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SUMMARY:SARIs : a new paradigm for turbulent accretion onto black holes - 
 Rony Keppens (KU Leuven\, Belgium)
DTSTART:20241104T140000Z
DTEND:20241104T150000Z
UID:TALK222919@talks.cam.ac.uk
CONTACT:Loren E. Held
DESCRIPTION:During the last three decades\, the astrophysical literature c
 ollected ever growing evidence that magnetic fields\, however weak\, play 
 a decisive role in triggering\nturbulent flow in accretion disks. A proces
 s referred to by its acronym MRI\, for magneto-rotational\ninstability\, p
 rovides a universally accepted explanation of why magnetized accretion dis
 ks turn turbulent at all. Magnetic fields and a rotation rate that decreas
 es away from the central object\, introduce a runaway process governed by 
 conservation of angular momentum.\n\nHowever [1]\, the MRI diverts our att
 ention to special (axisymmetric\, i.e. with no variation about the disk ce
 nter) natural eigenfrequencies of the disk. This focus\non eigenfrequencie
 s is physically sound\, just like we appreciate music through the natural 
 vibrations of a guitar string. Curiously enough\, a rigorous mathematical 
 analysis now reveals that we may as well go beyond these pure natural reso
 nances\, and look for any growing wave\npackage that just needs a tiny amo
 unt of added energy (just like the guitar player does by plucking the stri
 ng).\nThis opens up an entire new window on what drives turbulence in accr
 etion disks\, with a decisive role for very localized wave packages that s
 urf the disk at\nspeeds larger than the Nobel-prize winning Alfvén speed.
  These Super-Alfvénic rotational instabilities - SARIs for\nshort - are c
 ompletely unprecedented\, and since they no longer adopt artificial axisym
 metry\, may drive processes enhancing the magnetic field in-situ\, acting 
 as a dynamo. This new concept for turbulence in accretion disks truly repr
 esents a paradigm-shift\, but required the solution of intricate mathemati
 cal equations\, where singular behavior is key [1\,2]. Since singular beha
 vior of mathematical equations gave birth to black holes\, it is satisfyin
 g to\ndiscover that turbulent processes near black holes thrive on singula
 rities as well.\n\n[1] Hans Goedbloed & R. Keppens\, 2022\, ApJ Supplement
  Series 259\, 65 (41pp)\, Full paper\, doi:10.3847/1538-4365/ac573c\n[2] N
 . Brughmans\, R. Keppens & H. Goedbloed\, 2024\, ApJ 968\, 19 (20pp)\, Ful
 l paper\, doi:10.3847/1538-4357/ad3d52\n
LOCATION:MR14 DAMTP and online
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