PLASMA CONDENSATION IN SOLAR CORONAL LOOPS:II. 'CATASTROPHIC COOLING' AND HIGH-SPEED DOWNFLOWS

摘要

The second part of this work focuses on the application of the concept of plasma condensation to large coronal loops. In contrast to the short loops analyzed in M¨uller et al. (2003a), these models can more easily be compared to SOHO and TRACE observations. From our numerical calculations of coronal loops we find several classes of time-dependent solutions (static, periodic, irregular), depending on the spatial dependence of a temporally constant energy deposition in the loop. One of these classes is in remarkably close agreement with the features observed with TRACE, described by Schrijver (2001): Emission in CIV (154.8 nm), developing initially near the loop tops, cool plasma sliding down on both sides of the loop, downflow velocities of up to 100 km/s, and a downward acceleration which is substantially reduced with respect to the solar surface gravity. Furthermore, these results also offer an explanation for the observations of De Groof et al. (2003a,b). In contrast to earlier models, we suggest that the process of catastrophic cooling does not have to be initiated by a drastic decrease of the loop heating. It can also result from a loss of equilibrium at the loop apex which is a natural consequence if the loop is heated predominantly at the footpoints, but constant in time.