Magnetic clouds in the solar wind: A numerical assessment study of analytical models

TL;DR

This study uses 3D magnetohydrodynamic simulations to evaluate the validity of analytical models of magnetic clouds in the solar wind, revealing their limitations based on velocity and rotation effects.

Contribution

It provides a numerical assessment of an analytical magnetic cloud model, identifying conditions where the model's assumptions break down.

Findings

Analytical models are valid mainly when MCs move with solar wind velocity.

MCs exhibit different evolution when moving faster or slower than the background flow.

Rotation of MCs influences their dynamics and evolution.

Abstract

Magnetic clouds (MCs) are "magnetized plasma clouds" moving in the solar wind. MCs transport magnetic flux and helicity away from the Sun. These structures are not stationary but feature temporal evolution as they propagate in the solar wind. Simplified analytical models are frequently used for the description of MCs, and fit certain observational data well. The goal of the present study is to investigate numerically the validity of an analytical model which is widely used for the description of MCs, and to determine under which conditions this model's implied assumptions cease to be valid. A numerical approach is applied. Analytical solutions that have been derived in previous studies are implemented in a \textbf{3-D magnetohydrodynamic} simulation code as initial conditions. Initially, the analytical model represents the main observational features of the MCs. However, these characteristics prevail only if the structure moves with a velocity close to the velocity of the background flow. In this case an MC's evolution can quite accurately be described using an analytic, self-similar approach. The dynamics of the magnetic structures which move with a velocity significantly above or below that of the velocity of the solar wind is investigated in detail. Besides the standard case in which MCs only expand and propagate in the solar wind, the case of an MC rotating around its axis of symmetry is also considered, and the resulting influence on the MC's dynamics is studied.