Evidence for the Two-fluid Scenario in Solar Prominences

Abstract

This paper presents observational evidence of the different dynamical behavior of neutral and ionized species in solar prominences. The analysis of a time-series of Sr ii 4078 Å and Na D spectra in a quiescent prominence yields systematically larger Doppler shifts (line-of-sight velocities) for the ions V LOS (Sr ii ) = 1.22 ×  V LOS (Na D ). Both lines show a 30 minute oscillation of good coherence. Sixteen hours later the same prominence underwent marked morphological changes (with a rising dome), and the Sr ii velocity excess dropped to V LOS (Sr ii ) = 1.11 × V LOS (Na D ). The same excess is found for the line pair Fe ii 5018 Å and He i 5015 Å. The widths of the ionic lines, mainly non-thermally broadened, are not related to the macro-velocities. The emission ratio of Na D and Sr ii , a measure of the electron density, yields n e  = 4 × 10 10 cm −3 , shows no relation with the V LOS variation or with height above the limb, and seems to be reduced 16 hr later during the active phase. We apply a new wavelength reference from aureola spectra, which is independent of photospheric velocity fields.