超热木星大气中的亚临界发电机作用解释了西向热斑偏移
Westward hotspot offset explained by subcritical dynamo action in an ultra-hot Jupiter atmosphere
AI总结：
本文首次对约2400 K的超热木星大气进行磁流体动力学模拟，发现亚临界发电机过程触发层流平均流破裂并导致零或西向热斑偏移，为解释观测到的热斑偏移多样性提供了新机制。
摘要：
热木星是潮汐锁定的、靠近其宿主恒星的木星大小行星，平衡温度高于约1000 K。光度观测发现，热斑（大气中最热的位置）相对于星下点发生了偏移。一些观测显示东向偏移，另一些显示西向偏移，而流体动力学模拟由于特征性东向平均流的平流作用显示出东向偏移。特别是对于平衡温度高于2000 K的超热木星，必须考虑电磁效应，因为电离驱动的显著电导率以及随后的磁场感应很可能产生可观的洛伦兹力。我们在此提供了首个超热木星大气的磁流体动力学数值模拟，平衡温度约为2400 K，完全捕捉了非线性电磁感应效应。我们发现了一种新的湍流流态，这在热木星中迄今未知。其主要特征是众所周知的层流平均流的破裂。这种破裂由强局部磁场触发。这些磁场由亚临界发电机过程维持。它由一个假定的深层发电机区域提供的足够强的背景场启动，其振幅约为2.5 G，处于现实水平。我们的结果显示，在发电机情形下，热斑偏移为零或西向，具体取决于大气性质，而流体动力学情形则具有通常的东向偏移。由于我们的模拟在赤道处具有东向平均流，径向流对于产生零或西向热斑偏移必定很重要。亚临界发电机为解释观测到的热斑偏移多样性提供了一种新情景。在该情景中，发电机仅在过去的某个时间由足够强的场启动，且只针对部分行星群体。
英文摘要：
Hot Jupiters are tidally-locked Jupiter-sized planets close to their host star. They have equilibrium temperatures above about 1000 K. Photometric observations find that the hotspot, the hottest location in the atmosphere, is shifted with respect to the substellar point. Some observations show eastward and some show westward hotspot offsets, while hydrodynamic simulations show an eastward offset due to advection by the characteristic eastward mean flow. In particular for ultra-hot Jupiters with equilibrium temperatures above 2000 Kelvin, electromagnetic effects must be considered since the ionization-driven significant electrical conductivity and the subsequent induction of magnetic fields likely result in substantial Lorentz forces. We here provide the first magnetohydrodynamic numerical simulation of an ultra-hot Jupiter atmosphere at an equilibrium temperature of about 2400 K that fully captures non-linear electromagnetic induction effects. We find a new turbulent flow regime, hitherto unknown for hot Jupiters. Its main characteristic is a break-down of the well-known laminar mean flows. This break-down is triggered by strong local magnetic fields. These fields are maintained by a subcritical dynamo process. It is initiated by a sufficiently strong background field from an assumed deep dynamo region at a realistic amplitude around 2.5 G. Our results show a zero or westward hotspot offset for the dynamo case, depending on atmospheric properties, while the hydrodynamic case has the usual eastward offset. Since our simulation has an eastward mean flow at the equator, radial flows must be important for producing the zero or westward hotspot offset. A subcritical dynamo offers a new scenario for explaining the diversity of observed hotspot offsets. In this scenario, the dynamo has been initiated by sufficiently strong fields at some time in the past only for a part of the population.
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