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

[1] It is shown that in front of a coronal mass ejection, having a velocity u lower than the critical uC 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 uC in the disturbed region front part in radial distributions of the difference brightness. Since the uC 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 uC is exceeded may be identified with the formation of a shock wave.