Solar Vortices: Catalysts of Magnetoacoustic Wave Dissipation and Atmospheric Heating

Technical Summary: Solar Vortices as Catalysts of Magnetoacoustic Wave Dissipation and Atmospheric Heating

Problem Statement
A persistent challenge in solar physics is understanding the mechanisms that transport energy and momentum from the solar photosphere to the chromosphere and corona. While Magnetohydrodynamic (MHD) waves are widely recognized as fundamental carriers of this energy, the specific influence of photospheric vortex flows on the excitation, propagation, and dissipation of these waves remains insufficiently constrained. Specifically, the extent to which rotational flows modify the thermal structure of the lower solar atmosphere and the dynamics of slow magnetoacoustic waves—particularly their transition into shocks—requires systematic quantification.

Methodology
The authors employed high-resolution, three-dimensional radiative MHD simulations using the MURaM code to model a strong plage region with a mean vertical magnetic field of 200 G. The simulation domain covered 12 Mm × 12 Mm horizontally and 4 Mm vertically, resolving the upper convection zone, photosphere, and chromosphere with a 10 km grid spacing.

Key methodological approaches included:

• Field-Line Tracking: Seed points were placed at the photospheric surface ($z=0$) within magnetically concentrated regions. Magnetic field lines were traced dynamically through a 20-minute time series, advecting seed points with the local velocity field to follow the evolution of plasma parcels and wave propagation along the actual magnetic topology.
• Vortex Identification: Photospheric vortices were identified using the swirling strength diagnostic ($\lambda_v$) derived from the 3D velocity field. To capture coherent, expanding vortex structures while minimizing noise from small-scale turbulence, the velocity field was smoothed using a height-dependent Gaussian convolution.
• Comparative Analysis: The study systematically compared thermodynamic and dynamic variables (temperature, heating rates, parallel velocity) along magnetic field lines rooted in identified vortex regions versus non-vortex regions.
• Shock Diagnostics: Shock formation was tracked by analyzing the height-time evolution of the parallel Mach number ($M_\parallel$) and the gradient of the parallel velocity. Shock thickness was estimated using the ratio of velocity jump to maximum velocity gradient.

Key Results

1. Wave Evolution and Shock Formation: The simulations confirmed that slow magnetoacoustic waves originate in the photosphere with small amplitudes and propagate upward along magnetic field lines. Due to atmospheric stratification and decreasing density, wave amplitudes amplified significantly, leading to nonlinear steepening and the formation of shocks in the mid-to-upper chromosphere ($z' \gtrsim 1.3$–2 Mm). These shocks drove recurrent, quasi-ballistic plasma surges with a ~5-minute periodicity, consistent with solar p-mode oscillations.
2. Vortex Influence on Temperature: Magnetic field lines rooted in vortex regions exhibited a systematically enhanced temperature compared to non-vortex lines. While explicit viscous and Joule heating rates showed enhancement only at lower heights (0.2–1.2 Mm), the temperature elevation persisted into the upper chromosphere. The authors attribute this to the cumulative thermal history of the plasma, including advection of heated material and compressive heating from shock entropy production, rather than solely instantaneous local dissipation.
3. Shock Formation Height: A comparison of shock formation heights between vortex and non-vortex regions revealed no systematic difference. In both populations, disturbances became supersonic at an average height of approximately 1.5 Mm above the photosphere. This indicates that rotational flows do not significantly alter the altitude at which shocks form.
4. Shock Velocity Amplification: Although the formation height remained unchanged, supersonic upflows at vortex locations exhibited somewhat higher parallel velocities compared to non-vortex regions. This suggests that vortex-driven motions may amplify the velocity of propagating shocks, even if they do not fundamentally change the shock generation mechanism.

Significance and Claims
The paper claims that photospheric vortex flows play a coupled role with slow-mode shocks in the energy transport and heating of the lower solar atmosphere. The findings indicate that while vortices do not shift the height of shock formation, they contribute to:

• Enhanced Thermal Structure: Vortex regions sustain higher temperatures, likely due to more efficient channeling and amplification of wave energy, leading to stronger nonlinear steepening and greater conversion of wave energy into heat.
• Dynamical Modification: Rotational flows may modestly amplify the velocity of supersonic disturbances.

The authors emphasize that these results provide predictive diagnostics for forthcoming high-resolution observations, such as those from the Daniel K. Inouye Solar Telescope (DKIST). By bridging numerical modeling with high-resolution solar measurements, the study highlights the necessity of considering the interplay between local magnetic topology, vortex flows, and wave dissipation to fully understand chromospheric heating. The authors remain modest in their conclusions, noting that the specific mechanisms driving the elevated temperatures (e.g., adiabatic compression vs. radiative trapping) require further quantitative investigation, and that the shock thickness diagnostics used are effective measures of steepened structures rather than strict physical thicknesses of ideal shocks.