Resonant Absorption of Kink MHD Waves in Inclined and Asymmetric Coronal Loops

TL;DR

This study investigates how inclination and asymmetry in coronal loops affect the resonant absorption and properties of kink MHD oscillations, revealing that these factors influence oscillation frequencies, damping rates, and period ratios, which are crucial for coronal seismology.

Contribution

It provides a detailed numerical analysis of the effects of loop inclination and asymmetry on kink oscillations, highlighting their impact on observable parameters used in coronal seismology.

Findings

Inclined and asymmetric loops have lower oscillation frequencies and damping rates.

The period ratio P1/P2 increases with inclination but is less affected by asymmetry.

Mode frequency to damping rate ratios remain approximately unchanged, supporting their use in seismology.

Abstract

This paper separately evaluates the effects of inclination and asymmetry of solar coronal loops on the resonant absorption of kink magnetohydrodynamic (MHD) oscillations. We modelled a typical coronal loop by a straight and axisymmetric cylindrical magnetic flux tube filled with cold plasma. We solved the dispersion relation numerically for different values of the longitudinal mass density stratification. We show that, in inclined and asymmetric loops, the frequencies and their corresponding damping rates of the fundamental and first-overtone modes of kink oscillations are smaller in comparison with semi-circular uninclined loops with the same lengths. The results also indicate that, the period ratio $P_1/P_2$, increases with increasing the inclination of the loop, but it decreases less than $2\%$ while imposing the asymmetry to each loop side, up to $9.66\%$ of the loop length. The ratio of each mode frequency to its corresponding damping rate remain unchanged approximately while the inclination or the asymmetry imposed. Hence, we conclude that these ratios are reliable for inferring the physical parameters of coronal loops and coronal medium, regardless of the loop shape or the state of its inclination. In addition, in contrast with the effect of asymmetry which is not significant on the period ratio $P_1/P_2$, when an observed oscillating loop has a smaller apex height, the state of its inclination is an important factor that should be considered, especially when the period ratio $P_1/P_2$, is taken into consideration for coronal seismology.