Martian Thermosphere Density Variations and Mechanisms from MAVEN Observations
Creators: name: Liu, Jiandong
Publisher: name: Zenodo
Year: 2019
Subjects: Mars thermospheric density
Surface dust storm
Solar zenith angle
Gravity wave
GCM model

description: The thermosphere of Mars begins at the homopause (115-130 km) and reaches the exobase (200 km), which is complex by low-level atmospheric dynamics and external solar EUV/UV radiation. Meanwhile, the weak plasma formed by solar heating/dissociation escapes in different ways due to lack of effective intrinsic magnetic field protection, results in the loss of Mars volatiles. Therefore, accurate determination of the thermospheric density variations and investigation of its structures variations are essential for understanding the mechanism of Mars atmospheric loss, constructing the GCM model and determining the spacecraft orbit. However, the risks are higher and the flight control are more difficult for Mars exploration missions, so the scientific observation is short and the accuracy is low. Therefore, it is impossible to accurately estimate and fully understand the density variation of the Martian thermospheric layer. With the launch of the Mars Atmosphere and Volatiles Evolution Mission (MAVEN), new data are available for long-lasting study of the thermospheric density. The paper uses accelerometer-derived density data from MAVEN and three other earlier missions, as well as the number density of the five main components measured by MAVEN's Neutral Gas and Ion Mass Spectrometer (NGIMS) to study the structure and variation of the Martian thermospheric density. The main results and new phenomena are summarized as follows: 1) Combined with the MAVEN, MGS, MRO and ODY accelerometer-derived densities, the diurnal cycle of Mars thermospheric density was obtained. The seasonal magnitude and phase evolution of the cycles were analyzed, in which the diurnal items were obtained using principal component analysis (PCA). Density anomalies were found during the spring night. The diurnal density is greatly separated, especially under the influence of dust storm impact in autumn and winter. 2) The annual (Ls~0-360) harmonic structure of the Mars thermosphere between 150 km and 190 km was estimated using MAVEN and MGS density data. An improved fitting model is proposed with considering the Infrared Column Dust Optical Depth(IR-CDOD) dust storm activity index together with the Sun-Mars distance and the Solar Zenith Angle (SZA). Thus, the two major sources are found to excite and control the annual harmonic structure of the Mars thermospheric density. 3) The Martian themospheric density altitude-latitude structures in different seasons are estimated by the four orbiters accelerometer-derived densities. The consistency of the seasonal density amplitude and phase evolution of the Martian thermosphere is further verified by the vertical structure. The six principal components of the (relative) vertical structure were extracted by PCA method and compared with the numerical filtering results. It is indicated that the internal atmospheric gravity wave (IAGW) is coupled with the dust storm activity in autumn and winter: The autumnwinter IAGW fluctuations are narrowed due to thermospheric density increase caused by the surface dust storm. As a result, the energy carried by the gravity waves is transmitted to a higher layer or even extended to the exospheric layers, which might have an important influence on the exospheric density and wind field. 4) From the perspective of a global structure, the Martian thermospheric polar warming phenomenon has a seasonal evolution, which the density is higher at the high-latitudes region of the north polar in spring and summer (Ls~0-180 [deg] ) and vice versa in autumn and winter. Therefore, this trans-hemispheric atmospheric circulation leads to a density increase in the overall hemisphere, resulting in a northsouth asymmetric density distribution. The anomalous (higher density) distribution of the winter equatorial density is firstly discovered but could not be recovered by the GCM model. The global distribution of the stationary planetary waves and Kelvin waves are deduced from the original observations by numerical filters. The topographic caused equatorial stationary planetary waves are discovered and explained. 5) The CO2, N2, O, CO and Ar density measured by NGIMS data at both thermospheric and exospheric altitude are further confirmd the seasonal harmonic density cycle. Except that atomic O is obviously controlled by the cos (SZA) parameter (represent solar heating), the other major neutrals are controlled by dust storms and the solar irradiation. The dust effect extends at least to the 240 km altitude. The exosphericextension of the IAGW and dust storm coupling effect was verified at 240 km, demonstrating that the dust storm can be the excitation source of the IAGW. At the same time, these neutrals confirm the winter equatorial density anomalies and the fluctuation distribution in autumn and winter. The thesis presents the Mars thermospheric density variations characteristics using multi-satellite data. The diurnal, anniversary and vertical structure of the layer are found and compared with the GCM model. The results are important to show the difference and improve the GCM model as well as understand the density structure and evolution of the upper Martian layers.