MHD INTERPRETATION OF LASCO OBSERVATIONS OF A CORONAL MASS EJECTION AS A DISCONNECTED MAGNETIC-STRUCTURE

We present a qualitative and quantitative comparison of a single coron al mass ejection (CME) as observed by LASCO (July 28-29, 1996) with th e results of a three-dimensional axisymmetric time-dependent magnetohy drodynamic model of a Aux rope interacting with a helmet streamer. The particular CME considered was selected based on the appearance of a d istinct 'tear-drop' shape visible in animations generated from both th e data and the model. The CME event begins with the brightening of a p re-existing coronal streamer which evolves into a 'tear-drop' shaped l oop followed by a Y-shaped structure. The brightening moves slowly out ward with significant acceleration reaching velocities of similar to 4 50 km s(-1) at 30 R-.. The observed CME characteristics are compared w ith the model results. On the basis of this comparison, we suggested t hat the observed features were caused by the evacuation of a flux rope in the closed field region of the helmet streamer (i.e., helmet dome) . The flux rope manifests itself as the cavity of the quasi-static hel met streamer and the whole system becomes unstable when the flux rope reaches a threshold strength. The observed 'tear-drop' structure is du e to the deformed flux rope. The leading edge of the flux rope interac ts with the helmet dome to form the typical loop-like CME. The trailin g edge of this flux rope interacts with the local bi-polar field to fo rm the observed Y-shaped structure. The model results for the evolutio n of the magnetic-field configurations, velocity, and polarization bri ghtness are directly compared with observations. Animations have been generated from both the actual data and the model to illustrate the go od agreement between the observation and the model. These animations c an be found on the CD-ROM which accompanies this volume.