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arXiv 2408.01650astro-ph.EP
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巨行星中磁场对纬向急流影响的数值模拟

Numerical Simulations of Magnetic Effects on Zonal Flows in Giant Planets

Shanshan Xue, Yufeng Lin

AI总结:

本文通过含径向变化电导率的球壳磁流体动力学模拟,研究背景偶极磁场如何抑制或破坏巨行星深层纬向流,并给出与木星、土星观测相符的磁场—风速关系。

摘要:

木星和土星从表面可见交替出现的东西向急流。这些纬向流的起源已争论了数十年。Juno任务的高精度重力测量以及Cassini任务的“盛大终章”揭示,表面观测到的纬向流可能向下延伸数千公里,并在分子氢向金属氢过渡的区域附近停止,这表明磁场对纬向流具有磁制动作用。在本研究中,我们在一个具有径向变化电导率的球壳中开展了一组磁流体动力学模拟,以研究磁场与纬向流之间的相互作用。我们数值模型的一个关键特征是在可压缩旋转对流上施加一个背景偶极磁场。通过改变所施加磁场的强度和对流的剧烈程度,我们研究了磁场如何与对流运动以及由对流驱动的纬向流相互作用。模拟结果表明,磁场倾向于破坏金属氢中的纬向流,并抑制分子包层中的纬向流,同时磁场可能增强径向对流运动。我们通过模拟提取了磁场强度与表面纬向流振幅之间的定量关系,该关系与木星和土星观测到的磁场及纬向风速大致吻合。这一发现从新的视角为巨行星中由深对流驱动、并被限制在分子氢层内的纬向风情景提供了支持。

英文摘要:

Jupiter and Saturn exhibit alternating east-west jet streams as seen from surface. The origin of these zonal flows has been debated for decades. The high-precision gravity measurements by Juno mission and the grand finale of Cassini mission have revealed that the zonal flows observed at the surface may extend several thousand kilometres deep and stop around the transition region from molecular to metallic hydrogen, suggesting the magnetic braking effect on zonal flows. In this study, we perform a set of magnetohydrodynamic simulations in a spherical shell with radially variable electrical conductivity to investigate the interaction between magnetic fields and zonal flows. A key feature of our numerical models is that we impose a background dipole magnetic field on the anelastic rotating convection. By varying the strength of the imposed magnetic field and the vigor of convection, we investigate how the magnetic field interacts with the convective motions and the convection-driven zonal flows. Our simulations reveal that the magnetic field tends to destroy zonal flows in the metallic hydrogen and suppress zonal flows in the molecular envelope, while the magnetic field may enhance the radial convective motions. We extract a quantitative relation between the magnetic field strength and the amplitude of zonal flows at the surface through our simulations, which roughly matches the observed magnetic field and zonal wind speed of Jupiter and Saturn. This discovery provides support from a new perspective for the scenario of deep convection-driven zonal winds which are confined to the molecular hydrogen layers in giant planets.

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