Many phenomena in the Sun’s atmosphere are magnetic in nature and study of the atmospheric magnetic field plays an important part in understanding these phenomena. Tools to study solar magnetic fields include magnetic topology and features such as magnetic null points, separatrix surfaces, and separators. The theory of these has most robustly been developed under magnetic charge topology, where the sources of the magnetic field are taken to be discrete, but observed magnetic fields are continuously distributed, and reconstructions and numerical simulations typically use continuously distributed magnetic boundary conditions. This article investigates the pitfalls in using continuous-source descriptions, particularly when null points on the $z=0$ plane are obscured by the continuous flux distribution through, e.g., the overlap of non-point sources. The idea of null-like points on the boundary is introduced where the parallel requirement on the field $B_{\parallel }=0$ is retained but the requirement on the perpendicular component is relaxed, i.e. $B_{\perp }\ne 0$ . These allow the definition of separatrix-like surfaces which are shown (through use of a squashing factor) to be a class of quasi-separatrix layer, and separator-like lines which retain the x-line structure of separators. Examples are given that demonstrate that the use of null-like points can reinstate topological features that are eliminated in the transition from discrete to continuous sources, and that their inclusion in more involved cases can enhance understanding of the magnetic structure and even change the resulting conclusions. While the examples in this article use the potential approximation, the definition of null-like points is more general and may be employed in other cases such as force-free field extrapolations and MHD simulations.

中文翻译：

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日冕磁场的拓扑结构：使用类零点扩展磁性骨架

太阳大气中的许多现象本质上都是磁性的，对大气磁场的研究在理解这些现象方面起着重要作用。研究太阳磁场的工具包括磁拓扑和特征，例如磁零点、分离面和分离器。这些理论在磁电荷拓扑结构下得到了最稳健的发展，其中磁场源被认为是离散的，但观察到的磁场是连续分布的，重建和数值模拟通常使用连续分布的磁边界条件。本文研究了使用连续源描述的缺陷，特别是当 $z=0$ 平面上的零点被连续通量分布所掩盖时，例如，非点源的重叠。引入了边界上的类空点的想法，其中保留了对 $B_{\parallel }=0$ 场的平行要求，但放宽了对垂直分量的要求，即 $B_{\perp }\ne 0 $ . 这些允许定义（通过使用挤压因子）显示为一类准分离线层的类分离线表面，以及保留分离器 x 线结构的类分离线。给出的例子表明，使用类零点可以恢复在从离散源到连续源的过渡中消除的拓扑特征，并且将它们包含在更多涉及的案例中可以增强对磁结构的理解，甚至改变由此产生的结论. 虽然本文中的示例使用了潜在的近似值，