Turbulent Heating between 0.2 and 1 au: A Numerical Study

TL;DR

This study uses 3D MHD simulations with the expanding box model to demonstrate that turbulent dissipation can produce the observed 1/R radial temperature decrease in the solar wind between 0.2 and 1 AU.

Contribution

It shows that MHD turbulence simulations with specific initial conditions can reproduce the solar wind's temperature profile, advancing understanding of turbulent heating mechanisms.

Findings

Radial temperature profiles close to 1/R were obtained.

Turbulence starting at 0.2 AU with Mach number 1 can produce observed heating.

Limited inertial range due to simulation constraints.

Abstract

The heating of the solar wind is a key to understand its dynamics and acceleration process. The observed radial decrease of proton temperature in the solar wind is slow compared to the adiabatic prediction and it is thought to be caused by turbulent dissipation. To generate the observed 1/R decrease, the dissipation rate has to reach a specific level which varies in turn with temperature, wind speed, and heliocentric distance. We want to prove that MHD turbulent simulations can lead to the 1/R profile. We consider here the slow solar wind, characterized by a quasi-2D spectral anisotropy. We use the EBM (expanding box model) equations, which incorporate into 3D MHD equations the expansion due to the mean radial wind, allowing to follow the plasma evolution between 0.2 and 1 AU. We vary the initial parameters which are: Mach number, expansion parameter, plasma beta, and properties of the energy spectrum as the spectral range and slope. Assuming turbulence starts at 0.2 AU with a Mach number equal to unity, with a 3D spectrum mainly perpendicular to the mean field, we find radial temperature profiles close to 1/R in average. This is done at the price of limiting the initial spectral extent, corresponding to the small number of modes in the inertial range available, due to the modest Reynolds number reachable with high Mach numbers.