Horizontally polarized kink oscillations supported by solar coronal loops in an asymmetric environment

Abstract

Kink oscillations are ubiquitously observed in solar coronal loops, and understanding them is crucial in the contexts of coronal seismology and atmospheric heating. We studied kink modes supported by a straight coronal loop embedded in an asymmetric environment using 3D magnetohydrodynamic simulations. We implemented the asymmetric effect by setting different exterior densities below and above the loop interior and initiated the simulation using a kink-like velocity perturbation perpendicular to the loop plane, mimicking the frequently measured horizontally polarized kink modes. We find that the external velocity fields show fan-blade structures propagating in the azimuthal direction as a result of the successive excitation of higher azimuthal Fourier modes. Resonant absorption and phase-mixing can still occur despite an asymmetric environment, leading to the development of small-scale structures at loop boundaries. These small-scale structures nonetheless develop asymmetrically at the upper and lower boundaries due to the different gradients of the Alfv\'en speed. These findings enrich our understanding of kink modes in coronal loops embedded within an asymmetric environment, providing insights that will be helpful for future high-resolution observations.