Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, Institute of Space Sciences, Shandong University, Weihai, Shandong, 264209, China;
The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723.
Proc Natl Acad Sci U S A. 2020 Jul 14;117(28):16193-16198. doi: 10.1073/pnas.2000614117. Epub 2020 Jun 29.
A distinct class of aurora, called transpolar auroral arc (TPA) (in some cases called "theta" aurora), appears in the extremely high-latitude ionosphere of the Earth when interplanetary magnetic field (IMF) is northward. The formation and evolution of TPA offers clues about processes transferring energy and momentum from the solar wind to the magnetosphere and ionosphere during a northward IMF. However, their formation mechanisms remain poorly understood and controversial. We report a mechanism identified from multiple-instrument observations of unusually bright, multiple TPAs and simulations from a high-resolution three-dimensional (3D) global MagnetoHydroDynamics (MHD) model. The observations and simulations show an excellent agreement and reveal that these multiple TPAs are generated by precipitating energetic magnetospheric electrons within field-aligned current (FAC) sheets. These FAC sheets are generated by multiple-flow shear sheets in both the magnetospheric boundary produced by Kelvin-Helmholtz instability between supersonic solar wind flow and magnetosphere plasma, and the plasma sheet generated by the interactions between the enhanced earthward plasma flows from the distant tail (less than -100 R) and the enhanced tailward flows from the near tail (about -20 R). The study offers insight into the complex solar wind-magnetosphere-ionosphere coupling processes under a northward IMF condition, and it challenges existing paradigms of the dynamics of the Earth's magnetosphere.
一类独特的极光，称为跨极极光弧（TPA）（在某些情况下称为“theta”极光），出现在地球极区高层大气中，当行星际磁场（IMF）为北向时。TPA 的形成和演化提供了有关在北向 IMF 期间，能量和动量从太阳风转移到磁层和电离层的过程的线索。然而，它们的形成机制仍然知之甚少，存在争议。我们从多次仪器观测到的异常明亮的多个 TPA 以及来自高分辨率三维（3D）全球磁流体动力学（MHD）模型的模拟中报告了一种机制。观测和模拟结果非常吻合，表明这些多个 TPA 是由沿磁力线电流（FAC）片内沉降的高能磁层电子产生的。这些 FAC 片是由超音速太阳风流和磁层等离子体之间的开尔文-亥姆霍兹不稳定性在磁层边界产生的多流剪切片，以及来自远尾（小于-100R）的增强的向地等离子流和来自近尾（约-20R）的增强的向后等离子流之间相互作用产生的等离子片。该研究深入了解了北向 IMF 条件下复杂的太阳风-磁层-电离层耦合过程，并对地球磁层动力学的现有范式提出了挑战。
