Speaker
Description
An MHD solution of zero magnetic field can be unstable to the dynamo instability. It plays the perhaps most important role in virtually all of astrophysics! But it requires kinetic energy which can be tapped and converted into magnetic energy. In a rotating body, shear may sometimes be important to produce an excess of toroidal field over poloidal. In the Sun, shear is the result of turbulence, but in accretion discs, a Keplerian shear flow would be stable and would not be turbulent if there was not the magneto-rotational instability. Both dynamo and magneto-rotational instabilities can lead to a doubly-positive feedback to explain the generically magnetic nature of accretion discs. For stars, instabilities akin to the magneto-rotational one have long been applied to the radiative interior, but recently also applications to the turbulent convection zone have been discussed. It might alter the nature of the Omega effect in stellar mean-field dynamos, but this requires mean-field magnetic Reynolds numbers that are perhaps unrealistically large. In my talk, I will end with an assessment of where I believe we stand with regard to the solar dynamo after all these years.
