Ultraviolet radiation physics in astronomy and geophysics

Abstract

The astrophysicist has been compelled, in his studies of the solar and stellar atmospheres, to abandon in some cases the simpler “local thermodynamic equilibrium” approach. The new approach assumes statistical equilibrium and writes equations which concern the transfer of energy. Details of the atomic and electronic collisions must be taken into account, along with the effects of the radiation field itself. The need for values of excitation, ionization, and similar cross sections is therefore of paramount importance. For studies of the corona where high ionization and excitation prevail, u.v. radiation is a pertinent factor. The appropriate laboratory and theoretical parameters are badly needed.Detailed studies of the photochemical reactions and ionization in the earth's upper atmosphere frequently touch on u.v. spectroscopic theory and data. In particular, the investigation of ionization rates has pointed out the need for accurate total absurption cross sections of molecules and atoms for u.v. radiation as well as the need for better ionization efficiencies. The range below 300 Å should be emphasized. Accurate solar absolute intensity data is lacking for the same regions.The improvement of optical instruments for the u.v. region is progressing, but more work remains to be done. The greatest lack seems to be the absence of good absolute calibration for intensity in the far u.v. The causes of some of the inadequacies of modern reflection gratings, including the echelle, should be found by laboratory research, and the remedies, if any, put into effect. Transmission and reflectivity research in the u.v. has already contributed notably to improvement in optical instrumentation. A good physics theory remains to be developed in this field, and basic laboratory research is in order.