The irreversibility line in <$E roman {Bi sub 2 Sr sub 2 CaCu sub 2 O} sub y> single crystals is measured by three experimental methods: by measuring magnetization at direct current, by measuring microwave absorption and by registering harmonics in a high-frequency response of the superconductor. It is found that the irreversibility line position in the magnetic phase diagram depends essentially on measuring method and frequency. The analysis of the results suggests that the noncoincidence of the irreversibility line positions may be due to the difference in the hysteresis mechanisms of magnetic properties and that in the scales of measurement time. It is shown that the irreversibility line position at low temperatures is determined by the vortex creep rate which is limiting for this method of registering, and at high temperatures - by the time of overcoming the surface barrier by 2D vortices. It can be concluded that the irreversibility line is a dynamic phase transition. The first experimental evidence for the existance of the surface barrier for 2D vortices is obtained.

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