Phase mixed Alfven waves in partially ionised solar plasmas

The phase mixing of Alfven waves is one of the most promising mechanisms for explaining the heating of the solar atmosphere. The damping of waves in this case requires small transversal scales, relative to the magnetic field direction; this requirement is achieved by considering a transversal inhomogeneity in the equilibrium plasma density profile. Using both a single-fluid and a two-fluid approximation of a partially ionised chromospheric plasmas, we study the effectiveness of the damping of phase-mixed shear Alfven waves and investigate the effect of varying the ionisation degree and the type of wave driver on the dissipation of waves. Our results show that the dissipation length of shear Alfven waves strongly depends on the ionisation degree of the plasma, but, more importantly, in a partially ionised plasma, the damping length of shear Alfven waves is several orders of magnitude shorter than in the case of a fully ionised plasma, providing evidence that phase mixing could be a large contributor to heating the solar chromosphere. The effectiveness of phase mixing is investigated for various ionisation degrees, ranging from very weakly to very strongly ionised plasmas. In the case of the single fluid approximation we consider three distinct wave drivers; a single- and dual-frequency continuous driver and a finite lifetime pulse-like driver. Our results show that phase-mixed propagating Alfven waves with a modest amplitude of 2.5 kms-1 in a partially ionised plasma with ionisation degrees in the range mu = 0.518-0.657, corresponding to heights of 1916 - 2150 km above the solar surface, can provide sufficient heating to balance chromospheric radiative losses in the quiet Sun. In the case of the two-fluid approximation, we provide a proof of concept set of results, once again highlighting the efficiency of phase mixing at damping Alfven waves in partially ionised plasmas.