A Discussion on solar studies with special reference to space observations - Solar flare X-ray spectra

The solar X -ray spectrum provides a versatile method for determining physical conditions in the lower corona and corona—chromosphere interface which are associated with the chromospheric (Hα) flare phenomenon. Information is contained both in the continuum and line emission which exists at these wavelengths. Continuum emission is predominant below 0.13 nm because of the relatively low solar abundance of heavy elements capable of producing line emission at these wavelengths. During the initial phase of an X-ray event this continuum frequently appears in short, often quasi-periodic bursts whose spectrum is best described by a power law to 100 keV and decreasing more rapidly at high energies. The electron spectrum apparently responsible for these bursts has many similarities to that required for the production of type III radio bursts. The emission of flare-associated soft X-ray radiation (both line and continuum radiation) begins at the time of hard X-ray bursts but reaches maximum one to several minutes later. Line emission from ions up to Ni xxvii in the helium-like ion sequence and up to Fe xxvi in the hydrogen-like ion sequence has been observed during large flares. The evolution of the plasma in which this radiation originates can be studied by comparing emission lines in the same or adjacent stages of ionization of an element. From such observations we conclude that a steady-state condition rarely if ever exists in the X-ray emitting regions associated with a solar flare.