| 系外行星逃逸大气的多流 (磁) 流体模拟 |
| 其他题名 | Simulations of Exoplanet Escape Atmospheres using Multifluid (Magnetic) Fluid Dynamics
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| 邢磊
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| 学位类型 | 博士
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| 导师 | 李焱
; 郭建恒
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| 2024-07-01
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| 学位授予单位 | 中国科学院大学
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| 学位授予地点 | 北京
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| 学位专业 | 天体物理
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| 关键词 | 系外行星上层大气模拟
流体动力学
磁流体动力学
辐射转移过程
计算流体程序开发
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| 摘要 | 行星大气的研究在探索行星演化,生命起源,行星宜居性等关键问题中扮演着至关重要的角色。对太阳系内的行星而言,其大气对它们的温度、气候和表面环境着关键性影响。地球所拥有的优越大气环境甚至直接促成了高等文明的兴起。随着对系外行星的研究和探索不断深入,我们开始认识到地球在宇宙中可能并不孤独。除了太阳系内的行星,通过对系外行星大气的透射光谱的观测,发现许多热木星和热海王星也拥有着大气层。和太阳系行星大气不同的是,这种大气往往处于极端膨胀的状态,延伸到数个甚至数十个行星半径以外。这种膨胀大气很可能是由于恒星 X 射线和极紫外辐射(XUV)驱动行星大气发生流体动力学逃逸所形成的。在逃逸大气中除了氢(H)以外往往还可能观测到氦(He)、碳(C)、氧(O)、镁(Mg)、硅(Si)等元素的信号。然而,由于行星引力对大气逃逸的抑制效果,较重的粒子往往更难逃逸。考虑到大气中组分可能发生解耦,发现 H+ 和 O 可能在磁场的影响下发生较明显的解耦,速度解耦最多可能会达到50%,且解耦区域与磁场有明显的相关性。说明逃逸大气中重离子的解耦很可能会受到磁场的影响,这值得进一步研究。然而,我们的程序目前仍处于较初步的阶段,需要进一步的开发和改进。同时,我们也希望这个程序能在天文等离子体模拟的领域上发挥更大的作用。要模拟多组分大气的真实逃逸情况往往离不开多流体的数值模拟程序。 本文选择 PLUTO 作为多流体程序开发的基础。在将程序分解,重新设计,添加多流体功能后,目前已经开发出了初步的通用多流磁流体模拟程序 PLUTO-XMF。这个程序继承了 PLUTO 的优良特性,支持流体/磁流体的多流体模拟,在流体功能中支持电子压强的计算(基于电中性假设)。在磁流体模拟上,目前实现了基本的 MHD 方程求解功能以及八波控制 ∇ ⋅ B = 0 的方法。 目前,对于系外行星逃逸大气中氦异常这一现象还没有一个很好的解释,猜测原因可能是逃逸大气中氦发生了解耦。为了研究 HD209458 b 逃逸大气中氢和氦的解耦机制,我们利用了 PLUTO-XMF 多流体模拟工具模拟 HD209458 b 的逃逸大气。通过对这颗热木星大气在 He10830 波段的透射光谱进行拟合发现,在最佳拟合的大气模型中 He/H=0.039,这小于太阳中的 He/H=0.0846。这表明这颗热木星的逃逸大气中氦的确更难逃逸。我们相信这种解耦的过程对于更重粒子逃逸过程的研究可能更加重要。 利用多流体程序的磁流体模块,我们探索了磁场对热海王星 GJ436 b 逃逸大气中 H,H+ 解耦的影响。模拟结果表明,磁场的确对两种粒子的解耦产生了影响,并且更强的磁场会促使这两种粒子的解耦。不过由于 H 和 H+ 的电荷交换过程较强,使得两者的解耦程度相对较小。这也从侧面说明了,在纯氢大气的模拟中,将 H 和 H+ 合并成单流体模拟是合理的。进一步 H+ 和 O 解耦的测试案例却发现 H+ 和 O 可能在磁场的影响下发生较明显的解耦,速度解耦最多可能会达到50%,且解耦区域与磁场有明显的相关性。说明逃逸大气中重离子的解耦很可能会受到磁场的影响, 这值得进一步研究。 本文的主要工作是,针对系外行星大气的精细模拟需求,利用自主开发的多流磁流体模拟程序 PLUTO-XMF,完成了对热木星 HD209458 b 的多流流体模拟以及 GJ436 b 的多流磁流体模拟。前者的工作主要说明了这颗热木星的逃逸大气中氦可能发生解耦,后者的工作主要说明了磁场会影响逃逸大气中 H 和 H+的解耦。本文工作的主要创新点是 1.将单流体程序 PLUTO 开发为多流体程序 PLUTO-XMF 以适应更复杂的大气模拟物理场景 2.创新性的提出了使用差分黎曼求解器求解电子压强的方法,为等离子体的数值模拟提供了新的方法和思路。然而,我们的程序目前仍处于较初步的阶段,需要进一步的开发和改进。同时,我们也希望这个程序能在天文等离子体模拟的领域上发挥更大的作用。 |
| 其他摘要 | The study of planetary atmospheres plays a crucial role in exploring key questions such as planetary evolution, the origin of life, and planetary habitability. The atmosphere of planets within the solar system has a critical influence on their temperature, climate, and surface environment. The superior atmospheric conditions on Earth even directly contributed to the rise of advanced civilizations. As research and exploration of exoplanets continue to advance, we are beginning to realize that Earth may not be alone in the universe. Observations of the transmission spectra of exoplanet atmospheres have revealed that many hot Jupiters and hot Neptunes also possess atmospheres. Unlike the atmospheres of planets within the solar system, these atmospheres often exist in an extremely expanded state, extending several to tens of planetary radii. This expandedatmosphere is likely formed due to fluid dynamic escape driven by stellar X-ray and extreme ultraviolet (XUV) radiation. In addition to hydrogen (H), the escaping atmospheres often exhibit signals of elements such as helium (He), carbon (C), oxygen (O),magnesium (Mg), and silicon (Si). However, due to the inhibitory effect of planetary gravity on atmospheric escape, heavier particles often escape with more difficulty. Considering the possibility of decoupling of components in the atmosphere, simulating the true escape scenarios of multi-component atmospheres often requires the use of multi-fluid numerical simulation programs.In this study, PLUTO was chosen as the foundation for the development of a multi-fluid simulation program. After decomposing the program, redesigning it, and adding multi-fluid functionality, we have developed an initial version of the universal multi-fluid magnetohydrodynamics (MHD) simulation