Resonant Absorption of Fast Magnetoacoustic Waves due to Coupling into the Slow and Alfven Continua in the Solar Atmosphere

TL;DR

This study investigates how fast magnetoacoustic waves are absorbed through resonances in the solar atmosphere, revealing the influence of wave angles and magnetic field inclination on energy absorption efficiency.

Contribution

It introduces the concept of coupled resonances affecting wave absorption and provides numerical analysis of absorption coefficients at slow and Alfven resonances under solar conditions.

Findings

Efficient wave absorption at Alfven resonance in the corona.

Wave absorption depends on incident wave angle and magnetic field inclination.

Coupled resonances influence absorption efficiency, especially in chromospheric conditions.

Abstract

Resonant absorption of fast magnetoacoustic (FMA) waves in an inhomogeneous, weakly dissipative, one-dimensional planar, strongly anisotropic and dispersive plasma is investigated. The magnetic configuration consists of an inhomogeneous magnetic slab sandwiched between two regions of semi-infinite homogeneous magnetic plasmas. Laterally driven FMA waves penetrate the inhomogeneous slab interacting with the localised slow or Alfven waves present in the inhomogeneous layer and are partly reflected, dissipated and transmitted by this region. The presented research aims to find the coefficient of wave energy absorption under solar chromospheric and coronal conditions. Numerical results are analyzed to find the coefficient of wave energy absorption at both the slow and Alfven resonance positions. The mathematical derivations are based on the two simplifying assumptions that (i) nonlinearity is weak, and (ii) the thickness of the inhomogeneous layer is small in comparison to the wavelength of the wave, i.e. we employ the so-called long wavelength approximation. Slow resonance is found to be described by the nonlinear theory, while the dynamics at the Alfven resonance can be described within the linear framework. We introduce a new concept of coupled resonances, which occurs when two different resonances are in close proximity to each other, causing the incoming wave to act as though it has been influenced by the two resonances simultaneously. Our results show that the wave energy absorption is heavily dependent on the angle of the incident wave in combination with the inclination angle of the equilibrium magnetic field. In addition, it is found that FMA waves are very efficiently absorbed at the Alfven resonance under coronal conditions. Under chromospheric conditions the FMA waves are far less efficiently absorbed, despite an increase in efficiency due to the coupled resonances.