Comparison of the thin flux tube approximation with 3D MHD simulations

TL;DR

This paper compares the traditional thin flux tube approximation with 3D MHD simulations to evaluate its accuracy in modeling small magnetic flux concentrations in the solar photosphere.

Contribution

It provides a systematic comparison between the thin flux tube/sheet models and realistic 3D MHD simulations, assessing the approximation's validity.

Findings

Super-equipartition flux concentrations are well modeled by the approximation.

Differences are mainly due to asymmetry and dynamics of the structures.

The approximation is justified for small flux tubes with certain magnetic field strengths.

Abstract

The structure and dynamics of small vertical photospheric magnetic flux concentrations has been often treated in the framework of an approximation based upon a low-order truncation of the Taylor expansions of all quantities in the horizontal direction, together with the assumption of instantaneous total pressure balance at the boundary to the non-magnetic external medium. Formally, such an approximation is justified if the diameter of the structure (a flux tube or a flux sheet) is small compared to all other relevant length scales (scale height, radius of curvature, wavelength, etc.). The advent of realistic 3D radiative MHD simulations opens the possibility of checking the consistency of the approximation with the properties of the flux concentrations that form in the course of a simulation. We carry out a comparative analysis between the thin flux tube/sheet models and flux concentrations formed in a 3D radiation-MHD simulation. We compare the distribution of the vertical and horizontal components of the magnetic field in a 3D MHD simulation with the field distribution in the case of the thin flux tube/sheet approximation. We also consider the total (gas plus magnetic) pressure in the MHD simulation box. Flux concentrations with super-equipartition fields are reasonably well reproduced by the second-order thin flux tube/sheet approximation. The differences between approximation and simulation are due to the asymmetry and the dynamics of the simulated structures.