Hydromagnetic instability in differentially rotating flows.

We study the stability of compressible differentially rotating flows in the presence of the magnetic field, and we show that the compressibility profoundly alters the previous results for a magnetized incompressible flow. The necessary condition of newly found instability can be easily satisfied in various flows in laboratory and astrophysical conditions and reads BsBphiOmega' not equal to 0, where Bs and Bphi are the radial and azimuthal components of the magnetic field, Omega' = d Omega/ds with s being the cylindrical radius. Contrary to the well-known magnetorotational instability that occurs only if omega decreases with s, the instability considered in this paper may occur at any sign of Omega'. The instability can operate even in a very strong magnetic field that entirely suppresses the standard magnetorotational instability. The growth time of instability can be as short as a few rotation periods.