We investigate how representing active regions with bipolar magnetic regions (BMRs) affects the end-of-cycle polar field predicted by the surface flux transport model. Our study is based on a new database of BMRs derived from the SDO/HMI active region patch data between 2010 and 2020. An automated code is developed for fitting each active region patch with a BMR, matching both the magnetic flux and axial dipole moment of the region and removing repeat observations of the same region. By comparing the predicted evolution of each of the 1090 BMRs with the predicted evolution of their original active region patches, we show that the bipolar approximation leads to a 24% overestimate of the net axial dipole moment, given the same flow parameters. This is caused by neglecting the more complex multipolar and/or asymmetric magnetic structures of many of the real active regions, and may explain why previous flux transport models had to reduce BMR tilt angles to obtain realistic polar fields. Our BMR database and the Python code to extract it are freely available.

中文翻译：

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表面通量传输活动区域的双极近似有多好？

我们研究了用双极磁区 (BMR) 表示活动区如何影响由表面通量传输模型预测的循环结束极场。我们的研究基于一个新的 BMR 数据库，该数据库源自 2010 年至 2020 年间的 SDO/HMI 活动区域贴片数据。开发了一种自动化代码，用于将每个活动区域贴片与 BMR 拟合，同时匹配磁通量和轴向偶极矩区域并删除同一区域的重复观察。通过将 1090 个 BMR 中每个 BMR 的预测演化与其原始活动区域补丁的预测演化进行比较，我们表明，在相同的流动参数下，双极近似导致净轴向偶极矩高估 24%。这是由于忽略了许多实际活动区域的更复杂的多极和/或不对称磁结构造成的，并且可以解释为什么以前的通量传输模型必须减小 BMR 倾斜角才能获得真实的极场。我们的 BMR 数据库和提取它的 Python 代码是免费提供的。