分区和外推对位磁场连通性的影响
Effects of partitioning and extrapolation on the connectivity of potential magnetic fields
摘要：
日冕磁场的特征可能在于它的磁力线如何将相反光球通量的区域连接起来--它的连通性。连通性可以被量化为一旦识别出这样的区域，连接相对区域对的净通量。一种现有的算法将典型的活动区域划分成范围从几十到几百的单极区域的数量，这取决于算法参数。这项工作探讨了分区的属性如何取决于一些算法参数，以及连接性如何取决于一个特定的活动区域磁图分区的粗糙度。我们发现它们之间的连接数量与区域的数量成比例，即使可能的连接数量与其平方成比例。有几种方法可以产生日冕场，甚至是势场。场可以在导电边界内或在无限半空间上计算。对于连通性的计算，单极区域可以用点源代替，或者精确的磁图可以用作下边界条件。我们的调查显示，来自这些不同领域的连通性只有轻微的差异-不超过15%。最大的区别是在导电壁内的场和半空间内的场之间。随着更精细的划分产生更多的源区，它们的连通性变得更加不同。这也给出了一种定量的方法，确定传导边界必须放置多远，以便不显著影响外推。对于相同的外边界，使用点源而不是精确的磁图在连通性上产生较小的差异：通常为6%，与源区的数量无关。
英文摘要：
Coronal magnetic field may be characterized by how its field lines interconnect regions of opposing photospheric flux -- its connectivity. Connectivity can be quantified as the net flux connecting pairs of opposing regions, once such regions are identified. One existing algorithm will partition a typical active region into a number of unipolar regions ranging from a few dozen to a few hundred, depending on algorithmic parameters. This work explores how the properties of the partitions depend on some algorithmic parameters, and how connectivity depends on the coarseness of partitioning for one particular active region magnetogram. We find the number of connections among them scales with the number of regions even as the number of possible connections scales with its square. There are several methods of generating a coronal field, even a potential field. The field may be computed inside conducting boundaries or over an infinite half-space. For computation of connectivity, the unipolar regions may be replaced by point sources or the exact magnetogram may be used as a lower boundary condition. Our investigation shows that the connectivities from these various fields differ only slightly -- no more than 15%. The greatest difference is between fields within conducting walls and those in the half-space. Their connectivities grow more different as finer partitioning creates more source regions. This also gives a quantitative means of establishing how far away conducting boundaries must be placed in order not to significantly affect the extrapolation. For identical outer boundaries, the use of point sources instead of the exact magnetogram makes a smaller difference in connectivity: typically 6% independent of the number of source regions.
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