The effect of thermal misbalance on magnetohydrodynamic modes in coronal magnetic cylinders

TL;DR

This paper explores how thermal misbalance affects magnetohydrodynamic wave dispersion in coronal magnetic cylinders, revealing minor effects on fast modes but significant impacts on slow modes, especially at shorter wavelengths.

Contribution

It provides a detailed analysis of thermal misbalance effects on MHD wave dispersion in cylindrical solar coronal structures, including analytical expressions for frequency shifts.

Findings

Thermal misbalance slightly increases damping rates of fast sausage and kink modes at short wavelengths.

Significantly shifts the cusp frequency of slow magnetoacoustic modes, especially at small wavenumbers.

Analytical expressions accurately describe frequency shifts due to thermal misbalance, matching numerical and observational data.

Abstract

This study investigates the dispersion of magnetohydrodynamic waves influenced by thermal misbalance in a cylindrical configuration with a finite axial magnetic field within solar coronal plasmas. Specifically, it examines how thermal misbalance, characterized by two distinct timescales directly linked to the cooling and heating functions, influences the dispersion relation. This investigation is a key approach for understanding non-adiabatic effects on the behaviour of these waves. Our findings reveal that the effect of thermal misbalance on fast sausage and kink modes, consistent with previous studies on slabs, is small but slightly more pronounced than previously thought. The impact is smaller at long-wavelength limits but increases at shorter wavelengths, leading to higher damping rates. This minor effect on fast modes occurs despite the complex interaction of thermal misbalance terms within the dispersion relation, even at low-frequency limits defined by the characteristic timescales. Additionally, a very small amplification is observed, indicating a suppressed damping state for the long-wavelength fundamental fast kink mode. In contrast, slow magnetoacoustic modes are significantly affected by thermal misbalance, with the cusp frequency shifting slightly to lower values, which is significant for smaller longitudinal wavenumbers. This thermal misbalance likely accounts for the substantial attenuation observed in the propagation of slow magnetoacoustic waves within the solar atmosphere. The long-wavelength limit leads to an analytical expression that accurately describes the frequency shifts in slow modes due to misbalance, closely aligning with both numerical and observational results.