Fig. 2

Selected photographs of the NLC taken from a Nayoro, b Horokanai, c Monbetsu, d Rikubetsu, and e Otaru in Hokkaido in the early hours of morning on June 21, 2015. It is noted that uncertainty exists in the shooting time of the photograph (e) because the image was not taken for a scientific purpose at that time. The notation “**” in time code represents this uncertainty