2.2   SXT vs. CDS

Two high-resolution SXT images were taken after the flare, in the AlMg and Be119 filters. The images were taken around 18:40 UT. One of the CDS rasters was taken at about the same time, with the central exposure taken at 18:34 UT (the slit was moved from West to East in about 25 minutes).

This is the first time that a spectrum of a flare line is observed simultaneously with the SXT images. Note the similarity between the Fe XIX emission (log T=6.9) and the SXT images.

At first, one would be tempted to say that all SXT emission must then have come from lines emitted at similar temperatures as Fe XIX. In order to verify this, and the CDS-SXT cross-calibration, a central area (Solar X=250, Solar Y=300) in the CDS raster was selected, and averaged line intensities extracted. An emission measure was then calculated and synthetic X-ray spectra were constructed using CHIANTI v.3 (Dere et al., 2001), and folded with the SXT effective areas. Indeed most of the lines that SXT observes are emitted at similar temperatures.

The SXT filter ratio temperatures, calculated with the standard software, indicate an uniform distribution for the loop. However, the plasma along the line of sight is obviously not isothermal, as the CDS monochromatic images (e.g. Si XII) show, and the interpretation of the validity of SXT temperatures is quite complex (work is in progress). Also, uncertainties in elemental abundances, ionization fractions, and continuum emission should be taken into account when using filter ratio methods.