Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000a%26a...353..813r&link_type=abstract
Astronomy and Astrophysics, v.353, p.813-822 (2000)
Astronomy and Astrophysics
Astrophysics
19
Accretion, Accretion Disks, Magnetic Fields, Ism: Jets And Outflows, Turbulence
Scientific paper
The influence of large-scale magnetic fields on the structure of accretion disks is studied. The magnetic field is obtained by a self-consistent nonlinear dynamo model with magnetic pressure strongly influencing the density stratification which itself feeds back to the field generation. The resulting magnetic field geometry is discussed in relation to the accretion disk wind theory. Regarding new results of MHD turbulence simulations, both possible signs of the alpha -effect are allowed (Brandenburg & Donner 1997). In the canonical case of positive alpha the resulting field is of quadrupolar symmetry. The field strength is about 50% of the value for dynamo models nonlinearly limited by alpha -quenching. The temperature profiles as well as the disk geometry remain nearly unchanged. The viscous stress remains the key transporter of angular momentum driving the accretion inflow. For negative alpha , however, a stationary dipolar structure of the magnetic field results. The additional magnetic torque at the disk surface changes the profile of the effective temperature significantly to a profile which is more flat. The magnetic torque becomes of the same order as the radial viscous torque. The inclination angle of the poloidal field exceeds 30o even for a magnetic Prandtl number of order unity, and also the criterion for poloidal collimation after Spruit et al. (1997) is fulfilled. The dynamo-generated magnetic field configuration thus supports the magnetic wind launching concept for accretion disks for realistic turbulent magnetic Prandtl numbers.
Elstner Detlef
Rekowski M. v.
Rudiger Günther
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