Chang Tzu-Fang, Cheng Chio-Zong
Institute of Space and Plasma Sciences, National Cheng Kung University, Tainan, Taiwan ; Department of Physics, National Cheng Kung University, Tainan, Taiwan ; Plasma and Space Science Center, National Cheng Kung University, Tainan, Taiwan.
Institute of Space and Plasma Sciences, National Cheng Kung University, Tainan, Taiwan ; Plasma and Space Science Center, National Cheng Kung University, Tainan, Taiwan ; Department of Advanced Energy, University of Tokyo, Tokyo, Japan.
Earth Planets Space. 2015;67(1):168. doi: 10.1186/s40623-015-0334-8. Epub 2015 Oct 13.
Wave-like substorm arc features in the aurora and Pi2 magnetic disturbances observed in the near-Earth plasma sheet are frequently, and sometimes simultaneously, observed around the substorm onset time. We perform statistical analyses of the THEMIS ASI auroral observations that show wave-like bright spot structure along the arc prior to substorm onset. The azimuthal mode number values of the wave-like substorm arcs are found to be in the range of ~100-240 and decrease with increasing geomagnetic latitude of the substorm auroral arc location. We suggest that the azimuthal mode number is likely related to the ion gyroradius and azimuthal wave number. We also perform correlation study of the pre-onset wave-like substorm arc features and Pi2 magnetic disturbances for substorm dipolarization events observed by THEMIS satellites during 2008-2009. The wave-like arc brightness structures on the substorm auroral arcs tend to move azimuthally westward, but with a few exceptions of eastward movement, during tens of seconds prior to the substorm onset. The movement of the wave-like arc brightness structure is linearly correlated with the phase velocity of the Pi2 δ disturbances in the near-Earth plasma sheet region. The result suggests that the Pi2 transverse δ disturbances are related to the intensifying wave-like substorm onset arcs. One plausible explanation of the observations is the kinetic ballooning instability, which has high azimuthal mode number due to the ion gyroradius effect and finite parallel electric field that accelerates electrons into the ionosphere to produce the wave-like arc structure.
在近地等离子体片中观测到的极光中的波状亚暴弧特征以及Pi2磁扰动，经常且有时同时在亚暴开始时间前后被观测到。我们对THEMIS ASI极光观测数据进行了统计分析，这些观测显示在亚暴开始之前沿着弧存在波状亮点结构。发现波状亚暴弧的方位模数取值范围在~100 - 240之间，并且随着亚暴极光弧位置的地磁纬度增加而减小。我们认为方位模数可能与离子回旋半径和方位波数有关。我们还对2008 - 2009年期间THEMIS卫星观测到的亚暴偶极化事件的亚暴开始前波状亚暴弧特征与Pi2磁扰动进行了相关性研究。在亚暴开始前的几十秒内，亚暴极光弧上的波状弧亮度结构倾向于向西方位移动，但有少数向东移动的例外情况。波状弧亮度结构的移动与近地等离子体片区域中Pi2δ扰动的相速度呈线性相关。结果表明Pi2横向δ扰动与增强的波状亚暴开始弧有关。对这些观测结果的一个合理的解释是动力学气球模不稳定性，由于离子回旋半径效应和有限的平行电场，该不稳定性具有高方位模数，平行电场将电子加速到电离层以产生波状弧结构。
