Space Observations Needed for Improving Our Knowledge of Solar Flares

Extract

Up to the present time the technical possibilities of space research were not quite adequate for obtaining the ultraviolet spectrum of flares that occupy only a small portion (less than 10″ of arc) of the solar disk. Up to now all observations have been made in integrated sunlight. The importance of the knowledge of UV spectrum of flares is hardly necessary to emphasize. For example we just could mention that if ultraviolet spectra were available for a flare say from 850 Å up to 2000 Å, we would be able to estimate such extremely important parameters as the number of hydrogen atoms N₁ in the first quantum state (by using L-α, L-β, etc.), electronic temperature and density (from Lyman continuum) and other physical parameters of a flare which at the present time we try to derive by different indirect and inadequate methods. Several resonance (ultimate) lines are concentrated in this spectral interval and their careful examination in flares can bring additional important information about conditions prevailing in flares and in the underlying chromosphere. The same applies of course to the whole UV spectrum below 800 Å, and in particular to the resonance lines of HeI (λ 512 Å) and HeII (λ 304 Å), as well as to a number of lines of highly ionized atoms.