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

May 10-11, 1999, is known as an interval of very weak solar wind, with an i nterplanetary plasma density of <1 cm(-3). On May 12-13 the density increas ed to more than 10 cm-3 and a weak magnetic storm occurred. During the earl y recovery phase of the storm, we observed a stable auroral red (SAR) are a t Rikubetsu, Japan (43.5 degreesN, 143.8 degreesE, magnetic latitude = 34.7 degreesN), at 1500-1740 UT (0000-0240 LT) on May 13. It is unusual to obse rve a SAP, are at such a low-latitude station during a weak storm, which ha d a minimum Dst index of only -49 nT (1500 UT on May 13). The maximum SAR a rc intensity at a wavelength of 630 mn reached similar to 700 R in the nort hern sky of Rikubetsu. A weak emission (similar to7 R) at 427.8 nm (N-2(+)( 1NG)) was also observed. Ground magnetic field data indicate a clear substo rm starting at 1522 UT. The plasma density data obtained by the Akebono sat ellite (2000-8000 km altitude) show a steep plasmapause at similar to 40 de grees magnetic latitude during the recovery phase of the storm. The electro n temperature data obtained by the Defense Meteorological Satellite Program F14 satellite (800 km altitude) show enhancement of the temperature at sub auroral latitudes during the storm. Using these data, we discuss the genera tion of the SAR are during the weak storm.