Analysis of Mark 4 and LASCO C2 , C3 coronagraph data shows that , at the distance R \leq 6 R _ { \odot } from the center of the Sun , the thickness of a CME-generated shock-wave front ( \delta _ { F } ) may be of order of the proton mean free path .
This means that the energy dissipation mechanism in the shock front at these distances is collisional .
A new discontinuity ( thickness \delta _ { F } ^ { * } \ll \delta _ { F } ) is observed to appear in the anterior part of the front at R \geq 10 R _ { \odot } .
Within the limits of experimental error , the thickness \delta _ { F } ^ { * } \approx 0.1-0.2 R _ { \odot } does not vary with distance and is determined by the spatial resolution of the LASCO C3 instrument .
At the initial stage of formation , the discontinuity on the scale of \delta _ { F } ^ { * } has rather small amplitude and exists simultaneously with the front having thickness \delta _ { F } .
The relative amplitude of the discontinuity gradually increases with distance , and the brightness profile behind it becomes even .
Such transformations may be associated with the transition from a collisional shock wave to a collisionless one .