从磁图看地表流动
Surface Flows From Magnetograms
摘要：
从光球和/或色球矢量磁图的时间序列中估计速度可以用来确定磁图层的磁能通量（坡印廷通量）和螺旋度，并为该层上方的太阳大气的数据驱动模拟提供与时间相关的边界条件。垂直于磁场的速度分量对于计算这些输运率和导出模型边界条件都是必要的。在这里，我们讨论了一些可能的方法来估计垂直流动的磁图。由于多普勒频移包含平行于磁场的流动的贡献，因此垂直速度通常无法单独从多普勒频移中恢复。感应方程的垂直分量将$B_z$中的演化与垂直流场联系起来，但具有有限的零空间，这意味着一些"零"流，例如，沿正常视野轮廓的运动不影响$B_z$。因此，需要额外的信息来准确地指定垂直流场。分析邻域中的$\partial_t B_z$的跟踪方法具有悠久的传统，但是最近开发了其他方法。在最近的一篇论文中，使用MHD模拟的合成磁图测试了几种此类技术。在这里，我们使用相同的测试数据来表征：1）感应方程的法向分量本身估计流量的能力;以及2）跟踪方法恢复垂直于$\mathbf{B}$和平行于$B_z$的轮廓的流量分量的能力。这项工作得到了NASA太阳物理理论基金NNG 05G144G的支持。
英文摘要：
Estimates of velocities from time series of photospheric and/or chromospheric vector magnetograms can be used to determine fluxes of magnetic energy (the Poynting flux) and helicity across the magnetogram layer, and to provide time-dependent boundary conditions for data-driven simulations of the solar atmosphere above this layer. Velocity components perpendicular to the magnetic field are necessary both to compute these transport rates and to derive model boundary conditions. Here, we discuss some possible approaches to estimating perpendicular flows from magnetograms. Since Doppler shifts contain contributions from flows parallel to the magnetic field, perpendicular velocities are not generally recoverable from Doppler shifts alone. The induction equation's vertical component relates evolution in $B_z$ to the perpendicular flow field, but has a finite null space, meaning some ``null'' flows, e.g., motions along contours of normal field, do not affect $B_z$. Consequently, additional information is required to accurately specify the perpendicular flow field. Tracking methods, which analyze $\partial_t B_z$ in a neighborhood, have a long heritage, but other approaches have recently been developed. In a recent paper, several such techniques were tested using synthetic magnetograms from MHD simulations. Here, we use the same test data to characterize: 1) the ability of the induction equation's normal component, by itself, to estimate flows; and 2) a tracking method's ability to recover flow components that are perpendicular to $\mathbf{B}$ and parallel to contours of $B_z$. This work has been supported by NASA Heliophysics Theory grant NNG05G144G.
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