High-Temperature Plasmas in Active Regions Observed with the Soft X-Ray Telescope aboard Yohkoh

High-temperature plasmas reaching 5–6 × 106 K in solar active regions have been found with the soft X-ray telescope aboard Yohkoh. NOAA region 6919 was investigated in detail using five different X-ray filters: The temperature of a bright loop in the active region is 5.7 × 106 K, with an emission measure of 5.0 × 1028 cm−5; in a fainter part of the region plasma, we find 5.0 × 106 K and 4.0 × 1027 cm−5. This indicates that such high-temperature plasmas exist in the active region, irrespective of the brightness. Another observation of the quiet corona was conducted in order to investigate the reliability of a temperature analysis with the same filter pairs which show such high temperatures in active regions: The inferred temperature was 2.7 × 106 K, and the emission measure 1.3 × 1026 cm−5, which is consistent with the typical results of Skylab. Therefore, the high-temperature plasmas in solar active regions are considered to be real.