Uniturbulence and Alfvén Wave Solar Model in MPI-AMRVAC

TL;DR

This work introduces UAWSoM, a MPI-AMRVAC module that couples Alfvén and kink wave energy to the MHD equations to study wave-driven heating in a transversely structured solar atmosphere. By representing wave energy with Elsässer and Q-variables and incorporating radiative losses and anisotropic conduction, the authors demonstrate that kink-wave uniturbulence can sustain coronal-like temperatures in 1D without arbitrary background heating, while Alfvén-wave heating alone is insufficient and can destabilize the atmosphere under strong reflection. Cross-validation against a Python implementation confirms the overall kink-dominated heating behavior, though boundary-condition treatments introduce quantitative differences. Through parameter studies on Alfvén reflection and energy partition, the results highlight the higher heating efficiency of kink waves under realistic conditions and suggest a hybrid heating scenario may be viable if kink energy is sufficiently large. The findings offer a potential physical basis for reducing ad hoc heating terms in AWSoM-type models and motivate future 3D global simulations with forward-modeling to connect with solar observations.

Abstract

The coronal heating problem remains a fundamental challenge in solar physics. While AWSoM-type models (Alfvén Wave Solar Model) have proven highly successful in reproducing the large-scale structure of the solar corona, they inherently neglect contributions from additional wave modes that arise when the effects of transverse structuring is fully incorporated into the magnetohydrodynamic (MHD) equations. In this paper, we compare the roles of kink wave- and Alfvén wave-driven heating in sustaining a region of the solar atmosphere, using newly developed physics and radiative cooling modules within MPI-AMRVAC. We extend the existing MHD physics module in MPI-AMRVAC by incorporating additional Alfvén and kink wave energy contributions to the MHD equations. We examine their roles in heating the solar atmosphere and driving the solar wind. To validate our approach, we compare numerical results from Python-based simulations with those obtained using the UAWSoM module in MPI-AMRVAC. Furthermore, we assess the heating efficiency of kink waves relative to that of pure Alfvén waves through two parameter studies: (1) exploring how different Alfvén wave reflection rates impact the simulated atmosphere, and (2) varying the relative magnitudes of Alfvén and kink wave energy injections. Finally, we present results from a larger-scale domain, sustained entirely by kink wave-driven heating. Our results show that kink wave-driven (UAWSoM) models are able to sustain a stable atmosphere without requiring any artificial background heating terms, unlike traditional Alfvén-only models. We attribute this to the increased heating rate associated with kink waves compared with Alfvén waves, given the same energy injection. Kink waves can sustain a model plasma with temperature and density values representative of coronal conditions without resorting to ad hoc heating terms.

Figures (10)

• Figure 1: The initial profile of outward propagating kink wave energy is shown as a function of height over the $100$ Mm domain. Beyond $30$ Mm, the initial wave energy is assumed to be constant.
• Figure 2: From the top panel, working downwards, we present the results obtained from the new module UAWSoM in MPI-AMRVAC for the outward propagating kink wave energy ($\mathrm{J\ m^{-3}}$), temperature (in units MK), the velocity (in km s$^{-1}$) and the plasma density (in units g cm$^{-3}$) as functions of time (seconds) and space, $X$ (Mm). The left-hand column represents the results obtained from the MPI-AMRVAC implementation of UAWSoM, while the data presented in the right-hand column is obtained from the Python implementation.
• Figure 3: MPI-AMRVAC simulation as prescribed in Figure \ref{['fig:Py_AMRVAC_comp']} but now the simulation is allowed to run for much longer. We consider this to be a qausi-steady state. The quantities shown are the outward propagating kink wave energy ($\mathrm{J\ m^{-3}}$), temperature (in units MK), the velocity (in km s$^{-1}$) and the plasma density (in units g cm$^{-3}$) as functions of time (seconds) and space, $X$ (Mm).
• Figure 4: Temperature (in MK given by various line styles and colors) and velocity (in km s$^{-1}$ given by various thickness and color solid lines) are given as a function of height (Mm) plotted at time $t = 1800$ s, where the reflection coefficient ($\sigma$) corresponding to Alfvén wave energy reflection is varied. The reflection coefficients ($\sigma$) are 5 (pink), 1 (red), 0.5 (orange), 0.2 (green), 0.1 (blue) and 0 (black) that leads to no Alfvén wave heating. The various thicknesses of lines refer to the velocity, and the various line styles refer to the temperature.
• Figure 5: The heating rates (given in W m$^{-3}$) corresponding to Alfvén and kink waves given the same energy injection are shown as functions of height for the various reflection rates considered. The reflection coefficients ($\sigma$) are 5 (pink), 1 (red), 0.5 (orange), 0.2 (green), 0.1 (blue) and 0 (black) that leads to no Alfvén wave heating. The various thicknesses of lines refer to the Alfvén wave heating term, and the various line styles refer to the kink wave heating terms.