Radiatively driven downdrafts and redshifts in transition region lines

We address the hypothesis that downdrafts driven by radiatively-cooling condensations in the solar transition region are able to produce significant redshifts in UV lines as frequently observed on the solar disk and, more recently, on other stars. In a first paper, significant redshifted line components at several km/s have been found from modeling the evolution of an isobaric perturbation twice as dense as the unperturbed atmosphere, almost as large as the thickness of the transition region of an active region loop, and with central temperature higher than the formation temperature of the UV lines. In the present work we show the results of an extensive exploration of the space of the important parameters controlling the evolution of isobaric perturbations: density contrast (δ), dimensions, and ambient pressure. The center of most of the perturbations is placed where the temperature of the unperturbed medium is T0 = 4.5 105 K, but higher temperatures do not lead to substantially different results. From the hydrodynamic evolution we synthesize the line flux and effective speed along the line of sight, and examine the distribution of the most intense Dopplershifted components. In a wide region of the parameter space (δ > 0.5, dimensions of the order of the thickness of the transition region) we find 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, lead to qualitatively similar results. Our calculations suggest also that redshifts may occurmore easily in the higher pressure plasma, typical of active regions, in general consistency to observations.