The Role of Velocity Redistribution in Enhancing the Intensity of the He II 304 Å Line in the Quiet-Sun Spectrum

We present observational evidence of the effect of small-scale ("microturbulent") velocities in enhancing the intensity of the He II l304 line with respect to other transition region emission lines, a process we call "velocity redistribution,". We first show results from the 1991 and 1993 flights of SERTS (Solar EUV Rocket Telescope and Spectrograph). The spectral resolution of the SERTS instrument was sufficient to infer that, at the spatial resolution of 5'', the line profile is nearly Gaussian both in the quiet Sun and in active regions. We were then able to determine, for the quiet Sun, a lower limit for the amplitude of nonthermal motions in the region of formation of the 304 A line of the order of 10 km s-1. We estimated that, in the presence of the steep temperature gradients of the solar transition region (TR), velocities of this magnitude can significantly enhance the intensity of that line, thus at least helping to bridge the gap between calculated and observed values. We also estimated the functional dependence of such an enhancement on the relevant parameters (nonthermal velocities, temperature gradient, and pressure). We then present results from a coordinated campaign, using SOHO/CDS and Ha spectroheliograms from Coimbra Observatory, aimed at determining the relationship between regions of enhanced helium emission and chromospheric velocity fields and transition region emission in the quiescent atmosphere. Using these data, we examined the behavior of the He II l304 line in the quiet-Sun supergranular network and compared it with other TR lines, in particular with O III l600. We also examined the association of 304 A emission with the so-called coarse dark mottle, chromospheric structures seen in Ha red-wing images and associated with spicules. We found that all these observations are consistent with the velocity redistribution picture.