The Na i and Sr ii Resonance Lines in Solar Prominences

Abstract

We estimate the electron density, n_e, 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 (tau_alpha ~ 25) we obtain a mean n_e ~ 2*10^10 /cm^3; for a faint one (tau_alpha ~ 4) n_e ~ 4*10^10 /cm^3 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, T_kin and V_nth, required to deduce n_e from the emission ratio NaI/SrII cannot be unambiguously determined from observed widths of lines from atoms of different mass. The reduced widths, Delta-lambda_D / lambda_0, of Sr II 4078 show an excess over those from NaD2 and H-delta, assuming the same T_kin and V_nth. 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.