RADIATIVELY DRIVEN DOWNDRAFTS AND REDSHIFTS IN TRANSITION REGION LINES .2. EXPLORING THE PARAMETER SPACE

We address the hypothesis that downdrafts driven by radiatively-coolin g condensations in the solar transition region are able to produce sig nificant redshifts in UV lines as frequently observed on the solar dis k and, more recently, on other stars. In a first paper, significant re dshifted line components at several km/s have been found from modeling the evolution of an isobaric perturbation twice as dense as the unper turbed atmosphere, almost as large as the thickness of the transition region of an active region loop, and with central temperature higher t han the formation temperature of the UV lines. In the present work we show the results of an extensive exploration of the space of the impor tant parameters controlling the evolution of isobaric perturbations: d ensity contrast (delta), dimensions, and ambient pressure. The center of most of the perturbations is placed where the temperature of the un perturbed medium is T-0 = 4.5 10(5) K, but higher temperatures do not lead to substantially different results. From the hydrodynamic evoluti on we synthesize the line flux and effective speed along the line of s ight, and examine the distribution of the most intense Doppler-shifted components. In a wide region of the parameter space (delta > 0.5, dim ensions of the order of the thickness of the transition region) we fin d redshifted components at speeds of several km/s for ambient pressure values ranging from those typical of quiet Sun to active regions. The assumption of isotropic thermal conduction, or, alternatively, of 1-D hydrodynamics, i.e. mimicking the effect of strong magnetic fields, l ead to qualitatively similar results. Our calculations suggest also th at redshifts may occur more easily in the higher pressure plasma, typi cal of active regions, in general consistency to observations.