Direct Observation of Low Coronal Breakout: Does Breakout Precede or Follow Solar Eruption?

Abstract

In this Letter, we report a direct SOHO LASCO C1 observation of low coronal that occurred during the coronal mass ejection (CME) on 1998 March 23. The LASCO C1 images show that a slowly expanding, small coronal loop on the northeastern limb erupted, becoming a CME with the typical three-part structure (core, void, and front). Just after the CME front went out of the C1 field of view (about 2 solar radii), a wedgelike open structure is clearly formed. From this observation, together with Yohkoh SXT and SOHO MDI images, we inferred the change of the coronal magnetic field configuration during the eruption, which shows a morphological consistency with the breakout CME model. However, our observation shows that the initial acceleration (~100 m s-2) of the CME front began about 1 hr before the apparent field opening. This observation disagrees with the CME initiation mechanism of the breakout model. We note that the observed eruption progressed in four distinct phases: a slow rise of loop structures, the initial acceleration of the CME, the magnetic breakout and second acceleration, and the CME propagation at almost-constant speed.