Three-dimensional propagation of kink wave trains in solar coronal slabs

Abstract

ABSTRACT Impulsively excited wave trains are of considerable interest in solar coronal seismology. To our knowledge, however, it remains to examine the three-dimensional (3D) dispersive propagation of impulsive kink waves in straight, field-aligned, symmetric, low-beta, slab equilibria that are structured only in one transverse direction. We offer a study here, starting with an analysis of linear oblique kink modes from an eigenvalue problem perspective. Two features are numerically found for continuous and step structuring alike, one being that the group and phase velocities may lie on opposite sides of the equilibrium magnetic field ($\boldsymbol {B}_0$), and the other being that the group trajectories extend only to a limited angle from $\boldsymbol {B}_0$. We justify these features by making analytical progress for the step structuring. More importantly, we demonstrate by a 3D time-dependent simulation that these features show up in the intricate interference patterns of kink wave trains that arise from a localized initial perturbation. In a plane perpendicular to the direction of inhomogeneity, the large-time slab-guided patterns are confined to a narrow sector about $\boldsymbol {B}_0$, with some wavefronts propagating toward $\boldsymbol {B}_0$. We conclude that the phase and group diagrams lay the necessary framework for understanding the complicated time-dependent behaviour of impulsive waves.