Driven phase-mixed Alfv\'en waves in a partially ionized solar plasma

TL;DR

This study investigates how phase mixing of shear Alfvén waves in a partially ionized solar plasma depends on ionization degree and Alfvén speed gradients, revealing differences between pulse and continuous wave drivers in wave damping and heating efficiency.

Contribution

It introduces a model analyzing phase mixing in a partially ionized chromospheric plasma with a pulse wave driver, highlighting the impact of ionization and inhomogeneity on wave damping and heating.

Findings

Dissipation length depends strongly on ionization degree and Alfvén speed gradient.

Pulse-driven waves have similar initial heating rates as continuous waves.

Alfvén pulses decay algebraically, not exponentially, due to energy injection differences.

Abstract

Phase mixing has long been understood to be a viable mechanism for expediting the dissipation of Alfv\'en wave energy resulting in the subsequent heating of the solar atmosphere. To fulfil the conditions necessary for phase mixing to occur, we consider the cross-field gradient in the Alfv\'en speed as a free parameter in our model. Using a single-fluid description of a partially ionized chromospheric plasma, we explore the efficiency of damping of shear Alfv\'en waves subject to phase mixing when a pulse wave driver is employed. Our results demonstrate a strong dependence of the dissipation length of shear Alfv\'en waves on both the ionization degree of the plasma and the gradient of the Alfv\'en speed. When assessing the efficiency of phase mixing across various inhomogeneities, our findings indicate that waves originating from a pulse driver exhibit initially identical heating rates as those generated by a continuous wave driver. One key difference observed was that Alfv\'en pulses possess a lower overall decay rate due to a change in damping profile from exponential to algebraic. This discrepancy arises from the absence of a consistent injection of energy into the base of the domain, that preserves longitudinal gradients of the magnetic field perturbations more effectively. These findings demonstrate the importance of understanding the relations between the wave driver, damping mechanisms, and propagation dynamics in resolving the atmospheric heating problem.