Horizontally and vertically polarized kink oscillations in curved solar coronal loops
Astronomy & Astrophysics(2024)
摘要
Kink oscillations are frequently observed in coronal loops. This work aims to
numerically clarify the influence of loop curvature on horizontally and
vertically polarized kink oscillations. Working within the framework of ideal
MHD, we conduct 3D simulations of axial fundamental kink oscillations in curved
density-enhanced loops embedded in a potential magnetic field. Both horizontal
and vertical polarizations are examined, and their oscillation frequencies are
compared with WKB expectations. We discriminate two different density
specifications. In the first (dubbed"uniform-density"), the density is axially
uniform and varies continuously in the transverse direction toward a uniform
ambient corona. Some further stratification is implemented in the second
specification (dubbed"stratified"), allowing us to address the effect of
evanescent barriers. Examining the oscillating profiles of the initially
perturbed uniform-density loops, we found that the frequencies for both
polarizations deviate from the WKB expectation by ∼ 10%. In the
stratified loop, however, the frequency of the horizontal polarization deviates
to a larger extent (∼ 25%). We illustrate the lateral leakage of kink
modes through wave tunnelling in 3D simulations, for the first time. Despite
this, in both loops, the damping time-to-period ratios are similar and close to
the analytical predictions for straight configurations under the
thin-tube-thin-boundary (TTTB) assumption. The WKB expectation for straight
configurations can reasonably describe the eigenfrequency of kink oscillations
only in loops without an asymmetrical cross-loop density profile perpendicular
to the oscillating direction. Lateral leakage via wave tunnelling is found to
be less efficient than resonant absorption, meaning that the latter remains a
robust damping mechanism for kink motions even when loop curvature is included.
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