Fig. 7

Comparison of a ground-based and b satellite-based observations of auroras on December 7, 1999. The time series of full-time-resolution images were sliced along the onset meridian (23.2 MLT, ± 0.2 h average) and are shown as auroral keograms. The red vertical lines indicate the times of the Akasofu initial brightening (AIB) and the poleward expansion, which were identified in the original ground-based images. The poleward expansion (i.e., auroral breakup) at 2127:50 UT was simultaneously observed in both ASIs and GIs. The AIB was observed at 2124:50 UT in the ASIs but was not evident in the GIs against noise-level fluctuations