Coronal mass ejections as expanding force-free structures

TL;DR

This paper models solar coronal mass ejections as expanding force-free magnetic structures, deriving self-similar solutions in spherical and cylindrical geometries, and predicts their internal dynamics and magnetic field evolution.

Contribution

It introduces a novel self-similar model of CMEs as expanding force-free structures with analytical solutions in spherical and cylindrical geometries.

Findings

Structures depend only on radius and expansion rate.

Maximum magnetic field occurs before reaching the CME center.

Expansion requires equal rates in axial and radial directions for cylindrical fields.

Abstract

We mode Solar coronal mass ejections (CMEs) as expanding force-fee magnetic structures and find the self-similar dynamics of configurations with spatially constant \alpha, where {\bf J} =\alpha {\bf B}, in spherical and cylindrical geometries, expanding spheromaks and expanding Lundquist fields correspondingly. The field structures remain force-free, under the conventional non-relativistic assumption that the dynamical effects of the inductive electric fields can be neglected. While keeping the internal magnetic field structure of the stationary solutions, expansion leads to complicated internal velocities and rotation, induced by inductive electric field. The structures depends only on overall radius R(t) and rate of expansion \dot{R}(t) measured at a given moment, and thus are applicable to arbitrary expansion laws. In case of cylindrical Lundquist fields, the flux conservation requires that both axial and radial expansion proceed with equal rates. In accordance with observations, the model predicts that the maximum magnetic field is reached before the spacecraft reaches the geometric center of a CME.