Abstract
The magnetorotational instability (MRI) is a shear instability and thus its sensitivity to the shear parameter q = −d ln Ω/d ln r is of interest to investigate. Motivated by astrophysical discs, most (but not all) previous MRI studies have focused on the Keplerian value of q = 1.5. Using simulation with eight vertical density scaleheights, we contribute to the subset of studies addressing the effect of varying q in stratified numerical simulations. We discuss why shearing boxes cannot easily be used to study q > 2 and thus focus on q < 2. As per previous simulations, which were either unstratified or stratified with a smaller vertical domain, we find that the q dependence of stress for the stratified case is not linear, contrary to the Shakura–Sunyaev model. We find that the scaling agrees with Abramowicz, Brandenburg & Lasota who found it to be proportional to the shear to vorticity ratio q/(2 − q). We also find however that the shape of the magnetic and kinetic energy spectra are relatively insensitive to q and that the ratio of Maxwell stress to magnetic energy ratio also remains nearly independent of q. This is consistent with a theoretical argument in which the rate of amplification of the azimuthal field depends linearly on q and the turbulent correlation time τ depends inversely on q. As such, we measure the correlation time of the turbulence and find that indeed it is inversely proportional to q.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have