Helioseismology challenges models of solar convection
Helioseismology challenges models of solar convection
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日震学挑战太阳对流模型
DOI：
10.1073/pnas.1208875109
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发表时间：
2012
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对流是能量通过太阳最外层 30% 传输的机制 (1)。众所周知，太阳湍流对流很难在整个对流区进行建模，该区域的密度跨越多个数量级。在《美国国家科学院院刊》中，Hanasoge 等人(2) 使用最近的日震观测得出了对太阳内部大尺度对流速度幅度的严格经验约束。他们报告的上限远小于流行的流体动力学数值模拟的预测。从历史上看，我们对太阳内部认识的巨大进步归功于日震学，即对 5 分钟太阳内部振荡的研究 (3)。 20 世纪 80 年代中期，全球模式频率被用来测量 0.71 太阳半径处的太阳对流包层深度，比之前基于低估的不透明度的预期要深。另一项引人注目的成就是推断太阳自转与半径和纬度的函数关系。大部分对流包络线的旋转有差异，赤道处的旋转速度比高纬度地区的旋转速度更快。对流区的底部是一个旋转剪切区，称为速跃层，它现在在太阳发电机理论中发挥着核心作用 (4)。尽管进行了有价值的尝试，但模型都没有自信地预测出上述太阳特征。每当日震学打开一扇了解太阳内部的新窗口时，就有可能出现惊喜。 Hanasoge 等人的工作(2) 也许是自美国国家航空航天局太阳动力学观测站 (SDO) 发射日震和磁成像仪 (HMI)(5) 以来最引人注目的日震学成果。 HMI 测量由近地表对流激发的地震波随机叠加引起的太阳表面运动。 1600 万像素的相机每 45 秒捕获一次全日光多普勒速度图像。哈纳索吉等人。利用这种高分辨率和全空间覆盖的独特组合来开展大规模太阳对流的高精度日震学。
Convection is the mechanism by which energy is transported through the outermost 30% of the sun (1). Solar turbulent convection is notoriously difficult to model across the entire convection zone, where the density spans many orders of magnitude. In PNAS, Hanasoge et al.(2) use recent helioseismic observations to derive stringent empirical constraints on the amplitude of large-scale convective velocities in the solar interior. They report an upper limit that is far smaller than predicted by a popular hydrodynamic numerical simulation. Historically, great advances in our understanding of the solar interior have been due to helioseismology, the study of 5-minute solar internal oscillations (3). In the mid 1980s global-mode frequencies were used to measure the depth of the solar convective envelope at 0.71 solar radius, deeper than previous expectations based on underestimated opacities. Another spectacular achievement was the inference of solar rotation as a function of radius and latitude. The bulk of the convective envelope rotates differentially, faster at the equator than at high latitudes. At the base of the convection zone is a zone of rotational shear, known as the tachocline, which now plays a central role in theories of the solar dynamo (4). Despite valuable attempts, none of the above solar features were confidently predicted by models. Whenever helioseismology opens a new window into the solar interior, surprises are possible. The work of Hanasoge et al.(2) is perhaps the most notable helioseismology result since the launch of the Helioseismic and Magnetic Imager (HMI)(5) on board the National Aeronautics and Space Administration’s Solar Dynamics Observatory (SDO). HMI measures the motions on the solar surface caused by the random superposition of seismic waves excited by near-surface convection. Full-sun Doppler velocity images are captured every 45 seconds by a 16-million-pixel camera. Hanasoge et al. use this unique combination of high resolution and full spatial coverage to carry out highprecision helioseismology of large-scale solar convection.