CORONAL MASS EJECTION PROPAGATION AND EXPANSION IN THREE-DIMENSIONAL SPACE IN THE HELIOSPHERE BASED ON STEREO/SECCHI OBSERVATIONS

We report on several new findings regarding the kinematic and morphological evolution of coronal mass ejections (CMEs) in the inner heliosphere using the unprecedented STEREO/SECCHI observations. The CME tracking is based on the three-dimensional Raytrace model, which is free of the projection effect, resulting in true CME velocities. We also measure the cross section size of the CME and hence its expansion velocity. For the four major CME events investigated, we find that their leading edge (LE) velocity converges from an initial range between 400 km s(-1) and 1500 km s(-1) at 5-10 R(circle dot) to a narrow range between 500 km s(-1) and 750 km s(-1) at 50 R(circle dot). The expansion velocity is also found to converge into a narrow range between 75 km s(-1) and 175 km s(-1). Both LE and expansion velocities are nearly constant after 50 R(circle dot). We further find that the acceleration of CMEs in the inner heliosphere from similar to 10 to 90 R(circle dot) can be described by an exponential function, with an initial value as large as similar to-80 m s(-2) but exponentially decreasing to almost zero (more precisely, less than +/- 5 m s(-2) considering the uncertainty of measurements). These results provide important observational constraints on understanding CME dynamics in interplanetary space.