Magnetic storm-time red aurora as seen from Hokkaido, Japan on December 1, 2023 associated with high-density solar wind

We report a citizen science-motivated study on the cause of an unusually bright red aurora as witnessed from Hokkaido, Japan during a magnetic storm on December 1, 2023. Such an intense red aurora event has occurred in the Halloween 2003 super storm, but the Dst index peak of this December 2023 storm was only -107 nT. In spite of the moderate storm amplitude, the extremely high solar wind density of >50 /cc caused the aurora oval extension to 53 MLAT (L=2.8). We discuss that the drift loss of the ring current particles across the small-size magnetopause is important, and Hokkaido was at the right position to see the direct effect of the large particle injection of the storm-time substorm.