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Session 2 - Everything Else.
Display session, Friday, June 27
Ballroom C, Chair: Richard Canfield

[2.74] Conversion of YOHKOH X-ray Observations to Spectral Radiance

L. Acton, D. Weston (MSU), M. Bruner (LMSAL)

The soft x-ray telescope on YOHKOH has returned some 50-100 full-disk images of the sun per day since October 1991. This data set is useful as a source of information on soft x-ray radiance from the sun and irradiance of the atmosphere of the earth. The purpose of this paper is to examine the errors inherent in deriving absolute spectral radiance from the YOHKOH observations.

Precise conversion of broad band x-ray measurements to spectral radiance requires a knowledge of the spectrum of the radiation. For solar measurements the spectrum is usually known only in approximation and theoretical spectral models are used to compute spectral radiance. A further problem is our poor knowledge of the ever-changing distribution of temperature and emission measure of the solar corona. Only if the distribution of emission measure with temperature (the Differential Emission measure or DEM) is known can spectral models be computed which reflect the true solar x-ray spectrum with good accuracy.

We have attempted to estimate the uncertainty in x-ray spectral radiance derived from YOHKOH data by comparing the ``true'' and measured radiance values for solar DEM distributions from the literature. Apart from possible systematic error the YOHKOH-derived radiances are quite good (15% accuracy) for the primary spectral passband of the telescope. Extrapolating to spectral bands at longer wavelengths of interest to aeronomy gives increasing error.

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