Calibrated Solar EUV Spectrum from Serts

Abstract

The Solar EUV Rocket Telescope and Spectrograph (SERTS) provides spatially imaged high-resolution spectra at wavelengths of 235 — 450 Å in first order, including many emission lines formed over the temperature range of 4.7 ≤ log T ≤ 7.5 characteristic of the corona and upper transition region. The instrument utilizes a grazing-incidence Wolter-2 telescope feeding a normal-incidence toroidal grating spectrograph which is quasi-stigmatic throughout its broad bandpass. Photometric calibrations have now been performed for each of the optical components carried on the 1989 and 1991 flights of this experiment. Calculations have also been carried out to determine the residual atmospheric extinction in the EUV at rocket altitudes during each flight, resulting in relative intensity measurements good to ± 25% over the first-order spectral range. An absolute photometric scale accurate to within a factor of 2 was then derived by fitting our solar observations to reported values for the average He II quiet sun flux at 304 Å. In addition, an absolute wavelength scale for the spectrograph was established by fitting postflight laboratory measurements to precisely known EUV standards. Solar wavelengths were then determined to better than 5 mÅ for most lines, often representing the highest accuracy presently available in this spectral range. A catalog is being developed from these calibrated observations that will list the absolute wavelength, intensity, and width for all lines detected in the averaged EUV spectrum of a solar active region. For each identified line, the catalog will also indicate the responsible ion and atomic transition, as well as the temperature of formation.