Kinematics of coronal mass ejections between 2 and 30 solar radii

Abstract

Kinematics of more than 5000 coronal mass ejections (CMEs) measured in the distance range 2-30 solar radii is investigated. A distinct anticorrelation between the acceleration, a, and the velocity, v, is found. In the linear form, it can be represented as a = -k 1 (v - v 0 ), where v 0 = 400 km s -1 , i.e., most of CMEs faster than 400 km s -1 decelerate, whereas slower ones generally accelerate. After grouping CMEs into the width and mean-distance bins, it was found that the slope k 1 depends on these two parameters: k 1 is smaller for CMEs of larger width and mean-distance. Furthermore, the obtained CME subsets show distinct quadratic-form correlations, of the form a = -k 2 (v - v 0 )|v - v 0 |. The value of k 2 decreases with increasing distance and width, whereas v 0 increases with the distance and is systematically larger than the slow solar wind speed by 100-200 km s -1 . The acceleration-velocity relationship is interpreted as a consequence of the aerodynamic drag. The excess of v 0 over the solar wind speed is explained assuming that in a certain fraction of events the propelling force is still acting in the considered distance range. In most events the inferred propelling force acceleration at 10 solar radii ranges between a L = 0 and 10 m s -2 , being on average smaller at larger distances. However, there are also events that show a L > 50 m s -2 , as well as events indicating a L < 0. Implications for the interplanetary motion of CMEs are discussed, emphasizing the prediction of the 1 a.u. arrival time.