Quiet, discrete auroral arcs are an important and fundamental consequence of solar wind-magnetosphere interaction. We summarize the current standing of observations of such auroral arcs. We review the basic characteristics of the arcs, including occurrence in time and space, lifetimes, width and length, as well as brightness, and the energy of the magnetospheric electrons responsible for the optical emission. We briefly discuss the connection between single and multiple discrete arcs. The acceleration of the magnetospheric electrons by high-altitude electric potential structure is reviewed, together with our current knowledge of these structures. Observations relating to the potential drop, altitude distribution and lifetimes are reviewed, as well as direct evidence for the parallel electric fields of the acceleration structures. The current closure in the ionosphere of the currents carried by the auroral electrons is discussed together with its impact on the ionosphere and thermosphere. The connection of auroral arcs to the magnetosphere and generator regions is briefly touched upon. Finally we discuss how to progress from the current observational status to further our understanding of auroral arcs.

中文翻译：

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安静、离散的极光弧——观测

安静、离散的极光弧是太阳风-磁层相互作用的重要和基本结果。我们总结了目前对此类极光弧的观察结果。我们回顾了弧的基本特征，包括在时间和空间中的出现、寿命、宽度和长度以及亮度以及负责光发射的磁层电子的能量。我们简要讨论单个和多个离散弧之间的联系。回顾了高空电位结构对磁层电子的加速，以及我们目前对这些结构的了解。回顾了与电位降、高度分布和寿命有关的观察结果，以及加速结构平行电场的直接证据。讨论了极光电子携带的电流在电离层中的电流闭合及其对电离层和热层的影响。简要介绍了极光弧与磁层和发电机区域的联系。最后，我们讨论如何从当前的观测状态发展到进一步了解极光弧。