The role of torsional Alfven waves in coronal heating

Abstract

In the context of coronal heating, among the zoo of MHD waves that exist in the solar atmosphere, Alfven waves receive special attention. Indeed, these waves constitute an attractive heating agent due to their ability to carry over the many different layers of the solar atmosphere sufficient energy to heat and maintain a corona. However, due to their incompressible nature these waves need a mechanism such as mode conversion (leading to shock heating), phase mixing, resonant absorption or turbulent cascade in order to heat the plasma. New observations with polarimetric, spectroscopic and imaging instruments such as those on board of the japanese satellite Hinode, or the SST or CoMP, are bringing strong evidence for the existence of energetic Alfven waves in the solar corona. In order to assess the role of Alfven waves in coronal heating, in this work we model a magnetic flux tube being subject to Alfven wave heating through the mode conversion mechanism. Using a 1.5-dimensional MHD code we carry out a parameter survey varying the magnetic flux tube geometry (length and expansion), the photospheric magnetic field, the photospheric velocity amplitudes and the nature of the waves (monochromatic or white noise spectrum). It is found that independently of the photospheric wave amplitude and magnetic field a corona can be produced and maintained only for long (> 80 Mm) and thick (area ratio between photosphere and corona > 500) loops. Above a critical value of the photospheric velocity amplitude (generally a few km/s) the corona can no longer be maintained over extended periods of time and collapses due to the large momentum of the waves. These results establish several constraints on Alfven wave heating as a coronal heating mechanism, especially for active region loops.