Center for Magnetic Self Organization
in Laboratory and Astrophysical Plasmas
Spiral Magnetic Instabilities
Observation of a spiral magnetic instability in the Princeton MRI experiment
The MRI experiment, a cylindrical channel of magnetized, liquid gallium confined between two rotating cylinders, was designed to study instabilities relevant to angular momentum transport and magnetic field amplification in astrophysical disks. We discovered that when a sufficiently strong magnetic field is imposed on a hydrodynamically stable flow, coherent oscillations emerge in both azimuthal and radial flow components. We detected this through measurements by Ultrasonic Doppler Velocimetry (UDV) mounted at various heights and azimuthal angles. Initially, the oscillations exhibit higher azimuthal mode numbers, but later evolve into an m=1 mode, appearing as a spiral structure in the azimuthal velocity as shown in Figure 1 below.
Figure 1. The azimuthal velocity in a spiral mode spontaneously excited in the MRI experiment, relative to the angle-averaged mean. Although the mean flow varies with radius, the spiral shear is steady.
The dependence on the imposed magnetic field strength shows similarities with the predictions for MRI (Magnetorotational Instability), but the instability persists at much lower Reynolds numbers than the predictions. By comparing with MHD simulations in a similar geometry at similar parameters, it has been suggested that the observed spiral instability is a hydrodynamic instability either of a sheared azimuthal flow induced by an axial magnetic field over the speed gap between two end rings, or of a poloidal circulation (Ekman circulation) enhanced by the axial field. In either case, however, the instability is qualitatively different from the MRI. On the other hand, the discovery of this new instability has directly relevance to geophysics since both strong axial field and similar boundary conditions exist there.