Nonlinear Processes in Coronal Heating and Slow Solar Wind Acceleration

TL;DR

This paper investigates coronal heating and slow solar wind acceleration through 3D MHD simulations, revealing turbulence-driven heating mechanisms and magnetic island formation consistent with LASCO observations.

Contribution

It presents new scalings of physical quantities in coronal loops and models the formation of density enhancements in the slow solar wind with improved theoretical insights.

Findings

Turbulence makes dissipation rate independent of Reynolds number.

Magnetic islands form within streamers, matching observed density enhancements.

Critical magnetic field divergence distance constrains magnetic topology.

Abstract

This work consists of two parts: the first devoted to the study of the heating of the magnetically confined Solar Corona, and the second to the acceleration of the Slow Solar Wind. Direct 3D reduced MHD simulations are presented. They model the heating of coronal loops in the solar atmosphere via the tangling of coronal field lines by photospheric footpoints motions within the framework of the "Parker scenario". We have derived scalings of physical quantities with loop length, and the ratio of photospheric to coronal Alfven velocities. The development of a turbulent dynamics makes the dissipation rate independent of the Reynolds number. The dynamics in physical space are desribed by weak turbulence, which develops when an MHD system is embedded in a strong axial magnetic field. The slow wind originates in and around the coronal streamer belt. The LASCO instrument onboard the SOHO spacecraft has observed plasma density enhancements forming beyond the cusp of a helmet streamer. Previous theoretical models for the formation and initial motion of these density enhancements are improved. The average expansion suffered by a parcel of plasma propagating outward, and the diamagnetic force due to the overall magnetic field radial gradients are now included. It is found that the magnetized wake configuration is resistively unstable, that an outward accelerating magnetic island develops at the center of the streamer, and that density enhancements occur within the magnetic islands. The values of the acceleration and density contrasts can be in good agreement with LASCO observations, provided the spherical divergence of the magnetic lines starts beyond a critical distance from the Sun. This result provides a constraint on the topology of the magnetic field.