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.

Parker (Astrophys J 174:499, 1972) 提出了关于天体物理等离子体中平衡磁场的基本性质的假设。他提出，如果平衡磁场受到任意的小扰动，那么在理想等离子体动力学下，产生的磁场通常不会松弛到平滑平衡，而是松弛到包含切向磁场不连续的状态。即使在天体物理等离子体参数下，随着接近奇异态，耗散最终也必须变得重要，导致快速磁重联和能量耗散的开始。这种拓扑耗散机制仍然是一个有争议的问题，并且是日冕加热纳米耀斑模型的关键要素。我们回顾了试图证明或反驳帕克假设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，并讨论了研究磁场线编织是否负责维持观测到的日冕温度所采取的不同方法。我们讨论了已经取得的许多进展，并强调了悬而未决的问题。