An Evaluation of Coronal Heating Models for Active Regions Based on Yohkoh, SOHO, and TRACE Observations
Recent soft X-ray and EUV data from space observations with Yohkoh, the Solar and Heliospheric Observatory (SOHO), and the Transition Region and Coronal Explorer (TRACE) established three important observational constraints for coronal heating models: (1) coronal loops in active regions have an overdensity that can be supplied only by upflows of heated chromospheric plasma, (2) chromospheric upflows have been observed frequently in coronal loops, and (3) the coronal heating function has been localized in the lower corona within a height range of λH<~10 Mm above the photosphere. Although these three observational facts have been derived from active region loops, the part of the solar corona that is topologically connected to active regions makes up >~80% of the heating energy requirement (at a typical day around the maximum of the solar cycle) and thus constitutes the majority of the energy budget of the coronal heating problem at large. We discuss and compare a comprehensive set of theoretical models of coronal heating under the aspect of whether they can satisfy these observational constraints. We find that conventional direct current (DC) and alternating current (AC) coronal heating models that consider coronal loops as homogeneous flux tubes (in density and temperature) do not predict these observed effects, while refined models that include gravity and the transition region can reproduce them. In particular, magnetic reconnection models that spawn chromospheric evaporation satisfy the observational constraints the easiest. Our main conclusion is that the coronal heating problem can be solved only by tapping energization processes in the chromosphere and transition region.