RADIATIVELY-DRIVEN DOWNDRAFTS AND REDSHIFTS IN TRANSITION REGION LINES .1. REFERENCE MODEL

We show that downdrafts driven by radiatively-cooling subarcsecond per turbations in the solar transition region can produce significant UV l ine redshifts consistent with those frequently observed on the solar d isk, and, by extension, with those more recently observed on stars. We model in detail the thermo-hydrodynamic evolution of a non-linear iso baric spherical condensation inside an initially hydrostatic atmospher e, stratified both in density and temperature, and with initial condit ions typical of a solar active region. We study the evolution of the a tmosphere both neglecting the effect of magnetic fields and taking it into account by forcing conduction and motions to be only along the ve rtical direction. The synthesis of emitted flux and effective speed in four UV lines from model results shows the presence of intense redshi fted components at speeds between 5 and 10 km s(-1).