Intensities of Hydrogen H and Helium D3 in Solar Prominences. Paper III

An attempt is made to study the effect of internal variations of excitation within prominences by tracing the variation of the intensity ratio of I ( Hα )/ I (D ₃ ) throughout a particular prominence in which intensities of Hα and D ₃ have been measured at 123 individual points. The appearance of this prominence in the light of Hα and D ₃ has been represented on the basis of the measured equivalent widths by two sets of isophotes which give an approximate picture of its general structure. In studying the variation of the intensity ratio of I ( Hα )/ I (D ₃ ) within the various regions of the prominence, due allowance has been made for the influence of self-absorption on Hα . The intensity ratio R is found to be approximately constant over one part of the prominence with a mean value $$\bar R = 42$$ which is practically identical with the value used in former calculations of the relative abundance of hydrogen and helium. In another part of the prominence where the observed isophotes show the presence of some central condensation above the chromosphere, the value of R is found to be variable. In this region existing negative density gradients are found to be more strongly pronounced in hydrogen than in helium. Hα appears in particular to decrease between two and three times faster with height than D ₃ , apparently on account of the simultaneous increase in the ionisation of hydrogen.