On May 11, 2024, an extreme G5-class geomagnetic storm triggered a spectacular and unusual display of colorful auroras in mid- to low-latitude regions all over the world. In Japan, auroras were seen not only from Hokkaido, the northernmost island, but also from the northern and central parts of Honshu, the main island. The widespread availability of commercial digital cameras among the public led to a flooding of high-resolution auroral images on social media. This study reports on the appearance of a blue aurora during a storm-time substorm on this day and its magnetic field-aligned and longitudinal structures captured by citizen scientists. Observations from two photographers at different locations revealed that the aurora was located at approximately 40 degrees magnetic latitude and magnetic local time (MLT) of 23 h, spanned about 1200 km (1-h MLT) in the longitudinal direction with three separated structures, and ranged in altitude from 400 km to at least 900 km. Simultaneous photometric measurements suggested that this blue aurora likely included emissions at 427.8 nm. While previous studies have noted the existence of emissions at 427.8 nm in low-latitude auroras, this is the first time that a blue low-latitude aurora has been spatially visualized and shown to have longitudinal and field-aligned structures. Additionally, previous studies suggested that blue emissions in low-latitude auroras are produced by energetic neutral atoms from the ring current. However, identifying longitudinal and field-aligned structures in this study may need an alternative explanation.
