熱木星上熱潮汐和赤道環流的交互作用

Abstract

迄今已發現的系外行星中，大多數是暴露在強烈的恆星輻射下的巨大氣體行星，而且有可能已被母恆星潮汐鎖定。這些系外行星的3D大氣模擬，和部分觀測結果顯示其赤道區域的大氣有超自轉風的現象，即赤道區域的環流移動的比行星自轉還要快。此現象在太陽系內幾個自轉緩慢的行星，例如金星和泰坦(土衛六)上被觀察到。人們嘗試提出不同的物理機制來解釋赤道區域的超自轉風。我們採用首先由 Fels 和 Lindzen（1974）所提的假設，在金星上的赤道超自轉風的來源是熱潮汐引起的垂直動量傳送。熱潮汐是恆星加熱大氣所驅動的大氣中溫度，密度，壓力，和風速的週期性振盪，熱潮汐的垂直結構基本上就是週期對應於母恆星的大尺度內部重力波。在行星大氣中，在被加熱的區域(波的來源區)所產生的熱潮汐和重力波有往和母恆星視運動相反方向加速的效果，造成超自轉風。我們應用HD 209458b的參數進行線性分析，探討超自轉風的形成和維持。研究的結果指出，當被加熱的區域在環流的中心(垂直方向)附近時，熱潮汐可以解釋超自轉風的動量來源，並且在垂直方向上重新分布大氣中的水平動量。

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.