Deriving solar X ray irradiance from Yohkoh observations

The soft X ray telescope (SXT) on Yohkoh produces calibrated images which are used to derive X ray irradiance and coronal temperature. In this paper we compute the mean temperature and spectral irradiance for 25 coronal differential emission measure (DEM) models ranging from very quiet solar conditions to the main phase of large flares. These results are compared to what would have been derived from an SXT observation of the solar conditions represented by the DEM models. After applying an empirical correction algorithm, derived as part of this work, 24 of 25 SXT filter-ratio temperatures fall within 0.1 in log10(T) of the mean DEM temperatures. The irradiance error depends both on the chosen spectral band and the spectrum, here represented by a single plasma temperature. As expected, the irradiance is more accurate and less sensitive to spectrum for bands that match the band pass of the sensor. Even a crude color temperature approximation to the coronal X ray spectrum greatly improves the accuracy of derived radiance over values derived for an assumed temperature of 3 x 10(6) K: the mean temperature of all DEM. An RMS improvement of a factor of 12-17 was found for the cases studied. For two bands, which match the sensor band pass, the RMS accuracy is better than 10%. Yohkoh X ray irradiance data, converted with SXT color temperatures, will be placed in the National Geophysical Data Center for use by the solar and atmospheric physics communities.