Browsing by Author "Tuomisto, Simo"
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Tuomisto, Simo (2019)The solar dynamo is a highly complex system where the smallscale turbulence in the convection zone of the Sun gives rise to largescale magnetic fields. The meanfield theory and direct numerical simulations (DNSs) are widely used to describe these dynamo processes. In meanfield theory the smallscale and largescale fields are separated into a fluctuating field and a mean field with their own characteristic length scales. The evolution of the mean fields is governed by the contributions of fluctuating fields and these contributions are often written as socalled turbulent transport coefficients (TTCs). Thus obtaining accurate values for these coefficients is important for the evaluation of meanfield models. Recently, testfield method (TFM) has been used to obtain TTCs from DNSs. The aim of this work is to use meanfield dynamo simulations to validate a testfield module (TFMod), which is an implementation of TFM. Both the TFMod and a meanfield module (MFMod), which is used to calculate the meanfield simulations, are written as modules in Pencil Code, which is a versatile magnetohydrodynamics simulation software. The validation is done by comparing magnetic fields generated using DNS with magnetic fields generated using a meanfield simulation that uses TTCs obtained using the TFMod. In addition the TTCs measured by the TFMod are compared against expected values that are calculated using the secondorder correlation approximation (SOCA). The model chosen for this validation is an α 2 dynamo driven by helical forcing as described by Mitra et al. (2010). The accuracy of the MFMod is confirmed by replicating meanfield models by Krause and Rädler (1980), Steenbeck and Krause (1969) and Jouve et al. (2008). The measured TTCs replicate various expected properties with the exception of the βtensor, which does not appear to be isotropic. This finding was shown by Viviani et al. (2019) to be an artefact from the original TFM tensor decomposition. Using all TTCs the MFMod is able to replicate various features of the DNS such as the growth rate, oscillation period and the field configurations of the magnetic fields. The validation gives credence to the usage of the TFMod for the measurement of TTCs from DNSs, but it also shows that meanfield simulations such as the one described here are important for testing the validity of the TTCs produced by the TFM. It also suggests that utilizing the combination of the TFM and mean field simulations in conjunction with DNS on more complicated dynamos could be an area of interest for future research.
Now showing items 11 of 1