Three-dimensional simulation of the fast solar wind driven by compressible magnetohydrodynamic turbulence (1905.11685v3)

Abstract: Using a three-dimensional compressible magnetohydrodynamic (MHD) simulation, we have reproduced the fast solar wind in a direct and self-consistent manner, based on the wave/turbulence driven scenario. As a natural consequence of Alfvenic perturbations at its base, highly compressional and turbulent fluctuations are generated, leading to heating and acceleration of the solar wind. The analysis of power spectra and structure functions reveals that the turbulence is characterized by its imbalanced (in the sense of outward Alfvenic fluctuations) and anisotropic nature. The density fluctuation originates from the parametric decay instability of outwardly propagating Alfven waves and plays a significant role in the Alfven wave reflection that triggers turbulence. Our conclusion is that the fast solar wind is heated and accelerated by compressible MHD turbulence driven by parametric decay instability and resultant Alfven wave reflection.

Three-Dimensional Simulation of the Fast Solar Wind Driven by Compressible Magnetohydrodynamic Turbulence

This paper presents a comprehensive paper employing three-dimensional compressible magnetohydrodynamic (MHD) simulations to reproduce the fast solar wind, elucidating its heating and acceleration mechanisms. The research leverages the wave/turbulence-driven scenario to address longstanding astrophysical questions related to solar wind dynamics and turbulence characterization.

Overview of Methodology and Simulations

The authors conducted their simulations focusing on the fast solar wind emanating from the Sun's polar regions during solar minimum. The MHD equations, inclusive of gravity and thermal conduction, were solved within a spherical coordinate framework to account for compressibility and three-dimensionality—elements critical for capturing the authentic behavior of solar wind turbulence. The initial conditions considered an isothermal Parker wind model with radially extending magnetic fields, while the boundary conditions ensured realistic upward Alfvénic perturbations at the coronal base.

Key Findings

1. Turbulence Characteristics:
   • The solar wind turbulence exhibited imbalanced and anisotropic properties. The imbalance is captured through power spectra disparities between outward and inward Elsässer variables, with outward components demonstrating flatter spectra—indicative of finer structures.
   • Anisotropy in the turbulence is evident, with field-aligned structures being more frequent.
2. Role of Compressibility:
   • Density fluctuations, driven by the parametric decay instability (PDI) of Alfvén waves, play a significant role in causing reflections of anti-Sunward Alfvén waves, thereby enhancing turbulence.
   • These fluctuations contribute critically to the wave reflection process necessary for turbulence activation and ongoing solar wind heating.
3. Wave Dynamics:
   • The simulations successfully reproduced the fast solar wind with radial velocities approximating 600 km/s and temperatures exceeding 10^6 K. Local enhancements in wave dynamics were observed as large-amplitude slow mode waves.

Implications and Future Directions

The simulation outcomes underscore the necessity of considering both compressibility and three-dimensionality in solar wind modeling—challenging simpler models that ignore these factors. This paper advances the understanding of solar wind turbulence, contributing to models of solar corona heating and space weather forecasting by affirming the critical role density fluctuations have in turbulence generation.

Future research, as suggested by the authors, might explore additional models with full Spitzer-Harm conduction and more self-consistent inner boundary conditions accounting for the energy transfer from the chromosphere through the transition region. Such efforts could provide deeper insights into the mass loss rates and other dynamical aspects of solar wind, extending the applicability of these findings across different stellar environments.

The paper provides a robust framework for further numerical experiments and theoretical investigations into magnetohydrodynamic processes governing solar and possibly other astrophysical winds.