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⩽0 R⊙ from the center of the Sun, the thickness of a CME‐generated shock‐wave front (δF) 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 δ*F≪δF) is observed to appear in the anterior part of the front at R⩾10 R⊙. Within the limits of experimental error, the thickness δ*F≈0.1–0.2 R⊙ 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 δ*F has rather small amplitude and exists simultaneously with the front having thickness δF. 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.