Kink oscillations in a coronal loop arcade with finite plasma-β: effect of oblique propagation

Abstract

ABSTRACT Kink oscillations of a curved coronal slab with finite plasma-β, simulating a loop arcade, are examined. Perpendicular propagation, i.e. propagation along the arcade axis (ky > 0) is taken into account. Two surface modes, labelled as faster and slower mode, are found to exist in the model. In the zero-β limit, the faster mode is a vertically polarized kink mode and the slower mode produces bending motions polarized along the arcade axis, provided $k_y^{-1}$ is of the order of or larger than the slab thickness a. Otherwise, if $k_y^{-1}$ is much less than a, the faster mode results in periodic displacement of a loop arcade along its axis and the slower mode has mixed properties. The phase speeds of both modes are very similar when $k_y^{-1}\sim a$, and they tend to the external and internal Alfvén speeds when ky → 0. As the internal plasma-β becomes finite and grows, the phase speed of the faster mode increases and that of the slower mode decreases. When βi > 0, these modes are a superposition of vertical kink motions and those that are oriented along the arcade axis, both supplemented with the significant cross-averaged density perturbations. It seems promising to use the obtained results for interpreting quasi-periodic pulsations, in terms of kink oscillations of flaring high-β loops, provided the developed theory is applicable to the torroidal single loop model when choosing an appropriate ky.