Growth or decay: universality of the turbulent dynamo saturation

James R. Beattie, Christoph Federrath, Neco Kriel, Philip Mocz, Amit Seta

Submitted on 21 September 2022


The small-scale turbulent dynamo (SSD) is likely to be responsible for the magnetisation of the interstellar medium (ISM) that we observe in the Universe today. The SSD efficiently converts kinetic energy Ekin into magnetic energy Emag, and is often used to explain how an initially weak magnetic field with EmagEkin is amplified, and then maintained at a level EmagEkin. Usually, this process is studied by initialising a weak seed magnetic field and letting the turbulence grow it to saturation. However, in this study, using three-dimensional, non-ideal magnetohydrodynamical turbulence simulations, we show that the same saturated state can also be achieved if initially EmagEkin or EmagEkin. This is realised through a two-stage exponential decay (1. a slow backreaction that converts Emag into Ekin, and 2. Ohmic dissipation concentrated in anisotropic current sheets) into the saturated state, for which we provide an analytical model. This means that even if there are temporary local enhancements of Emag in the ISM, such that Emag>Ekin, e.g., through amplifications such as compressions, over a long enough time the field will decay into the saturated state set by the SSD, which is determined by the turbulence and magnetic dissipation. However, we also provide analytical models for the decay times and utilise wait-time statistics from compressive supernova events to show that if the magnetic field is enhanced above the saturated state, it will not have enough time to decay the field before the next supernova event. Hence, unless there exists a mechanism for destroying magnetic fields that is not in our non-ideal MHD models, the amplitudes of interstellar magnetic fields may also be a... (abridged).


Comment: 18 pages. 14 figures. Submitted to MNRAS. Comments welcome

Subject: Astrophysics - Astrophysics of Galaxies