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

¹ Institut für Weltraum- und Astrophysik, Ruhr-Universität Bochum, Germany
e-mail: giorgi@tp4.rub.de; jk@tp4.rub.de; hf@tp4.rub.de
² Centre for Plasma Astrophyics, Katholieke Universiteit Leuven, Belgium
e-mail: Stefaan.Poedts@wis.kuleuven.be
³ Center for Theoretical Astrophysics, Institute of Theoretical Physics, Ilia State University, Tbilisi, Georgia

Received: 24 July 2010
Accepted: 13 September 2011

Abstract

Context. 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 to describe MCs, and they fit certain observational data well.

Aims. The goal of the present study is to numerically investigate the validity of an analytical model that is widely used to describe MCs, and to determine under which conditions this model’s implied assumptions cease to be valid.

Methods. A numerical approach is applied. Analytical solutions derived in previous studies are implemented in a 3D magnetohydrodynamic simulation code as initial conditions. 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.

Results. 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 that move with a velocity significantly above or below that of the velocity of the solar wind is investigated in detail.

Conclusions. Comparison of the numerical results with observational data indicates reasonable agreement especially for the intermediate case, in which the MC’s bulk velocity and the velocity of the background flow are equal. In this particular case, analytical solutions obtained on the basis of a self-similar approach indeed describe the MC’s evolution quite accurately. In general, however, numerical simulations are necessary to investigate the evolution as a function of a wide range of the parameters, which define the initial conditions.