Radiative transfer in cylindrical threads with incident radiation

Abstract

Context. Relatively cool and dense structures embedded in the solar corona (filaments, prominences, spicules, etc.) may be observed in hydrogen lines. Sometimes they last during several solar rotations. Aims. Our goal is to evaluate the lifetime of cool structures of the solar corona, determine their evolution from given physical conditions, and compute models in thermal equilibrium. Methods. We use numerical methods to simultaneously solve the equations of NLTE radiative transfer, statistical equilibrium of hydrogen level populations, and electric neutrality. Radiative transfer equations are solved using cylindrical coordinates and prescribed solar incident radiations. The computation of internal energy and radiative losses and gains yields the rates of temperature evolution. Results. For isothermal-isobaric cylinders with prescribed physical conditions, we determine the lifetimes and evolution rates for different positions along the radius. For models with prescribed diameter and pressure, we determine the run of temperature vs. radius corresponding to thermal equilibrium. This equilibrium is found to be stable for the whole range of parameters under investigation. Conclusions. The cores of large and high-pressure cylinders are found to evolve very slowly. This opens the possibility of observing these cool structures at temperatures somewhat different from that corresponding to theoretical radiative equilibrium.