Enhanced phase mixing of Alfv\'en waves propagating in stratified and divergent coronal structures

Corrected analytical solutions describing the enhanced phase mixing of Alfven waves propagating in divergent stratified coronal structures are presented. These show that the enhanced phase mixing mechanism can dissipate Alfven waves at heights less than half that is predicted by the previous analytical solutions. The enhanced phase mixing of 0.1 Hz harmonic Alfven waves propagating in strongly divergent, H_b=5 Mm, stratified coronal structures, H_rho=50 Mm, can fulfill 100% of an active region heating requirement, by generating viscous heating fluxes of F_H~2100 J /m^2 /s. The Alfven waves in this configuration are fully dissipated within 20 Mm, which is six times lower than would occur as a result of standard phase mixing in uniform magnetic fields. This results in the heating scale height, s_H, being lowered by a factor of six, to less than half of an active regions density scale height. Using the corrected analytical solutions it was found that, for a given wave frequency, the generation of a heating scale height of s_H<=50 Mm, by enhanced phase mixing in strongly divergent magnetic fields, requires a shear viscosity eight orders of magnitude lower, than required by standard phase mixing in uniform magnetic fields. It was also found that the enhanced phase mixing of observable, 0.01 rads /s Alfven waves, in strongly divergent magnetic fields, H_b=5 Mm, can generate heating scale heights within a density scale height, H_rho=50 Mm, using classical Braginskii viscosity. It is therefore not necessary to invoke anomalous viscosity in corona, if phase mixing takes place in strongly divergent magnetic fields. This study shows that the importance of enhanced phase mixing as a mechanism for dissipating Alfven waves in the solar corona (a stratified and divergent medium), has been seriously underestimated.
Comment: Accepted by A&A - 24th September 2007