KU Leuven Centre for mathematical Plasma-Astrophysics Seminar
Title: The Super-Alfvenic rotational instability in accretion disks: A new paradigm for 3D turbulence!
Speaker: Ronny Keppens from CmPA
Abstract
During the last three decades, the astrophysical literature collected ever growing evidence that magnetic fields, however weak, play a decisive role in triggering turbulent flow in accretion disks. A process now simply referred to by its acronym MRI, for magneto-rotational instability, provides a universally accepted explanation of why magnetized accretion disks turn turbulent at all. Magnetic fields, and a rotation rate that decreases away from the central accreting object, together introduce a runaway process governed by conservation of angular momentum.
However, (see arXiv https://arxiv.org/pdf/2201.11551.pdf), the MRI diverts our attention to special (axisymmetric, i.e. with no variation about the disk center) natural eigenfrequencies of the disk. This focus on eigenfrequencies 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 resonances, and look for any growing wave package that just needs a tiny amount of added energy (just like the guitar player does by plucking the string). This opens up an entire new window on what drives turbulence in accretion disks, with a decisive role for very localized wave packages that surf the disk at speeds larger than the Nobel-prize winning Alfvén speed. These Super-Alfvénic rotational instabilities - SARIs for short - are completely unprecedented, and since they no longer adopt artificial axisymmetry, may drive processes enhancing the magnetic field in-situ, acting as a dynamo. This new concept for turbulence in accretion disks truly represents a paradigm-shift, but required the solution of intricate mathematical equations, where singular behavior is key. Since singular behavior of mathematical equations gave birth to black holes, it is satisfying to discover that turbulent processes near black holes thrive on singularities as well. The theory is applicable to magnetized disks and rotating laboratory plasmas, opening new pathways to study of gas and fluid turbulence, witnessed routinely in our everyday life.
The seminars are in hybrid mode, you can follow in person in room 200B 00.07 or online at the (permanent) link:
https://eu.bbcollab.com/guest/7406a5ec00dc4ec6948200f9c769d454
Date and time: Thursday, March 3, 2022 - 14:00 to 15:00
