Estimating the magnetic Prandtl number: application to solar dynamo simulations
Staff - Faculty of Informatics
Date: 7 July 2021 / 16:30 - 17:30
Fabio Riva, Istituto Ricerche Solari (IRSOL), Switzerland
Radiative magneto-hydrodynamic (MHD) simulation codes represent a fundamental tool for investigating the complex nonlinear processes observed on the Sun. Unfortunately, the discretization of an MHD model and its implementation in a simulation code always introduce numerical errors in the system, which in turn add numerical viscosity and resistivity to the model equations. This poses two critical problems. First, it is currently extremely difficult, not to say impossible, to simulate the solar atmosphere at realistic Reynolds (Re) and magnetic Reynolds (Rm) numbers, since these two parameters are extremely large and their ratio Rm/Re, known as the magnetic Prandtl number (Pm), is very small. Second, the intrinsic diffusivities of a simulation code complicate the interpretation of the results obtained with it, since the effective Reynolds and magnetic Reynolds numbers stemming from a simulation are generally unknown. These two issues are particularly critical when simulating turbulent dynamos in the solar atmosphere, as it is still unclear how to extrapolate the numerical results to the Sun, in particular because of the disparate regimes of Pm between numerical simulations and the real Sun.
In this work, a general methodology for estimating the effective diffusivities stemming from radiative MHD simulations is proposed. The methodology is based on the method of Projection on Proper elements, initially introduced in the plasma physics community to verify plasma turbulence simulation codes. It relies on using different, higher order accuracy, numerical operators to post-process the simulation results. Applications of this procedure to small-scale solar dynamo simulations are then discussed.
Dr. Fabio Riva is a postdoc associate at the Istituto Ricerche Solari (IRSOL). He works with Dr. Oskar Steiner on numerical simulations of solar and stellar atmospheres, investigating the origin of small-scale magnetic fields and their interplay with plasma flows. Prior to this, after graduating at École Polytechnique Fédérale de Lausanne (EPFL) in physics, he obtained a Ph.D. in plasma physics at EPFL with title "Verification and validation procedures with applications to plasma-edge turbulence simulations" and worked as a postdoc fellow at the Culham Centre for Fusion Energy.
Host: Prof. Rolf Krause