Two-Dimensional Mapping of the He D(3)/Hb Emission Ratio in Solar Prominences

Solar prominences have been simultaneously observed in the integrated light of the He D(3) and the Hb emissions on two successive days, using the SST on La Palma with its tip-tilt mirror locked on a nearby white-light limb facular grain. The spatial and the temporal variation of the integrated line intensities and their ratio shows mainly two characteristics: (A) Constant emission ratio (even) in regions with substantial intensity variations and (B) varying emission ratio (often) tightly related to intensity structures of the prominence. (A) May be explained by a different number of superposing threads along the line of sight having very similar physical state. (B) Indicates threads with different intrinsic physical states; these may depend on the gas pressure or the inner structure of each thread, i.e., the 'packing density,' affecting the penetration of ionizing EUV radiation, which affects the He i level populations and thus the rate of the triplet excitation.