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 coronal mass ejection (CME) as observed by LASCO (July 28–29, 1996) with the results of a three-dimensional axisymmetric time-dependent magnetohydrodynamic model of a flux rope interacting with a helmet streamer. The particular CME considered was selected based on the appearance of a distinct ‘tear-drop’ shape visible in animations generated from both the data and the model. The CME event begins with the brightening of a pre-existing coronal streamer which evolves into a ‘tear-drop’ shaped loop followed by a Y-shaped structure. The brightening moves slowly outward with significant acceleration reaching velocities of ∼450 km s^-1 at 30 R⊙. The observed CME characteristics are compared with the model results. On the basis of this comparison, we suggested that 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 helmet streamer and the whole system becomes unstable when the flux rope reaches a threshold strength. The observed ‘tear-drop’ structure is due to the deformed flux rope. The leading edge of the flux rope interacts with the helmet dome to form the typical loop-like CME. The trailing edge of this flux rope interacts with the local bi-polar field to form the observed Y-shaped structure. The model results for the evolution of the magnetic-field configurations, velocity, and polarization brightness are directly compared with observations. Animations have been generated from both the actual data and the model to illustrate the good agreement between the observation and the model. These animations can be found on the CD-ROM which accompanies this volume. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1004923016322