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Lecture Talk
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6/1 (15:00-16:30)
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Anders Johansen (Leiden Observatory)
"The magnetorotational instability and accretion disc turbulence: an
overview"
"Accretion discs are behind some of the most fascinating phenomena in
the Universe. Viscous mass transport feed supermassive black holes and
young stars, and accretion discs are sites of both planet formation
and star formation. Keplerian shear flows are linearly unstable in the
presence of a weak magnetic field. The application of this
magnetorotational instability to accretion discs by Balbus & Hawley
(1991) led to an explosion in numerical investigations of the
non-linear state of this instabilitity. Magnetorotational turbulence
is characterised by positive Reynolds and Maxwell stresses,
transporting angular momentum outwards while allowing mass to flow
in. At scales larger than a scale height coherent magnetic field
structures dominate the turbulent energy. The arisal of such large
scale magnetic fields from small scale turbulent motion is still
poorly understood and is numerically frustrated by the difficulty of
getting the correct separation between viscous and resistive length
scales. An interplay between improved numerical algorithms and
increased numerical resolution on the one hand, and analytical models
of the excitation of large scale magnetic fields on the other hand, is
currently opening up for an increased understanding of the turbulent
dynamics of magnetised accretion discs."