Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, RH5 6NT, UK.
Department of Meteorology, University of Reading, Reading, RG6 6BB, UK.
Nat Commun. 2018 Nov 15;9(1):4806. doi: 10.1038/s41467-018-07086-0.
During geomagnetic substorms, stored magnetic and plasma thermal energies are explosively converted into plasma kinetic energy. This rapid reconfiguration of Earth's nightside magnetosphere is manifest in the ionosphere as an auroral display that fills the sky. Progress in understanding of how substorms are initiated is hindered by a lack of quantitative analysis of the single consistent feature of onset; the rapid brightening and structuring of the most equatorward arc in the ionosphere. Here, we exploit state-of-the-art auroral measurements to construct an observational dispersion relation of waves during substorm onset. Further, we use kinetic theory of high-beta plasma to demonstrate that the shear Alfven wave dispersion relation bears remarkable similarity to the auroral dispersion relation. In contrast to prevailing theories of substorm initiation, we demonstrate that auroral beads seen during the majority of substorm onsets are likely the signature of kinetic Alfven waves driven unstable in the high-beta magnetotail.
在磁暴期间，储存的磁能和等离子体热能会被爆炸式地转化为等离子体动能。地球夜侧磁层的这种快速重新配置在电离层中表现为极光显示，充满了天空。由于缺乏对磁暴起始的单一一致特征（即电离层中最接近赤道的弧形的快速增亮和结构化）的定量分析，对磁暴的理解进展受到阻碍。在这里，我们利用最先进的极光测量来构建磁暴起始期间波的观测色散关系。此外，我们使用高-β等离子体的动理论来证明切向阿尔文波的色散关系与极光的色散关系具有显著的相似性。与流行的磁暴起始理论相比，我们证明了在大多数磁暴起始期间看到的极光珠很可能是在高-β磁尾中不稳定驱动的动力阿尔文波的特征。