program, PLUTO-XMF. This program inherits the excellent features of PLUTO, supporting multi-fluid simulations of fluid/MHD. In fluid simulations, it supports the calculation of electron pressure (based on the assumption of charge neutrality). In MHD simulations, basic MHD equation-solving capabilities and the eight-wave control method for ∇ ⋅ B = 0 have been implemented.Currently, there is still no satisfactory explanation for the helium anomaly in the escaping atmospheres of exoplanets. It is speculated that this anomaly may be due to helium decoupling in the escaping atmospheres. To study the decoupling mechanism of hydrogen and helium in the escaping atmosphere of HD209458 b, we utilized the PLUTO-XMF multi-fluid simulation tool to simulate the escaping atmosphere of HD209458 b. By fitting the transmission spectra of this hot Jupiter in the He10830 band, we found that in the best-fit atmospheric model, He/H=0.039, which is less than He/H=0.0846 in the Sun. This indicates that helium in the escaping atmosphere of this hot Jupiter is indeed more difficult to escape. We believe that this decoupling process may be more important for the study of the escape process of heavier particles.Using the MHD module of the multi-fluid program, we explored the effect of the magnetic field on the decoupling of H and H+ in the escaping atmosphere of GJ436 b, a hot Neptune. The simulation results show that the magnetic field does indeed affect the decoupling of these two particles, and a stronger magnetic field promotes their decoupling. However, due to the strong charge exchange process between H and H+, the degree of decoupling between them is relatively small. This also indirectly indicates that in the simulation of a pure hydrogen atmosphere, it is reasonable to merge H and H+ into a single fluid simulation. Further test cases of H+ and O decoupling revealed that H+ and O may undergo more significant decoupling under the influence of the magnetic field, with a maximum velocity decoupling of up to 50%, and there is a clear correlation between the decoupling region and the magnetic field. This indicates that the decoupling of heavy ions in the escaping atmosphere may be influenced by the magnetic field, which is worth further investigation.The main work of this paper is to address the demand for fine-scale simulation of exoplanet atmospheres. We utilized the self-developed multi-fluid magnetohydrodynamic simulation program PLUTO-XMF to conduct multi-fluid simulations of the hot Jupiter HD209458 b and the magnetohydrodynamic simulations of GJ436 b. The former work mainly illustrates the possibility of helium decoupling in the escaping atmosphere of this hot Jupiter, while the latter work mainly demonstrates that the magnetic field affects the decoupling of H and H+ in the escaping atmosphere. The main innovations of this work are:1.The development of the single-fluid program PLUTO into the multi-fluid program PLUTO-XMF to adapt to more complex atmospheric simulation physical scenarios.2.The innovative proposal of using a differential Riemann solver to solve the electron pressure, providing new methods and ideas for the numerical simulation of plasmas.However, our program is still in a relatively preliminary stage and requires further development and improvement. At the same time, we also hope that this program can play a greater role in the field of astronomical plasma simulation. |
| 学科领域 | 天文学
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| 学科门类 | 理学
; 理学::天文学
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| 页数 | 0
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| 语种 | 中文
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| 文献类型 | 学位论文
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| 条目标识符 | http://ir.ynao.ac.cn/handle/114a53/28050
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| 专题 | 恒星物理研究组
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| 作者单位 | 中国科学院云南天文台
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| 第一作者单位 | 中国科学院云南天文台
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推荐引用方式
GB/T 7714 |
邢磊. 系外行星逃逸大气的多流 (磁) 流体模拟[D]. 北京. 中国科学院大学,2024.
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