Ieda Akimasa, Kauristie Kirsti, Nishimura Yukitoshi, Miyashita Yukinaga, Frey Harald U, Juusola Liisa, Whiter Daniel, Nosé Masahito, Fillingim Matthew O, Honary Farideh, Rogers Neil C, Miyoshi Yoshizumi, Miura Tsubasa, Kawashima Takahiro, Machida Shinobu
1Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, Aichi Japan.
2Finnish Meteorological Institute, Helsinki, Finland.
Earth Planets Space. 2018;70(1):73. doi: 10.1186/s40623-018-0843-3. Epub 2018 May 4.
Substorm onset has originally been defined as a longitudinally extended sudden auroral brightening (Akasofu initial brightening: AIB) followed a few minutes later by an auroral poleward expansion in ground-based all-sky images (ASIs). In contrast, such clearly marked two-stage development has not been evident in satellite-based global images (GIs). Instead, substorm onsets have been identified as localized sudden brightenings that expand immediately poleward. To resolve these differences, optical substorm onset signatures in GIs and ASIs are compared in this study for a substorm that occurred on December 7, 1999. For this substorm, the Polar satellite ultraviolet global imager was operated with a fixed-filter (170 nm) mode, enabling a higher time resolution (37 s) than usual to resolve the possible two-stage development. These data were compared with 20-s resolution green-line (557.7 nm) ASIs at Muonio in Finland. The ASIs revealed the AIB at 2124:50 UT and the subsequent poleward expansion at 2127:50 UT, whereas the GIs revealed only an onset brightening that started at 2127:49 UT. Thus, the onset in the GIs was delayed relative to the AIB and in fact agreed with the poleward expansion in the ASIs. The fact that the AIB was not evident in the GIs may be attributed to the limited spatial resolution of GIs for thin auroral arc brightenings. The implications of these results for the definition of substorm onset are discussed herein.
亚暴起始最初被定义为在地面全天空图像（ASI）中呈现纵向扩展的突然极光增亮（赤松初始增亮：AIB），几分钟后随之出现极光向极地方向扩展。相比之下，在基于卫星的全球图像（GI）中，这种明显的两阶段发展并不明显。相反，亚暴起始被识别为立即向极地方向扩展的局部突然增亮。为了解决这些差异，本研究针对1999年12月7日发生的一次亚暴，比较了GI和ASI中的光学亚暴起始特征。对于此次亚暴，极地卫星紫外全球成像仪以固定滤光片（170纳米）模式运行，从而能够获得比平常更高的时间分辨率（37秒），以分辨可能的两阶段发展。这些数据与芬兰穆奥尼奥的20秒分辨率绿线（557.7纳米）ASI进行了比较。ASI显示在协调世界时2124:50出现AIB，随后在协调世界时2127:50出现向极地方向扩展，而GI仅显示在协调世界时2127:49开始的起始增亮。因此，GI中的起始相对于AIB有所延迟，实际上与ASI中的向极地方向扩展一致。GI中未明显出现AIB这一事实可能归因于GI对薄极光弧增亮的空间分辨率有限。本文讨论了这些结果对亚暴起始定义的影响。
