Compressibility and local instabilities of differentially rotating magnetized gas

We study the stability of compressible cylindrical differentially rotating flows in the presence of the magnetic field and show that compressibility qualitatively alters the stability properties of flows. Apart from the well-known magnetorotational instability that can occur even in the incompressible flow, there exists a new instability caused by compressibility. The necessary condition of the newly found instability can easily be satisfied in various flows in laboratory and astrophysical conditions and reads BsB phi Omega' not equal 0, where B-s and B-phi are the radial and azimuthal magnetic fields, respectively, and Omega' = d Omega/ds with s being the cylindrical radius. Contrary to the magnetorotational instability that occurs only if Omega decreases with s, the newly found instability operates at any sign of Omega'. The considered instability can arise even in a very strong magnetic field that suppresses the magnetorotational instability.