Coronal implosion

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Introduction

During flares and coronal mass ejections (CMEs), magnetic reconnection results in the rapid release to the plasma of free energy stored in the large-scale structure of the coronal magnetic field. Specifically, the free energy is believed to be stored in the strongly nonpotential, i.e., sheared or twisted, field of a filament channel. Eruptions ensue if the force balance between the upward magnetic pressure force of the sheared filament channel field, and the downward magnetic tension force of the overlying quasi-potential field, is disrupted. As Hudson (2000) first pointed out, a reduction of magnetic energy during the transients in the flaring region, therefore the reduction of the upward magnetic pressure, would inevitably result in the contraction of the overlying field. The large-scale motions observed in almost all flares or CMEs, however, are explosive rather than implosive, which poses an obvious observational dilemma.

On the other hand, the contraction of flaring loops themselves during the early phase of flares, which is manifested as the converging conjugate footpoints and descending looptop emission, have been reported in X-ray, EUV, H-alpha, and microwave observations. The contraction can be explained by the relaxation of the sheared magnetic field, a scenario consistent with the Hudson conjecture.

One may wonder how the coronal magnetic field responds to the flare loop contraction. In this Nugget, observations of a coronal implosion is presented, showing that the EUV coronal loops overlying an eruptive filament push inward during the early phase of a flare, which is associated with the converging motion of the conjugate HXR footpoints and the downward motion of the HXR looptop source.

Observation

Discussion

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