Processes and mechanisms governing the initiation and propagation of CMEs

B Vršnak

doi:10.5194/angeo-26-3089-2008

Abstract

Abstract. The most important observational characteristics of coronal mass ejections (CMEs) are summarized, emphasizing those aspects which are relevant for testing physical concepts employed to explain the CME take-off and propagation. In particular, the kinematics, scalings, and the CME-flare relationship are stressed. Special attention is paid to 3-dimensional (3-D) topology of the magnetic field structures, particularly to aspects related to the concept of semi-toroidal flux-rope anchored at both ends in the dense photosphere and embedded in the coronal magnetic arcade. Observations are compared with physical principles and concepts employed in explaining the CME phenomenon, and implications are discussed. A simple flux-rope model is used to explain various stages of the eruption. The model is able to reproduce all basic observational requirements: stable equilibrium and possible oscillations around equilibrium, metastable state and possible destabilization by an external disturbance, pre-eruptive gradual-rise until loss of equilibrium, possibility of fallback events and failed eruptions, relationship between impulsiveness of the CME acceleration and the source-region size, etc. However, it is shown that the purely ideal MHD process cannot account for highest observed accelerations which can attain values up to 10 km s−2. Such accelerations can be achieved if the process of reconnection beneath the erupting flux-rope is included into the model. Essentially, the role of reconnection is in changing the magnetic flux associated with the flux-rope current and supplying "fresh" poloidal magnetic flux to the rope. These effects help sustain the electric current flowing along the flux-rope, and consequently, reinforce and prolong the CME acceleration. The model straightforwardly explains the observed synchronization of the flare impulsive phase and the CME main-acceleration stage, as well as the correlations between various CME and flare parameters.

Highlights

Coupling of the solar differential rotation, convective motions, and magnetic field results in magnetohydrodynamic (MHD) dynamo processes at different scales
It is worth noting that solar eclipse observations have revealed an analogous prominence/corona structure: the prominence is usually found in a coronal “void” enclosed by the coronal arch that is surmounted by the coronal streamer (Engvold, 1987; Koutchmy et al, 2004, and references therein). Such a prominence-corona structure, sometimes observed prior to the coronal mass ejections (CMEs) take-off by soft X-ray imaging instruments (e.g., Hudson et al, 1999, and references therein) and white-light coronagraphs, (Gibson, 2008; Burkepile, 2008), indicates that the basic CME morphology has its roots in the pre-eruption magnetic field configuration (Low, 1996, and references therein)
White light coronagraphic observations frequently show patterns consistent with the interpretation of CMEs in terms of flux-rope eruption (e.g., Krall, 2007, and references therein). Another line of evidence can be found in the in situ measurements, showing that interplanetary CMEs (ICMEs) frequently have flux-rope characteristics

Summary

Introduction

Coupling of the solar differential rotation, convective motions, and magnetic field results in magnetohydrodynamic (MHD) dynamo processes at different scales. Shearing and twisting motions induce electric currents, storing free energy into current-carrying magnetic field structures. A part of this energy is transferred through the solar surface into the corona, where it is partly spent for the coronal heating and partly released in eruptive processes, taking the form of coronal mass ejections (CMEs) and/or solar flares (cf., Priest, 1982). Eruptions start with a phase of a slow rise, most often seen in measurements of the associated prominence eruption. In this gradual pre-eruption stage, typical velocity of the eruptive prominence is in the order of 10 km s−1 (e.g., Rompolt, 1990). The overlying coronal arches have several times larger velocities (e.g., Maricicet al., 2004)

Objectives

Findings

Discussion

Conclusion