A stable auroral red arc observed over Japan after an interval of very weak solar wind

Abstract

May 10–11, 1999, is known as an interval of very weak solar wind, with an interplanetary plasma density of <1 cm−3. On May 12–13 the density increased to more than 10 cm−3 and a weak magnetic storm occurred. During the early recovery phase of the storm, we observed a stable auroral red (SAR) arc at Rikubetsu, Japan (43.5°N, 143.8°E, magnetic latitude = 34.7°N), at 1500–1740 UT (0000–0240 LT) on May 13. It is unusual to observe a SAR arc at such a low‐latitude station during a weak storm, which had a minimum Dst index of only −49 nT (1500 UT on May 13). The maximum SAR arc intensity at a wavelength of 630 nm reached ∼700 R in the northern sky of Rikubetsu. A weak emission (∼7 R) at 427.8 nm (N2+(1NG)) was also observed. Ground magnetic field data indicate a clear substorm starting at 1522 UT. The plasma density data obtained by the Akebono satellite (2000–8000 km altitude) show a steep plasmapause at ∼40° magnetic latitude during the recovery phase of the storm. The electron temperature data obtained by the Defense Meteorological Satellite Program F14 satellite (800 km altitude) show enhancement of the temperature at subauroral latitudes during the storm. Using these data, we discuss the generation of the SAR arc during the weak storm.