Effect of coronal loop structure on wave heating through phase mixing

Abstract

Context. The mechanism(s) behind coronal heating still elude(s) direct observation and modelling of viable theoretical processes and the subsequent effect on coronal structures is one of the key tools available to assess possible heating mechanisms. Wave heating via the phase mixing of magnetohydrodynamic (MHD) transverse waves has been proposed as a possible way to convert magnetic energy into thermal energy, but MHD models increasingly suggest this is not an efficient enough mechanism. Aims. We modelled heating by phase mixing transverse MHD waves in various configurations in order to investigate whether certain circumstances can enhance the heating sufficiently to sustain the million degree solar corona and to assess the impact of the propagation and phase mixing of transverse MHD waves on the structure of the boundary shell of coronal loops. Methods. We used 3D MHD simulations of a pre-existing density enhancement in a magnetised medium and a boundary driver to trigger the propagation of transverse waves with the same power spectrum as measured by the Coronal Multi-Channel Polarimeter. We consider different density structures, boundary conditions at the non-drive footpoint, characteristics of the driver, and different forms of magnetic resistivity. Results. We find that different initial density structures significantly affect the evolution of the boundary shell and that some driver configurations can enhance the heating generated from the dissipation of the MHD waves. In particular, drivers coherent on a larger spatial scale and higher dissipation coefficients can generate significant heating, although it is still insufficient to balance the radiative losses in this setup. Conclusions. We conclude that while phase mixing of transverse MHD waves is unlikely to sustain the thermal structure of the corona, there are configurations that allow for an enhanced efficiency of this mechanism. We provide possible signatures to identify the presence of such configurations, such as the location of where the heating is deposited along the coronal loop.

Weak Damping of Propagating MHD Kink Waves in the Quiescent Corona

“…This potentially rules out the energy pathway of resonant absorption to phase mixing as a viable mechanism for dissipating the kink wave energy in the quiescent corona. The numerical studies of Pagano & Moortel (2017); Pagano & De Moortel (2019); Pagano et al (2020) appear to show that phase mixing is inefficient for heating even when the rate of energy transfer is likely at its largest, i.e., under active region conditions. Moreover, the rate at which small-scales develop due to phase mixing is governed by the density gradient through the inhomogeneous boundary layer.…”

“…The density difference between the internal and external plasma of the coronal structures leads to a gradient in the Alfvén speed across the wave-guide boundary. The spatially-varying Alfvén frequency in the boundary layer enables phase mixing of rotational motions, which are described by Alfvén waves, and hence creates small spatial scales that eventually enables dissipation of the wave energy (e.g., Soler & Terradas 2015;Pagano & Moortel 2017;Pagano et al 2020). Moreover, a resonance is created at the locations where the Alfvén frequencies in the boundary layer match that of the global kink frequencies.…”

AWSoM Magnetohydrodynamic Simulation of a Solar Active Region. II. Statistical Analysis of Alfvén Wave Dissipation and Reflection, Scaling Laws, and Energy Budget on Coronal Loops

“…In addition, they found that nano-flare-like intermittent and localized heating is possible, even with just the wave heating mechanism, through collisions between counter-propagating Alfvén wave packets. Some of the other recent simulations with coronal heating in ARs include those of Berger & AsgariTarghi (2009), Antolin & Shibata (2010), Bingert & Peter (2011), AsgariTarghi & van Ballegooijen (2012), Hansteen et al (2015), Reep et al (2018), Cheung et al (2019), Guo et al (2019), Pagano et al (2020), Shi et al (2021), Breu et al (2022), andSarp Yalim et al (2023). Reviews of wave heating mechanisms, as well as recent progress in both observation and numerical models are available in Arregui (2015) and Van Doorsselaere et al (2020).…”

How Transverse Waves Drive Turbulence in the Solar Corona

“…Further numerical simulations of propagating transverse waves excited by continuous boundary motions also show that wave driving can significantly disrupt a pre-existing density structure [162]. In this case, the evolution of the density profile permits wave energy dissipation over a large cross-sectional area, and not just the initial boundary of the loop.…”

“…This is a positive result in view of the criticism outlined in [163], that wave heating is not able to dissipate energy throughout the loop nor sustain the assumed density profile. However, whilst the heating in such simulations can be significant for some configurations, it has not been able to balance radiative losses in a dense flux tube [50,162], even for highly enhanced dissipation coefficients. Further, it remains unclear whether similar wave driving can support any form of density structure in a fully stratified atmosphere (e.g., [163,164]).…”