Stellar magnetic fields in a jar

The magneto-rotational instability—the creation of an induced magnetic field in a rotating electrically conducting fluid immersed in an external magnetic field—has been known from astrophysical theory and numerical simulations. Now, physicists at the University of Maryland, College Park, report the first experimental observation of this phenomenon. With a baseball-sized copper ball rotating within a spherical vessel that contained liquid sodium, the researchers created conditions similar to those in Earth’s core, the outer envelopes of stars, and accretion disks surrounding black holes. In each case, a differentially rotating fluid (inner parts of the fluid rotating faster than outer parts) is destabilized by small magnetic fields, with the result that angular momentum is carried radially outward. Among other findings, the Maryland experiment demonstrates that the instability can arise even in the presence of preexisting turbulence. (D. R. Sisan, et al., Phys. Rev. Lett. 93 , 114502, 2004 http://dx.doi.org/10.1103/PhysRevLett.93.114502 .)