Oblique Quasi-kink Modes in Solar Coronal Slabs Embedded in an Asymmetric Magnetic Environment: Resonant Damping, Phase and Group Diagrams

Abstract

There has been considerable interest in magnetoacoustic waves in static, straight, field-aligned, 1D equilibria where the exteriors of a magnetic slab are different between the two sides. We focus on trapped, transverse fundamental, oblique quasi-kink modes in pressureless setups, where the density varies continuously from a uniform interior (with density ρ i) to a uniform exterior on either side (with density ρ L or ρ R), assuming ρ L ≤ ρ R ≤ ρ i. The continuous structuring and oblique propagation make our study new, relative to pertinent studies, and lead to wave damping via the Alfvén resonance. We compute resonantly damped quasi-kink modes as resistive eigenmodes, and isolate the effects of system asymmetry by varying ρ i/ρ R from the “Fully Symmetric” (ρ i/ρ R = ρ i/ρ L) to the “Fully Asymmetric” limit (ρ i/ρ R = 1). We find that the damping rates possess a nonmonotonic ρ i/ρ R-dependence as a result of the difference between the two Alfvén continua, and resonant absorption occurs only in one continuum when ρ i/ρ R is below some threshold. We also find that the system asymmetry results in two qualitatively different regimes for the phase and group diagrams. The phase and group trajectories lie essentially on the same side (different sides) relative to the equilibrium magnetic field when the configuration is not far from a “Fully Asymmetric” (“Fully Symmetric”) one. Our numerical results are understood by making analytical progress in the thin-boundary limit, and discussed for imaging observations of axial standing modes and impulsively excited wavetrains.