Theory of X-ray emission of the Sun

First the temperature T of the corona is discussed, since the spectral distribution of the X-ray radiation depends on the value of T. The cross sections for electronic collisions computed by Schwartz and Zirin for s waves yield an upper limit of T which is 1.3 to 1.4·106 degrees Kelvin. In pursuance of earlier calculations of the author the X-ray emission of the corona by free-free and free-bound as well as by line emission is discussed. The theoretical results are compared with the observations. The effect of the radiation in the atmosphere of the earth and the intensity distribution across the solar disk is summed up. The emission of the transient region to the chromosphere and of hot spots is considered. Finally the X-ray radiation during a flare is investigated; some conclusions are drawn from the observations on the physical state of the emitting gas.