  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.
