Magnetic Field Transport in Accretion Disks How accretion

Magnetic Field Transport in Accretion Disks – How accretion disks suck up magnetic fields Amir Jafari (Johns Hopkins) Ethan Vishniac (Johns Hopkins/AAS) ethan. vishniac@aas. org (410 303 5854)

Where does the launching field come from? Sucked in from the surrounding universe (along with the gas that feeds the accretion disk). Field is dragged (slowly) inward Rapidly mixed vertically. Field stays straight and uncompressed.

Magnetic Buoyancy We have calculated the effects of turbulence on the motion of the magnetic field. Several different effects push magnetic fields in different directions. The dominant effect in disks is buoyancy. The magnetic field is compressed and stretched. Where the magnetic field is concentrated, gas is expelled. Freed from the weight of the gas, the magnetic field moves upward.

Magnetic Buoyancy In an accretion disk this means that the field lines are pushed away from the midplane, preventing them from reconnecting and suppressing their tendency to move radially outward. The field lines become sharply bent and dragged inwards.

What does this mean? Typical accretion disks will suck up magnetic field from their surroundings and compress it towards their centers. In the outer parts of the disk, the bent field lines will accommodate winds. In the inner regions, the field lines will be powerful and more nearly straight. (This “inner region” usually exists for accretion onto black holes. )

What does this mean? Magnetically driven winds are typical. A strongly magnetized inner region with a jet will be more likely for systems that accrete weakly, have stronger fields around them, or are larger (relative to the size of the inner object). Thank you ethan. vishniac@aas. org (410 303 5854)