Magnetorotational Instability (MRI) Experiment To study fundamental physics of fast angular momentum transport in accretion disks Can hydrodynamic turbulence support fast accretion? No! Does MRI exist in the pure MHD form transporting angular momentum? We found its precursors! HH30 By HST Accretion Disks MRI Experiment Protostellar Disk

Nonaxisymmetric Modes Appear When Imposing Bz on Hydro-unstable Flows Movie shows radial magnetic field measured just outside outer cylinder induced by fluctuations in flow. A nonaxisymmetric traveling wave is found to grow and fluctuate in amplitude On the right are plots of 1) the integrated signal from a B-dot coil, 2) The amplitude of the (0,1) and (1,1) modes in time (first index is axial mode number and second index is azimuthal mode number), and 3) the phase of the two modes in time. Toroidal angle (radians) Bz = 3.30 kG Br measurements at surface show azimuthal (m=1) mode Two m=1 Modes Rotate at Different Speeds

Mode rotation increases with magnetic field Fast and Slow Magnetocoriolis Waves Identified; the Slow Wave Becomes the MRI Shear Alfvén waves modified by rotation: fast magnetocoriolis waves slow magnetocoriolis waves Alfvén waves Still damped waves, but slow waves will grow due to MRI if rotation rate is doubled We can use results to ask how fast plasma rotation must be to split Alfvén branch From the rate of change of the mode phase we obtain a rotation speed for each mode. They have the same dependence on magnetic field as the Alfven wave. In a rotating system, the Alfven wave is split into a fast and slow wave called the fast and slow magnetocoriolis wave due to the presence of the Coriolis force which may be in phase or out of phase with the Lorentz force. The growth rate for the fast and slow waves can be determined by matching the rotation rate of the observed modes to the fast and slow magnetocoriolis wave through the dispersion relation. It is found that the waves are damped, and hence must be driven, but that the slow wave becomes unstable if we double the rotation rate, thus obtaining the MRI. Mode rotation increases with magnetic field