The dynamic formation of quasi-static active region loops

Abstract

A hydrodynamic model is used to analyse the formation of an active region coronal loop. After an impulse of energy that ‘lifts' part of a cool uniform plasma into the domain of radiative instability (T> 105 K) the atmosphere is allowed to relax dynamically to a new quasi-steady equilibrium. Although the radiative-hydrodynamic coupling leads to quite complex physical phenomena, the final state of the plasma shows excellent agreement with previous quasi-static loop calculation. The bearing of this analysis on the evolution of flare plasmas is then discussed. In particular, it is shown that the energy flux emanating from an excited coronal source leads to the ‘dynamic evaporation’ of cool transition-zone and chromospheric material. The analysis indicates, however, that an accurate description of the transition zone in solar flares requires considerably more sophistication than has hitherto been employed. Finally, a discussion is given of the uniqueness and stability of the quasi-static loop. It is concluded that, despite thermal instability, a quasi-static model should adequately describe the gross features of quiescent coronal loops.