The Prominence-Corona Transition Region in transverse magnetic fields

Abstract

An emission measure analysis is performed for the Prominence-Corona Transition Region (PCTR) under the assumption that the cool matter of quiescent filaments is contained in long, thin magnetic flux loops imbedded in hot coronal cavity gas. Consequently, there is a transition region around each thread. Comparison of the model and observations implies that the temperature gradient is perpendicular to the magnetic lines of force in the lower part of the PCTR (T < 105 K). It is shown that in this layer the heating given by the divergence of the transverse conduction fails to account for the observed UV and EUV emission by several orders of magnitude. It is, therefore, suggested that the heating of these layers could be due to dissipation of Alfven waves. In the high-temperature layers (T ≥ 105 K), where the plasma β ≥ 1, the temperature gradient is governed by radiative cooling balancing conductive heating from the surrounding hot coronal gas. Also in these outer layers the presence of magnetic fields reduces notably the thermal conduction relative to the ideal field-free case. Numerical modelling gives good agreement with observed DEM; the inferred value of the flux carried by Alfven waves, as well as that of the damping length, greatly support the suggested form of heating. The model assumes that about 1/3 of the volume is occupied by threads and the rest by hot coronal cavity matter. The brightness of the EUV emission will depend on the angle between the thread structure and the line of sight, which may lead to a difference in brightness from observations at the limb and on the disk.