X-ray spectra of solar flares

A review of recent observations of the solar corona in X-rays and an introduction to the physics of solar flares and the atomic processes producing the X-radiation from the sun, are presented. Results from the graphite crystal spectrometer on board OSO-8 are then used to determine electron temperatures from observations of X-ray lines and continua during solar flares. The iron emission at 1.9 Angstroms and the helium-like sulphur and calcium lines are then examined in detail and the possible variation of the electron density and ionization balance during a flare is discussed and a simple flare model derived to fit the observations. The absolute coronal abundances of silicon, sulphur, argon and calcium are derived from line: continuum intensity ratios. The importance of diffusion in the solar atmosphere is discussed and the process of ambipolar diffusion is suggested as a cause for this variation of elemental abundances with height. (author)