| Summary: | 碩士 === 國立臺灣大學 === 天文物理研究所 === 99 === The large majority of exoplanets discovered thus far are gas planets exposed to strong stellar irradiation, and are likely to be tidally locked by their parent stars. Both 3D models and observations of the tidally locked gas giants have suggested the phenomena of equatorial superrotation in the atmosphere; i.e. the equatorial winds blow faster than self rotation of the planets. The phenomena have exhibited in several slow rotating planets such as Venus and Titan. Some mechanisms have been proposed to explain the equatorial superrotation. In our work, we adopt the theory first suggested by Fels and Lindzen (1974), that the equatorial superroatation on Venus resulted from the thermal tides pumping the momentum via vertical transport. Thermal tides are oscillations in temperature, density, pressure, and wind velocity driven by the stellar heating. The vertical structure of thermal tides is basically internal gravity waves with the pattern speed equal to the apparent motion of the parent star. Thermal tides or waves generated in the stellar heating region have the effect of acceleration to the direction opposite to the stellar motion, leading to the maintenance of superrotational winds. A linear analysis is applied to HD209458b to demonstrate the superrotation formation and maintenance. Our model suggests that when the stellar heating is absorbed near the center of the equatorial jet, the superrotation can be explained by thermal tides, which provide the momentum redistribution between different altitudes of the atmosphere.
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