Resonant absorption of kink magnetohydrodynamic waves by a magnetic twist in coronal loops

Abstract

There is ample evidence of twisted magnetic structures in the solar corona. This motivates us to consider the magnetic twist as the cause of Alfvén frequency continuum in the coronal loops, which can support the resonant absorption as a rapid damping mechanism for the observed coronal kink magnetohydrodynamic (MHD) oscillations. We model a coronal loop with a straight cylindrical magnetic flux tube which has constant but different densities in the interior and exterior regions. The magnetic field is assumed to be constant and aligned with the cylinder axis everywhere except a thin layer near the boundary of the flux tube which has an additional small magnetic field twist. Then, we investigate a number of possible instabilities that may arise in our model. In the thin tube thin boundary approximation, we derive the dispersion relation and solve it analytically to obtain the frequencies and damping rates of the fundamental (l = 1) and first/second overtone (l = 2, 3) kink (m = 1) MHD modes. We conclude that the resonant absorption by the magnetic twist can justify the rapid damping of kink MHD waves observed in coronal loops. Furthermore, the magnetic twist in the inhomogeneous layer can cause deviations from P 1 /P 2 = 2 and P 1 /P 3 = 3 which are comparable with the observations.

Observations of Excitation and Damping of Transversal Oscillations in Coronal Loops by AIA/SDO

“…This result shows that oscillation in a coronal loop cannot be excited by a low-speed shock wave of flare, since the values of the Alfvén speed are mostly more than 800 kms −1 (see, e.g. Nakariakov and Ofman 2001;Stepanov et al 2012;De Moortel and Nakariakov 2012;Zimovets and Nakariakov 2015). Table 1.…”

“…A large number of theoretical studies investigating the damping time of the transverse oscillations in a coronal loop have focused on the effects of phase mixing, resonant absorption, gravitational stratification, magnetic field divergence (see, e.g. Aschwanden et al 2003;Safari et al 2007;Ebrahimi and Karami 2016). In general, phase mixing and resonant absorption are shown to be the main physical mechanisms for damping of the standing transverse oscillations of coronal loops.…”

Resonant absorption of kink oscillations in coronal flux tubes with continuous magnetic twist

“…• As stated by Aschwanden et al (2003), the typical values of the density ratio ζ for coronal flux tubes are in the range [2,10]. Here, we take the extreme values ζ = 2 and ζ = 10 in our calculations.…”

The Effect of Weak Magnetic Twist on Resonant Absorption of Slow Sausage Waves in Magnetic Flux Tubes

“…They showed that the period ratio P 1 /P 2 of the fundamental and its first-overtone surface waves for kink (m = 1) and fluting (m = 2, 3) modes is lower than 2 in the presence of twisted magnetic field. Ebrahimi & Karami [18] investigated resonant absorption of kink MHD waves by a magnetic twist in coronal loops. They concluded that the resonant absorption by the magnetic twist can justify the rapid damping of kink MHD waves observed in coronal loops.…”