warming reflects the energy-materials interactions from the lower atmosphere layers. In this paper, we show a new phenomenon that enhances Mars upper atmosphere density in the equatorial region during the winter periods. First, both accelerometer-derived density and NGIMS-measured species from MAVEN show that the winter equatorial region has a secondary warming peak compared with that of the high-latitude polar warming area. Second, the major neutrals (CO2, Ar, CO, N2, and O) indicate that the phenomenon extends at least up to 240 km during both day and night sides. Furthermore, the topographic-related longitudinal structure emerged in the equatorial sector indicates that the variations are more dynamical than we expected. The warming found in this study suggest to be dust-related and influenced by the evolution of the seasonal solar insolation. Both local factors, upward small-scale gravity waves and CO2 IR-thermal effect transfer, may play key roles in shaping the warming structure.

Mars thermosphere is responsive to solar activities. Previous studies have not examined the neutral density responses to the solar flares thoroughly due to lack of observations. In this paper, Martian thermospheric mass density responses to the February and March 2017 C4.1 and B3.3 solar flare-induced Extreme Ultraviolet (EUV)-enhancement are studied using Mars Atmosphere and Volatile EvolutioN (MAVEN) accelerometer (ACC)-derived observations. We focus on analyzing the relationship between the reactions of density and the solar flare indexes. Firstly, the density experiences 2 to 5 times increases during the event, which is equivalent to a temperature increment of 150 to 250 K. Secondly, both nightside and dayside density rise with the EUV-enhancement, which is coupled with the solar flares at all the observed latitudes. Thirdly, the Martian Crustal Magnetic Field (CMF) could alter the thermospheric background thermal structure, which might also have influence on the heating-process caused by the EUV-enhancement. The nightside density inflation during the February event is due to day-night atmospheric transport, while the dayside density expansion is heated by EUV (17-22 nm)-enhancement during the March event directly. The region with an intensive CMF magnitude could restrain the precipitating ions,resulting in cooling of the atmosphere in that area. The thermal structure perturbations caused by these flares-induced EUV-enhancement might trigger considerable augmentation of the Jeans Escape rates of some light volatiles, such as atomic Hydrogen (H) or even short-term hydrodynamic escape in some extreme cases.