The Na i and Sr ii Resonance Lines in Solar Prominences

Abstract

We estimate the electron density, $n_{\mathrm{e}}$, and its spatial variation in quiescent prominences from the observed emission ratio of the resonance lines Na i 5890 Å (D₂) and Sr ii 4078 Å. For a bright prominence ($\tau_{\alpha}\approx25$) we obtain a mean $n_{\mathrm{e}}\approx2\times10^{10}~\mbox{cm}^{-3}$; for a faint one ($\tau _{\alpha }\approx4$) $n_{\mathrm{e}}\approx4\times10^{10}~\mbox{cm}^{-3}$ 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, $T_{\mathrm{kin}}$ and $V_{\mathrm{nth}}$, required to deduce $n_{\mathrm{e}}$ from the emission ratio Na i/Sr ii cannot be unambiguously determined from observed widths of lines from atoms of different mass. The reduced widths, $\Delta\lambda_{\mathrm{D}}/\lambda_{0}$, of Sr ii 4078 Å show an excess over those from Na D₂ and $\mbox{H}\delta\,4101$ Å, assuming the same $T_{\mathrm{kin}}$ and $V_{\mathrm{nth}}$. 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 Sr ii 4078 Å as compared to those from Na D₂.