Acceleration and Deceleration of Coronal Mass Ejections: Theory and Observations

Dr. James Chen
Plasma Physics Division
Naval Research Laboratory

Much new understanding of the coronal mass ejection (CME) phenomenon has beend gained based on both theoretical work and recent LASCO observations. This talk explores two aspects of CME dynamics: the initial acceleration of CMEs which is often hidden behind the occulting disk and deceleration in the early times (i.e., within the field of view of coronagraphs) and during propagation in the heliosphere. These aspects intimately pertain to the nature of forces that determine the dynamics of CMEs and their heliospheric counterparts. An important conclusion of a series of recent studies is that there exist a significant class of observed CMEs that are describable as erupting magnetic flux ropes. This conclusion is well constrained by observations, being based on comparing in detail the predictions of a theoretical model of flux ropes and the observed dynamics of CMEs as well as on the generic morphological features of flux ropes projected onto the plane of the sky. In particular, the model calculates the forces acting on erupting flux ropes and produces motion and expansion of CMEs in good agreement with observations. This model will be used to study the forces acting on CMEs and their dynamics at heights below 2-3 Rs from Sun center. It is found that CME acceleration typically reaches maximum and decreases within approximately 1 Rs of the surface. Beyond this height, acceleration is significantly less than the peak values lower down. It is shown that this height is determined by the inductive properties of 3-D flux ropes. For deceleration, drag coupling to the coronal/SW gas and the tension force of the flux-rope magnetic field provide the dominant contributions. Theoretical results for the early-time dynamics will be constrained by C1 and MK3 data, while the deceleration results will be tested using C2/C3 and interplanetary magnetic cloud data.