CORONAL LOOP HEATING AND CATASTROPHIC COOLING

摘要

There is plenty of clear evidence for frequent and very localizedrnheating events near the chromosphere,which mayrnbe responsible for the observable high temperatures ofrnthe coronal plasma. Here we perform 1D hydrodynamicrnsimulations of the evolution of a coronal loop undergoingrnimpulsive heating through the release of localized Gaussianrnenergy pulses near the loop's footpoints. We findrnthat when a discrete number of randomly spaced pulsesrnis released, the loop heats up and stays at coronal temperaturesrnfor the whole duration of the impulsive heatingrnstage provided that the elapsing time between successivernheat injections is less than 215 s. For elapsingrntimes longer than this critical or threshold value, coronalrntemperatures can no longer be maintained and thernloop apex cools down catastrophically well below the initialrnstate, reaching chromospheric temperatures (～104K)and leading to the typical hot-cool temperature pro-file characteristic of a cool condensation. As long as thernheating concentrates more and more at the loop's footpoints,rnthe temperature variation is seen to make a transitionrnfrom that of a uniformly heated loop to a flat, isothermalrnprofile along the loop length. Concentration of thernheating at the footpoints also results in the more frequentrnappearance of rapid and significant depressions of thernapex temperature during the loop evolution, most of themranging from and (～1.5×106 to ～104 K)lasting fromabout 3 to 10 minutes. This behaviour strongly resemblesrnthe intermittent variability of coronal loops inferred fromrnSOHO observations in active regions of the solar atmosphere.