Solar flares and focused energy transport by MHD waves

Abstract

Context. Transport of flare energy from the corona to the chromosphere has traditionally been assigned to electron beams; however, interest has recently been renewed in magnetohydrodynamic (MHD) waves as a complementary or alternative mechanism. Aims. We determine whether, and under what conditions, MHD waves deliver spatially localised energy to the chromosphere, as required if MHD waves are to contribute to emission from flare ribbons and kernels. This paper also highlights several properties of MHD waves that are relevant to solar flares and demonstrates their application to the flare problem. Methods. Transport is investigated using a magnetic arcade model and 2.5D MHD simulations. Different wave polarisations are considered and the effect of fine structuring transverse to the magnetic field is also examined. Ray tracing provides additional insight into the evolution of waveguided fast waves. Results. Alfven waves are very effective at delivering energy fluxes to small areas of chromosphere, localisation being enhanced by magnetic field convergence and phase mixing. Fast waves, in the absence of fine coronal structure, are more suited to powering emission from diffuse rather than compact sources; however, fast waves can be strongly localised by coronal waveguides, in which case focused energy is best transported to the chromosphere when waveguides are directly excited by the energy release. Conclusions. MHD waves pass an important test for inclusion in future flare models.