时距日震学：测量流涡度的一种新的平均方案
Time-distance helioseismology: A new averaging scheme for measuring flow vorticity
摘要：
时距日震学通过测量太阳表面各点之间的波传播时间，提供有关太阳近表层矢量流的信息。有人提出了具体的旅行时间空间平均数，以区分东西和南北方向的流动，并测量流动的水平发散。然而，还没有专门的测量技术被开发来测量流动涡度。在这里，我们提出了一种新的测量技术，专门用于测量涡度的垂直分量。流体涡度是太阳对流区动力学的基本性质，特别是旋转湍流对流。该方法包括测量波的旅行时间沿太阳表面上的一个封闭的轮廓，以近似的流通沿此轮廓。垂直涡度与顺时针和逆时针传播时间之差有关。我们应用的方法来表征太阳对流的旋涡运动使用日震数据从太阳动力学天文台（SDO/HMI）和迈克尔逊多普勒成像仪板载太阳和日光层天文台（SOHO/MDI）的日震和磁成像仪。远离赤道，垂直涡度和水平辐散之间有明显的相关性。水平外流与北半球的负涡度和南半球的正涡度有关。HMI观测的信号比MDI观测的信号强得多。我们通过与噪声模型的比较来表征信号的空间功率谱。利用这种日震技术可以探测水平波数低于250/RSun的垂直涡度。
英文摘要：
Time-distance helioseismology provides information about vector flows in the near-surface layers of the Sun by measuring wave travel times between points on the solar surface. Specific spatial averages of travel times have been proposed for distinguishing between flows in the east-west and north-south directions and measuring the horizontal divergence of the flows. No specific measurement technique has, however, been developed to measure flow vorticity. Here we propose a new measurement technique tailored to measuring the vertical component of vorticity. Fluid vorticity is a fundamental property of solar convection zone dynamics and of rotating turbulent convection in particular. The method consists of measuring the travel time of waves along a closed contour on the solar surface in order to approximate the circulation of the flow along this contour. Vertical vorticity is related to the difference between clockwise and counter-clockwise travel times. We applied the method to characterize the vortical motions of solar convection using helioseismic data from the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory (SDO/HMI) and from the Michelson Doppler Imager onboard the Solar and Heliospheric Observatory (SOHO/MDI). Away from the equator, a clear correlation between vertical vorticity and horizontal divergence is detected. Horizontal outflows are associated with negative vorticity in the northern hemisphere and positive vorticity in the southern hemisphere. The signal is much stronger for HMI than for MDI observations. We characterize the spatial power spectrum of the signal by comparison with a noise model. Vertical vorticity at horizontal wavenumbers below 250/R_Sun can be probed with this helioseismic technique.
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