Alexander Soenen

Modelling the initiation of coronal mass ejections: magnetic flux emergence versus shearing motions

Context. Coronal mass ejections (CMEs) are enormous expulsions of magnetic flux and plasma from the solar corona into the interplanetary space. These phenomena release a huge amount of energy. It is generally accepted that both photospheric motions and the emergence of new magnetic flux from below the photosphere can put stress on the system and eventually cause a loss of equilibrium resulting in an eruption. Aims. By means of numerical simulations we investigate both emergence of magnetic flux and shearing motions along the magnetic inversion line as possible driver mechanisms for CMEs. The pre-eruptive region consists of three arcades with alternating magnetic flux polarity, favouring the breakout mechanism. Methods. The equations of ideal magnetohydrodynamics (MHD) were advanced in time by using a finite volume approach and solved in spherical geometry. The simulation domain covers a meridional plane and reaches from the lower solar corona up to 30 

Initiation of Coronal Mass Ejections by Magnetic Flux Emergence in the Framework of the Breakout Model

R_\odot

Numerical simulations of homologous coronal mass ejections in the solar wind

. When we applied time-dependent boundary conditions at the inner boundary, the central arcade of the multiflux system expands, leading to the eventual eruption of the top of the helmet streamer. We compare the topological and dynamical evolution of the system when driven by the different boundary conditions. The available free magnetic energy and the possible role of magnetic helicity in the onset of the CME are investigated. Results. In our simulation setup, both driving mechanisms result in a slow CME. Independent of the driving mechanism, the overall evolution of the system is the same: the actual CME is the detatched helmet streamer. However, the evolution of the central arcade is different in the two cases. The central arcade eventually becomes a flux rope in the shearing case, whereas in the flux emergence case there is no formation of a flux rope. Furthermore, we conclude that magnetic helicity is not crucial to a solar eruption.

Magnetic helicity and active filament configuration

The possible role of magnetic flux emergence in the initiation of coronal mass ejections (CMEs) is investigated in the framework of the breakout model. The ideal MHD equations are solved numerically on a spherical, axisymmetric (2.5-dimensional) domain. An initial multiflux system in steady equilibrium containing a pre-eruptive region consisting of three arcades with alternating magnetic flux polarity is kept in place by the magnetic tension of the overlying closed magnetic field of a helmet streamer. The emergence of new magnetic flux in the central arcade is simulated by means of a time-dependent boundary condition on the vector potential applied at the solar base. Height-time plots of the ejected material, as well as time evolution of the magnetic, kinetic and internal energy in the entire domain as functions of flux emergence rate, are produced. The results show that the emergence of new magnetic flux in the central arcade triggers a CME. The obtained eruption corresponds to a slow CME, and conversion of magnetic energy into kinetic energy is observed.

Magnetic Flux Emergence and Shearing Motions as Trigger Mechanisms for Coronal Mass Ejections

Context. Coronal mass ejections (CMEs) are enormous expulsions of magnetic flux and plasma from the solar corona. Most scientists agree that a coronal mass ejection is the sudden release of magnetic free energy stored in a strongly stressed field. However, the exact reason for this sudden release is still highly debated. Aims. In an initial multiflux system in steady state equilibrium, containing a pre-eruptive region consisting of three arcades with alternating magnetic flux polarity, we study the initiation and early evolution properties of a sequence of CMEs by shearing a region slightly larger than the central arcade. Methods. We solve the ideal magnetohydrodynamics (MHD) equations in an axisymmetrical domain from the solar surface up to 30 R-circle dot. The ideal MHD equations are advanced in time over a non uniform grid using a modified version of the Versatile Advection Code (VAC). Results. By applying shearing motions on the solar surface, the magnetic field is energised and multiple eruptions are obtained. Magnetic reconnection first opens the overlying field and two new reconnections sites set in on either side of the central arcade. After the disconnection of the large helmet top, the system starts to restore itself but cannot return to its original configuration as a new arcade has already started to erupt. This process then repeats itself as we continue shearing. Conclusions. The simulations reported in the present paper, demonstrate the ability to obtain a sequence of CMEs by shearing a large region of the central arcade or by shearing a region that is only slightly larger than the central arcade. We show, be it in an axisymmetric configuration, that the breakout model can not only lead to confined eruptions but also to actual coronal mass ejections provided the model includes a realistic solar wind model.

Magnetic flux emergence and shearing motions as CME trigger mechanisms

Context. The role of magnetic helicity in active filament formation and destabilization is still under debate. Aims. Although active filaments usually show a sigmoid shape and a twisted configuration before and during their eruption, it is unclear which mechanism leads to these topologies. In order to provide an observational contribution to clarify these issues, we describe a filament evolution whose characteristics seem to be directly linked to the magnetic helicity transport in corona. Methods. We applied different methods to determine the helicity sign and the chirality of the filament magnetic field. We also computed the magnetic helicity transport rate at the filament footpoints. Results. All the observational signatures provided information on the positive helicity and sinistral chirality of the flux rope containing the filament material: its forward S shape, the orientation of its barbs, the bright and dark threads at 195 A. Moreover, the magnetic helicity transport rate at the filament footpoints showed a clear accumulation of positive helicity. Conclusions. The study of this event showed a correspondence between several signatures of the sinistral chirality of the filament and several evidences of the positive magnetic helicity of the filament magnetic field. We also found that the magnetic helicity transported along the filament footpoints showed an increase just before the change of the filament shape observed in Hα images. We argued that the photospheric regions where the filament was rooted might be the preferential ways where the magnetic helicity was injected along the filament itself and where the conditions to trigger the eruption were yielded.

SIDE MAGNETIC RECONNECTIONS INDUCED BY CORONAL MASS EJECTIONS: OBSERVATIONS AND SIMULATIONS

We study the initiation and early evolution of coronal mass ejections (CMEs) in the framework of numerical ideal magnetohydrodynamics (MHD). The magnetic field of the active region possesses a topology in order for the “breakout” model to work. An initial multi‐flux system in steady equilibrium containing a pre‐eruptive region consisting of three arcades with alternating flux polarity is kept in place by the magnetic tension of the overlying closed magnetic field of the helmet streamer. Both foot point shearing and magnetic flux emergence are used as a triggering mechanism in this model. The boundary conditions cause the central arcade to expand and lead to the eventual ejection of the top of the helmet streamer. We compare the topological and dynamical evolution of the two triggering mechanisms and find that the overall evolution of the systems are similar.

Observational and numerical study of the 25 July 2004 event

We present recent developments in the mathematical modeling and numerical simulations of the initiation and interplanetary evolution of CMEs in the framework of ideal magneto‐hydrodynamics (MHD). In earlier work, we reconstructed simple, axisymmetric (2.5D) solar wind models for the quiet Sun. Next, we mimicked fast CME events by superposing high‐density plasma blobs on the background wind and launching them in a given direction at a certain speed, enabling the study of the evolution of the fast CME shocks, their effects on the coronal field and background solar wind. Here, more realistic CME onset models are presented to investigate the possible role of magnetic foot point shearing and magnetic flux emergence/disppearence as triggering mechanisms of the instability. In particular, the well‐known breakout model has been superposed on a solar wind model and it is shown that both foot point shearing and magnetic flux emergence can be used as a triggering mechanism in this model.