Features of the Dynamics of a Shock Excited by Fast Coronal Mass Ejections

Abstract

The event on January 27, 2012 is an example of the features of the dynamics of formation of a shock, excited by a fast coronal mass ejection (CME) at a velocity higher than 2000 km/s, are investigated. The following data were used: (a) images of the Sun in the UV channel of 13.1 nm from the AIA instrument at distances of 1.0–1.4 solar radii from the Sun’s center, (b) the images of the white corona from the LASCO C2 and C3 coronographs at R ≈ 2.1–30 solar radii. Investigations indicated the validity of regularities, established earlier for the cases of relatively slow CMEs (at velocities <1500 km/s): (1) The shock front is formed, when the velocity of a leading part of a CME relative to the undisturbed solar wind becomes greater than the local Alfven velocity of wind, which corresponds to the phenomenon of “transition through the sound velocity” for magnetized plasma. (2) The change in the width of a shock front and, accordingly, in the energy dissipation mechanism occurs in a shock front from a “collisional” one, at distances from the Sun’s center lower than 10 solar radii, to a “collisionless” one, at distances greater than ten solar radii. During the event of January 27, 2012, it was possible to investigate in more detail the process of transition from the collisional to collisionless shock front. It was found that the longitudinal length of a shock front, excited by a fast CME at distances lower than 6 solar radii, increased almost 10 times as compared to a slower CME. One of the main causes for this increase, along with a high velocity, is the fact that the motion of CME has occurred in the plane of the belt of coronal streamers. It is concluded that, at distances lower than 6 solar radii, the shock front structure ahead of the CME is of “parallel” type; that is, the angle between the vector of an undisturbed magnetic field and the normal to the front was close to 0°.