Empirical relation between angular momentum transport and thermal-to-magnetic pressure ratio in shearing box simulations

By combining data from different published 3D simulations of Keplerian shearing boxes unstable to the magneto-rotational instability (MRI), we highlight tight anti-correlations between the total effective inferred angular momentum transport parameter, alpha(tot), its separate Maxwell and Reynolds contributions alpha(mag) and alpha(kin), and the kinetic to magnetic pressure ratio beta, defined with the initial or saturated (when available) thermal pressure. Plots of Log(alpha(kin)), Log(alpha(mag)), and Log(alpha(tot)) vs. Log(beta) are well fit by straight lines even as alpha(kin), alpha(mag), and alpha(tot) vary by four orders of magnitude over the simulations included. The ratio alpha(kin)/alpha(mag) and the product alpha(tot)beta are quite constant and largely independent of the presence or absence of weak mean fields, the choice of initial and boundary conditions, and the resolution. In short, simulations have more strongly constrained the product alpha(tot)beta than alpha(tot) itself. (C) 2007 Elsevier B.V. All rights reserved.