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The corotation radius is another important characteristic of a magnetic rotator. Suppose that an accreting plasma penetrates the light cylinder and is stopped by the magnetic field at a certain distance given by the balance between the static magnetic field pressure and the plasma pressure. Suppose that the plasma is ``frozen'' in the rotator's magnetic field. This field will drag the plasma and force it to rotate rigidly with the angular velocity of the star. The matter will fall on to the stellar surface only if its rotational velocity is smaller than the Keplerian velocity at the given distance :
Otherwise, a centrifugal barrier emerges and the rapidly rotating magnetic field impedes the accretion of matter (Schwartzman, 1970a; Pringle and Rees, 1972; Davidson and Ostriker, 1973; Lamb et al., 1973; Illarionov and Sunyaev, 1975). The latter authors assumed that if , the magnetic field throws the plasma back beyond the capture radius. They called this effect the ``propeller'' regime. In fact, matter may not be shed (Lipunov 1980, 1982d[97, 101]), but it is important to note that a stationary accretion is also not possible.
The corotation radius is thus defined as
where P is the rotational period of the star.
If , rotation influences the accretion insignificantly. Otherwise, a stationary accretion is not possible for .