Solar Coronal Heating: MHD Models and Observational Signatures

Abstract

Although it is generally accepted that the mechanism for maintaining high coronal temperatures is magnetic, understanding of the process is still far from complete. It is likely that different heating mechanisms operate in various large-scale observable structures (loops, coronal holes, X-ray bright points, etc). All heating theories broadly divide into two classes: wave models, if the time-scale of the driving photospheric motions is fast compared with Alfven transit time, and quasi-static models, if the driving time is slow (Browning, 1991; Sakurai, 1996). In the latter case, more relevant for compact and strongly magnetized active regions, the coronal field evolves through a series of magnetostatic equilibria: photospheric motions generate electric currents (approximately field-aligned), and this is a source of excess magnetic energy available for heating. However, high electrical conductivity of the hot coronal plasma makes classical Ohmic dissipation on global length scales completely inefficient. Thus, for heating to be effective, the coronal magnetic field must possess fine-scale structure such as current sheets (Parker, 1972), within which even small resistivity can break the topology constraints of ideal MHD, allowing fast transition of coronal configuration to a state of lower magnetic energy via magnetic reconnection. Such a scenario of storage and release of magnetic energy is reminiscent of flares, and indeed the coronal heating process can be viewed as a superposition of numerous “nanoflare” events (Parker, 1988).