Parker (Astrophys J 174:499, 1972) put forward a hypothesis regarding the fundamental nature of equilibrium magnetic fields in astrophysical plasmas. He proposed that if an equilibrium magnetic field is subjected to an arbitrary, small perturbation, then—under ideal plasma dynamics—the resulting magnetic field will in general not relax towards a smooth equilibrium, but rather, towards a state containing tangential magnetic field discontinuities. Even at astrophysical plasma parameters, as the singular state is approached dissipation must eventually become important, leading to the onset of rapid magnetic reconnection and energy dissipation. This topological dissipation mechanism remains a matter of debate, and is a key ingredient in the nanoflare model for coronal heating. We review the various theoretical and computational approaches that have sought to prove or disprove Parker’s hypothesis. We describe the hypothesis in the context of coronal heating, and discuss different approaches that have been taken to investigating whether braiding of magnetic field lines is responsible for maintaining the observed coronal temperatures. We discuss the many advances that have been made, and highlight outstanding open questions.

中文翻译：

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帕克问题：光滑无力场和日冕加热

Parker（Astrophys J 174：499，1972）提出了一个关于天体等离子体中平衡磁场基本性质的假设。他提出，如果平衡磁场受到任意的小扰动，则在理想的等离子体动力学下，所得的磁场通常不会趋向于平滑的平衡，而是趋向于包含切向磁场不连续的状态。即使在天体等离子体参数下，随着接近奇异状态，耗散最终也必须变得很重要，从而导致快速的磁重新连接和能量耗散。这种拓扑耗散机制尚有争议，并且是纳米火炬的关键成分冠状加热模型。我们回顾了试图证明或反驳帕克假说的各种理论和计算方法。我们在日冕加热的背景下描述了这一假设，并讨论了已采取的不同方法来研究磁场线的编织是否负责维持观测到的日冕温度。我们讨论了已取得的许多进步，并突出了悬而未决的悬而未决的问题。