SIPHON FLOWS IN ISOLATED MAGNETIC-FLUX TUBES .5. RADIATIVE FLOWS WITH VARIABLE IONIZATION

Our calculations of steady siphon flows in arched, isolated magnetic flux tubes in the solar atmosphere are extended to include radiative transfer between the flux tube and its surroundings and variable ionization Of the flowing gas. For the radiative transfer we use the Spiegel formulation (valid in both the optically thick and optically thin limits) in the layers below the photosphere and the formulation of Ulmschnelder et al. for an optically thin flux tube above the solar surface. We show that the behavior of a siphon flow is strongly determined by the degree of radiative coupling between the flux tube and its surroundings in the superadiabatic layer just below the solar surface. We calculate critical siphon flows with adiabatic tube shocks in the downstream leg, illustrating the radiative relaxation of the temperature jump downstream of the shock. For flows in arched flux tubes reaching up to the temperature minimum, where the opacity is low, the gas inside the flux tube is much cooler than the surrounding atmosphere at the top of the arch. We suggest that gas cooled by siphon flows contributes to the cool component (T < 4000 K) of the solar atmosphere at the height of the temperature minimum implied by observations of the infrared CO bands at 4.6 and 2.3 mum. We have already suggested that siphon flows are a possible mechanism for producing some of the intense magnetic elements observed in the solar photosphere. In Paper IV we pointed out that the observational signature of this mechanism at the solar surface would be a pair of magnetic elements of opposite polarity, separated by a distance of from one to several arcseconds, with magnetic fields slightly inclined to the vertical in each element (consistent with their being connected by an arched flux tube) and with an upflow in one element (the upstream footpoint of the arch) and a downflow and a somewhat greater magnetic field strength in the other clement (the downstream footpoint). This specific prediction has now received support in the recent observations of Ruedi, Solanki, & Rabin.