YOHKOH (SOLAR-A) OBSERVATIONS OF SOLAR-ACTIVITY

The Yohkoh satellite, launched 31 August 1991, observes solar X-rays a nd gamma-rays with four instruments. This paper briefly introduces Yoh koh, emphasizing its recent observations of the solar corona as seen i n soft X-radiation. The Yohkoh soft X-ray telescope (SXT) has a CCD se nsor and a grazing-incidence mirror with greatly reduced scattering in comparison with earlier telescopes of this type. With these and other advantages it provides the most sensitive and detailed views of the X -ray emission corona. Long exposures regularly show detectable emissio n from heights exceeding one solar radius above the photosphere. The m ain observational objectives of SXT are in Rare research, but observat ions of other coronal structures are equally significant. We anticipat e important applications of the Yohkoh/SXT data in defining physical p arameters of the corona during the Ulysses and SOHO observations of th e solar wind and corona. This instrument was recently used to make uni que X-ray observations of a transit of Mercury.