Faint red auroras as seen from Japan associated with intense magnetospheric compression

We report four low-latitude auroral events in 2024 as observed from Hokkaido, Japan (June 28, August 4, September 12, and November 9). These auroral events occurred during moderately intense magnetic storms, with the peak Dst index of approximately −110 nT, accompanied by significant magnetospheric compression. We estimate the altitudes of these red auroras to be > ~500 km, via widespread citizen science efforts. During the red aurora appearances in Japan, the ASYM-H index increased significantly to around 150 nT, which was approximately 1.3–2.0 times larger than the SYM-H peak amplitude for all events, suggesting that the actual storm intensities were underestimated. We further propose that the very dense solar wind of >~30 /cc is a key for causing the majority of extended red auroras during moderately intense magnetic storms, possibly via the stronger-than-usual enhancement of the atmospheric Joule heat in the subauroral latitude.