Abstract
Detailed calculations of the physical structure of accretion disk
boundary layers, and thus their inferred observational properties, rely
on the assumption that angular momentum transport is opposite to the
radial angular frequency gradient of the disk. The standard model for
turbulent shear viscosity satisfies this assumption by construction.
However, this behavior is not supported by numerical simulations of
turbulent magnetohydrodynamic (MHD) accretion disks, which show that
angular momentum transport driven by the magnetorotational instability
(MRI) is inefficient in disk regions where, as expected in boundary
layers, the angular frequency increases with radius. In order to shed
light on physically viable mechanisms for angular momentum transport in
this inner disk region, we examine the generation of hydromagnetic
stresses and energy density in differentially rotating backgrounds with
angular frequencies that increase outward in the shearing-sheet
framework. We isolate the modes that are unrelated to the standard MRI
and provide analytic solutions for the long-term evolution of the
resulting shearing MHD waves. We show that, although the energy density
of these waves can be amplified significantly, their associated stresses
oscillate around zero, rendering them an inefficient mechanism to
transport significant angular momentum (inward). These findings are
consistent with the results obtained in numerical simulations of MHD
accretion disk boundary layers and challenge the standard assumption of
efficient angular momentum transport in the inner disk regions. This
suggests that the detailed structure of turbulent MHD accretion disk
boundary layers could differ appreciably from those derived within the
standard framework of turbulent shear viscosity
| Original language | English |
|---|---|
| Journal | Astrophysical Journal |
| Volume | 751 |
| Issue number | 1 |
| Pages (from-to) | 48 |
| ISSN | 0004-637X |
| DOIs | |
| Publication status | Published - 20 May 2012 |