On The Possible Mechanism Of Energy Dissipation In Shock-Wave Fronts Driven Ahead Of Coronal Mass Ejections

Analysis of Mark 4 and LASCO C2, C3 coronagraph data shows that, at the distance $R\le 6$ 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 $\delta_F^* \ll \delta_F$)isobservedtoappearintheanteriorpartofthefrontat$R \geq 10$R$\odot$\mathrm{.}Withinthelimitsofexperimentalerror,thethickness$\delta_F^* \approx$0.1-0.2R$\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 ${\delta }_F^{\ast }$ 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.