Kinematics of coronal mass ejections between 2 and 30 solar radii. What can be learned about forces governing the eruption?

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=-k₁(v-v₀), where v₀=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 k₁ depends on these two parameters: k₁ is smaller for CMEs of larger width and mean-distance. Furthermore, the obtained CME subsets show distinct quadratic-form correlations, of the form a= -k₂ (v-v₀)| v-v₀|. The value of k₂ decreases with increasing distance and width, whereas v₀ 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 v₀ 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.

Appendices A and B are only available in electronic form at http://www.edpsciences.org