Heating of the solar corona

Abstract

We review a number of models which are currently being considered for coronal heating, but we consider also heating of the chromosphere which requires nearly as much energy as the active corona, and more energy than coronal holes or the quiet corona. There are basically two types of models, which are motivated by a variety of observations. (1) Models which invoke MHD waves generated by the convective motions are motivated by observations of the ubiquitous presence of Alfvén waves in the solar wind. There is evidence that these waves heat and accelerate the solar wind protons and heavy ions. The solar wind thus provides one example of wave heating. Waves have the advantage of being able to heat the chromosphere and photospheric magnetic flux tubes on their way to the corona. MHD turbulence (as observed in the solar wind) or resonance absorption seem to provide adequate dissipation mechanisms. A problem with wave theories is that the waves tend to be reflected by the steep Alfén speed gradient in the chromosphere and transition region, but it is estimated that adequate energy fluxes can enter the open corona, or closed coronal loops if global loop resonances can be excited. Short coronal loops (L≲10⁴ km) can also receive adequate wave energy fluxes even if the loop resonances are not excited, but a problem exists with getting enough energy into intermediate length loops (L≈10⁴-5×10⁴ km) since their resonant frequencies are possibly to high to be excited. (2) Models which invoke the gradual buildup of coronal magnetic energy due to random walks of the photospheric flux tubes, and the subsequent release of that energy via current sheet information and reconnection, are supported by observations indicating that localized impulsive heating and dynamic events occur in the transition region and corona. These models cannot explain the chromospheric heating or the coronal heating on open field lines. They require substantial random walks of the photospheric footpoints, which still need to observationally verified. A third possibility, which has not been studied in detail, is that the chromospheric and coronal heating is associated with emergence and cancellation of magnetic flux. All types of models are ripe for further studies using numerical simulations, and along the way we shall offer several suggestions for fruitful numerical studies.