Coronal temperature diagnostics from high-resolution soft X-ray spectra

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The Flat Crystal Spectrometer (FCS) on the Solar Maximum Mission has accumulated over 300 spectral atlas scans of the soft X-ray spectrum (1.4 to 23Å). We present here the results of a study which compares these data with theoretical predictions of the relative intensities of some of the brighter lines to determine which line intensity ratios give the most reliable temperature diagnostics for the coronal plasma. The soft X-ray spectrum contains a vast selection of bright lines formed by a variety of elements and their various ionisation stages. The atomic physics is comparatively well understood, since these transitions originate from ions stripped of most of their electrons (e.g, H-like and He-like ions, which dominate this part of the spectrum). For this reason, soft X-ray lines are often used to characterise the temperature and emission measure of the coronal plasma. The purpose of this study is to investigate how accurately and consistently the temperature of the coronal plasma can be derived from soft X-ray spectra. For derivation of coronal temperatures, the plasma is often assumed to be isothermal, sometimes even for flares! While this is a convenient assumption, a full differential emission measure model is the best way of representing the distribution of temperatures in the plasma. Unfortunately, it is not always possible to get enough data to calculate a differential emission model. Using these data, we investigate under what circumstances the isothermal assumption becomes invalid. However, we find that the accuracy of the atomic data and the measured elemental abundances limit the interpretation of the observations.