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- Title
Dawn‐Dusk Asymmetries in the Martian Upper Atmosphere.
- Authors
Gupta, Neha; Venkateswara Rao, N.; Kadhane, Umesh R.
- Abstract
In this paper, we report the first comprehensive observations of local time asymmetries in densities and scale heights (temperatures) of the Martian upper atmosphere (between 150 and 300 km) measured by the Mars Atmosphere and Volatile Evolution mission/Neutral Gas and Ion Mass Spectrometer. For this purpose, we use the densities and temperatures of Ar. In general, the daytime densities and temperatures are greater than the nighttime values. The maximum and minimum values, however, are observed at the dusk and dawn terminators, respectively. An enhancement at the dusk terminator is persistently observed at all altitudes; however, the time of the peak enhancement shifts toward sunlit hours with increasing altitude. At the dawn terminator, a minimum in density is observed at altitudes of 150–170 km. At higher altitudes, the minimum is observed close to midnight. Accordingly, the dawn‐dusk asymmetry is more prominent at 150–170 km and decreases with increasing altitude. A maximum ratio of six is observed between the dusk and dawn densities at 160 km. In addition, the local time for the maximum ratio at each altitude moves toward sunlit hours with increasing altitude. The observed asymmetry is explained in terms of dynamical heating and cooling due to convergent and divergent winds at the dusk and dawn terminators, respectively. In addition, upward propagating gravity waves generated by the solar terminator wave and O/CO2 radiative cooling are also proposed as important mechanisms contributing to the observed asymmetry. Plain Language Summary: The densities and temperatures of neutral species in the Martian upper atmosphere are expected to reach a maximum at noon and a minimum at midnight, with nearly equal values in the morning and evening hours. In the present study, we demonstrate that the densities and temperatures instead reach a maximum in the evening and a minimum in the morning hours. This difference in neutral densities and temperatures between the morning (dawn) and evening (dusk) is referred to as dawn‐dusk asymmetry. This asymmetry is more apparent at 150–170 km than at higher altitudes. The asymmetry is observed at all latitudes and for all Martian seasons. To explain such a consistent feature, one requires a constant source that can produce the appropriate asymmetry. The observed asymmetry can be attributed to warming on the evening side and cooling on the morning side, caused by atmospheric circulation. In addition, upward propagating wave disturbances generated in the lower atmosphere at the day‐night boundaries may also contribute to the observed asymmetry. Finally, cooling due to radiation released by CO2 molecules colliding with atomic oxygen may also play an important role in the observed asymmetry. Key Points: Local time asymmetries in the Martian upper atmosphere are studied using NGIMS/MAVEN neutral densities, scale heights, and temperaturesThe densities, scale heights, and temperatures of the major species are higher at the dusk terminator than at the dawn terminatorDynamical heating and cooling due to thermospheric winds, terminator wave, and O/CO2 radiative cooling are proposed as plausible mechanisms
- Subjects
UPPER atmosphere; MARTIAN exploration; MARS Atmosphere &; Volatile Evolution (Artificial satellite); MASS spectrometers; SPACE sciences
- Publication
Journal of Geophysical Research. Planets, 2019, Vol 124, Issue 12, p3219
- ISSN
2169-9097
- Publication type
Article
- DOI
10.1029/2019JE006151