The behaviour of the diamagnetic current modified MRI in different rotation profiles

Abstract

Magnetohydrodynamic turbulence driven by magnetorotational instability (MRI) has been widely acceptedas an efficient mechanism for transporting angular momentum radially outward in the discs. Previously, we haveshown the triggering effect of the magnetization currents and the gradients produced by these currents on MRI forKeplerian discs. In this work, we examine the properties of the diamagnetic current modified MRI for the discs wherethe epicyclic frequencies deviate from Keplerian behaviour. We show that the diamagnetic effect modifies the instabilityboth for an increasing and for a decreasing angular momentum with radius. The novel unstable region produced bymagnetization effects still exists even if dΩ2 /dR > 0, where Ω(R) is the angular velocity. Inclusion of the diamagneticeffect increases the maximum dimensionless growth rate sm = 0.25 which was found from the classical MRI to sm =0.59 for an outwardly increasing angular velocity profile with κ2 > 4Ω2 , where κ is the epicyclic frequency. Moreover,the maximum dimensionless growth rate is obtained as sm = 8.58 for a an outwardly decreasing angular velocity profilewith 0 < κ2 < Ω2 . Decreasing the magnetic field strength which is already weak, leads to more rapid growth in anydifferential rotation profile.