On formation of a shock wave in front of a coronal mass ejection with velocity exceeding the critical one

It is shown that in front of a coronal mass ejection, having a velocity u lower than the critical u C relative to the surrounding coronal plasma, there is a disturbed region expended along a direction of the CME propagation. The time difference brightness (plasma density) in the disturbed region smoothly decreases to larger distances in front of the CME. A discontinuity forms at u higher than u C in the disturbed region front part in radial distributions of the difference brightness. Since the u C value is close to the local fast‐mode MHD velocity, which in corona approximately equal to the Alfven one, the formation of such a discontinuity when u C is exceeded may be identified with the formation of a shock wave.