BRANCHING OF THE HELIUM-TO-BALMER EMISSION RATIO IN SOLAR PROMINENCE STRUCTURES

The H-alpha, H-beta, and He D-3 emission lines are simultaneously observed with high spatial resolution at the VTT on Tenerife. It is found that the emission ratios He D-3/H-beta and He D-3/H-alpha show typical mean values for each individual prominence. This branching is related to the mean kinetic temperature and to the structuring of each prominence. Whereas the Balmer excitation is almost uniformly distributed throughout prominences, the He excitation is enhanced in structured prominences of low Palmer brightness or outer parts of unstructured bright prominences, where exciting and ionizing EUV radiation can freely penetrate. In these regions the He ionization may exceed that of hydrogen. The atmospheric prominence parameters are not yet sufficiently known to allow a reliable determination of the solar Helium abundance.