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 Sol ar and Heliospheric Observatory (SOHO), and the Transition Region and Coron al Explorer (TRACE) established three important observational constraints f or coronal heating models: (1) coronal loops in active regions have an over density 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 coron a within a height range of lambda (H) less than or similar to 10 Mm above t he photosphere. Although these three observational facts have been derived from active region loops, the part of the solar corona that is topologicall y connected to active regions makes up greater than or similar to 80% of th e heating energy requirement (at a typical day around the maximum of the so lar cycle) and thus constitutes the majority of the energy budget of the co ronal heating problem at large. We discuss and compare a comprehensive set of theoretical models of coronal heating under the aspect of whether they c an satisfy these observational constraints. We find that conventional direc t current (DC) and alternating current (AC) coronal heating models that con sider coronal loops as homogeneous flux tubes (in density and temperature) do not predict these observed effects, while refined models that include gr avity and the transition region can reproduce them. In particular, magnetic reconnection models that spawn chromospheric evaporation satisfy the obser vational constraints the easiest. Our main conclusion is that the coronal h eating problem can be solved only by tapping energization processes in the chromosphere and transition region.