The Na I and Sr II Resonance Lines in Solar Prominences
Abstract
We estimate the electron density, ne, and its spatial variation in quiescent prominences from the observed emission ratio of the resonance lines Na I 5890 (D2) and Sr II 4078. For a bright prominence (taualpha ~ 25) we obtain a mean ne ~ 2*1010 /cm3; for a faint one (taualpha ~ 4) ne ~ 4*1010 /cm3 on two consecutive days with moderate internal fluctuation and no systematic variation with height above the solar limb. The thermal and non-thermal contributions to the line broadening, Tkin and Vnth, required to deduce ne from the emission ratio NaI/SrII cannot be unambiguously determined from observed widths of lines from atoms of different mass. The reduced widths, Delta-lambdaD / lambda0, of Sr II 4078 show an excess over those from NaD2 and H-delta, assuming the same Tkin and Vnth. We attribute this excess broadening to higher non-thermal broadening induced by interaction of ions with the prominence magnetic field. This is suggested by the finding of higher macro-shifts of SrII4078 as compared to those from NaD2.