Solar Wind Drag and the Kinematics of Interplanetary Coronal Mass Ejections

Coronal mass ejections (CMEs) are large-scale ejections of plasma and magnetic field from the solar corona, which propagate through interplanetary space at velocities of ∼100–2500 km s −1 . Although plane-of-sky coronagraph measurements have provided some insight into their kinematics near the Sun (<32 R ☉ ), it is still unclear what forces govern their evolution during both their early acceleration and later propagation. Here, we use the dual perspectives of the STEREO spacecraft to derive the three-dimensional kinematics of CMEs over a range of heliocentric distances (∼2–250 R ☉ ). We find evidence for solar wind (SW) drag forces acting in interplanetary space, with a fast CME decelerated and a slow CME accelerated toward typical SW velocities. We also find that the fast CME showed linear (δ = 1) dependence on the velocity difference between the CME and the SW, while the slow CME showed a quadratic (δ = 2) dependence. The differing forms of drag for the two CMEs indicate the forces responsible for their acceleration may be different.