Solar and Magnetospheric MHD Theory Group - University of St Andrews


Eruption of a flux rope driven by flux emergence


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We have studied the formation of flux ropes in a flux emergence region and their rise into the outer atmosphere of the Sun. A sub-photospheric twisted flux tube rises from the solar interior and expands into the corona. A flux rope is formed within the expanding field, due to shearing and reconnection of field lines at low atmospheric heights. If the tube emerges into a non-magnetized atmosphere, the flux rope rises, but remains confined inside the expanding magnetized volume. In contrast, if the expanding tube is allowed to reconnect with a pre-existing coronal field, the flux rope experiences a full eruption with a rise profile that is in qualitative agreement with erupting filaments and Coronal Mass Ejections.
Figure 1. 3D visualization of the two magnetic flux ropes: the erupting rope (magnetic flux system surrounding the red fieldlines) and the low-lying rope (white magnetic fieldlines), which is the original axis of the emerging field. The coloured-scaled map represents the base of the photosphere. Time is t=50 min.

The formation of the new rope is independent of the initial flux and twist of the original, buoyant magnetic field. The dominant force driving the rise of the new rope is the magnetic pressure force. However this force is balanced by the magnetic tension force of the expanding field of the original tube. This expanding field consists an ambient field for the erupting rope. When the ambient field expansion stops, the magnetic field strength drops only slightly with height above the flux rope. As a result, the magnetic tension of the overlying field stops the rise of the rope. The confinement of the flux rope may fail if the ambient field drops more rapidly with height above the flux rope, or is removed (e.g. via reconnection). To confirm this, we performed experiments with a pre-existing coronal field, which was suitable to reconnect with the expanding magnetic field of the emerging tube. Thus, the tension of the fieldlines of the expanding field is released and the new rope that erupt finds the way open to escape. Reconnection above and underneath the eruption flux rope is important. It leads to a runaway situation where the flux rope may accelerate to super-alfvenic velocities. During the eruption, dense plasma is lifted up against gravity. The dense plasma is accumulated at the dips of the twisted fieldlines of the erupting flux tube. The following movies show the eruption of the flux rope in the experiments without and with pre-existing coronal magnetic field. The colormap shows density. Arrows stand for the magnetic field. The center of the rope is shown by the black dot.

2D movie (animated gif) of eruption without coronal field.

2D movie (animated gif) of eruption with pre-existing coronal field.

3D movie (avi) of eruption without coronal field.

3D movie (animated gif) of eruption without coronal field.

3D movie (avi) of eruption with pre-existing coronal field.

3D movie (animated gif) of eruption with pre-existing coronal field.

The results of our numerical experiments appear in Astronomy and Astrophysics (2009). This is a copy of the paper.