1203.0823 (Eric G. Blackman)

Accretion Disks and Dynamos: Toward a Unified Mean Field Theory    [PDF]

Eric G. Blackman

Conversion of gravitational energy into radiation near stars and compact objects in accretion disks the origin of large scale magnetic fields in astrophysical rotators have long been distinct topics of active research in astrophysics. In semi-analytic work on both problems it has been useful to presume large scale symmetries, which necessarily results in mean field theories; magnetohydrodynamic turbulence makes the underlying systems locally asymmetric and highly nonlinear. Synergy between theory and simulations should aim for the development of practical, semi-analytic mean field models that capture the essential physics and can be used for observational modeling. Mean field dynamo (MFD) theory and alpha-viscosity accretion disc theory have exemplified such distinct pursuits. Both are presently incomplete, but 21st century MFD theory has nonlinear predictive power compared to 20th century MFD. in contrast, alpha-viscosity accretion theory is still in a 20th century state. In fact, insights from MFD theory are applicable to accretion theory and the two are really artificially separated pieces of what should ultimately be a single coupled theory. I discuss pieces of recent progress that provide clues to progress toward a unified theory. A key concept is that large scale magnetic fields can be sustained via magnetic helicity fluxes and via relaxation of small scale magnetic fluctuations, without appealing to the traditional kinetic helicity driver of 20th century textbooks. This is likely important for explaining the formation of large scale fields that supply non-local angular momentum transport via coronae and jets in a unified theory of accretion and dynamos.

View original: http://arxiv.org/abs/1203.0823