Generation of low-latitude red aurora during the initial phase of magnetic storm

Using highly sensitive optical instruments, twenty events of low-latitude auroras have been identified in Japan (35° MLAT) during the high solar-activity period of 1999-2004. Some of these low-latitude auroras appeared less than 2 hours after the onset of a magnetic storm (e.g., April 6, 2000 and October 21, 2001). We investigate energetic ion data obtained by the NOAA satellites during the magnetic storm of October 21, 2001, the sudden commencement of which triggered a large substorm. The low-latitude red aurora was observed at 1800-1930 UT (0300-0430 LT) at ~45° MLAT. The IMAGE FUV auroral images show that the auroral oval expands to ~50° MLAT at this time. Trapped ion flux at energies of 30-80 keV significantly increased just after this storm sudden commencement at mid- and low-latitudes, suggesting acceleration of ring current ions at the initial phase of the storm. The IMAGE HENA data also show sudden enhancement of ring current ion flux at this timing at radial distances of L=2-7 in the nightside sector. The IMAGE EUV images of the plasmasphere indicate that the plasmapause was located around L=2 at this time. These observations suggest a scenario that the overlap between the ring-current particles and plasmaspheric low-energy particles at L=2 causes heating of the plasmaspheric particles to generate the low-latitude red aurora at low latitudes.