Kinematics of coronal mass ejections between 2 and 30 solar radii^*

What can be learned about forces governing the eruption?

¹ Hvar Observatory, Faculty of Geodesy, Kačićeva 26, 10000 Zagreb, Croatia e-mail: bvrsnak@geodet.geof.hr
² NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

Received: 30 January 2004
Accepted: 21 April 2004

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 , where  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 . 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 and 10 m s^-2, being on average smaller at larger distances. However, there are also events that show  m s^-2, as well as events indicating . Implications for the interplanetary motion of CMEs are discussed, emphasizing the prediction of the 1 a.u. arrival time.