一种快速合成Stokes剖面的方法--ZDI辐射转移模拟及Stokes剖面反演
A fast method for Stokes profile synthesis -- Radiative transfer modeling for ZDI and Stokes profile inversion
摘要：
全极化辐射转移驱动的塞曼-多普勒成像方法的主要挑战仍然是巨大的计算需求。在每一个循环的迭代之间的相互作用的前向过程（光谱合成）和逆过程（基于导数的优化）的斯托克斯轮廓合成需要几千个评估的偏振辐射转移方程为一个给定的恒星表面模型。为了应付这些计算需求，并允许纳入一个完整的斯托克斯轮廓合成到多普勒和塞曼多普勒成像应用程序，以及到大规模的太阳斯托克斯轮廓反演，我们提出了一种新的快速和准确的合成方法计算本地斯托克斯轮廓。我们的方法是基于人工神经网络模型，我们用它来近似最重要的大气参数和相应的斯托克斯配置文件之间的复杂的非线性映射。许多专门的人工神经网络，用于模拟模型大气，磁场强度，场倾斜度和场方位角之间的函数关系，一方面和斯托克斯轮廓的各个组成部分（I，Q，U，V），另一方面。我们进行了广泛的统计评估，并表明我们的新方法在广泛的大气条件下产生准确的本地以及磁盘集成斯托克斯配置文件。与精确数值解相比，斯托克斯I和V剖面的平均均方根误差远低于0.2%。斯托克斯Q和U的误差在1%的范围内。我们的方法不仅提供了对LTE极化辐射转移的准确近似，而且还将合成加速了1000倍以上。
英文摘要：
The major challenges for a fully polarized radiative transfer driven approach to Zeeman-Doppler imaging are still the enormous computational requirements. In every cycle of the iterative interplay between the forward process (spectral synthesis) and the inverse process (derivative based optimization) the Stokes profile synthesis requires several thousand evaluations of the polarized radiative transfer equation for a given stellar surface model. To cope with these computational demands and to allow for the incorporation of a full Stokes profile synthesis into Doppler- and Zeeman-Doppler imaging applications as well as into large scale solar Stokes profile inversions, we present a novel fast and accurate synthesis method for calculating local Stokes profiles. Our approach is based on artificial neural network models, which we use to approximate the complex non-linear mapping between the most important atmospheric parameters and the corresponding Stokes profiles. A number of specialized artificial neural networks, are used to model the functional relation between the model atmosphere, magnetic field strength, field inclination, and field azimuth, on one hand and the individual components (I,Q,U,V) of the Stokes profiles, on the other hand. We performed an extensive statistical evaluation and show that our new approach yields accurate local as well as disk-integrated Stokes profiles over a wide range of atmospheric conditions. The mean rms errors for the Stokes I and V profiles are well below 0.2% compared to the exact numerical solution. Errors for Stokes Q and U are in the range of 1%. Our approach does not only offer an accurate approximation to the LTE polarized radiative transfer it, moreover, accelerates the synthesis by a factor of more than 1000.
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