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The solar X-ray continuum measured by RESIK

The solar X-ray continuum emission at five wavelengths between 3.495 Angstroms and 4.220 Angstroms for 19 flares in a seven-month period in 2002--2003 was observed by the RESIK crystal spectrometer on CORONAS-F. In this wavelength region, free--free and free--bound emissions have comparable fluxes. With a pulse-height analyzer having settings close to optimal, the fluorescence background was removed so that RESIK measured true solar continuum in these bands with an uncertainty in the absolute calibration of plus or minus 20 per cent. With an isothermal assumption, and temperature and emission measure derived from the ratio of the two GOES channels, the observed continuum emission normalized to an emission measure of 10^48 cm^-3 was compared with theoretical continua using the CHIANTI atomic code. The accuracy of the RESIK measurements allows photospheric and coronal abundance sets, important for the free--bound continuum, to be discriminated. It is found that there is agreement to about 25 per cent of the measured continua with those calculated from CHIANTI assuming coronal abundances in which Mg, Si, Fe abundances are four times photospheric.