Dice Question Streamline Icon: https://streamlinehq.com

Determine the thermal Prandtl number of density fluctuations in the intracluster medium

Determine the effective thermal Prandtl number (the ratio of kinematic viscosity to thermal conductivity) relevant for density fluctuations in the high-beta, weakly collisional intracluster medium of galaxy clusters, in order to accurately interpret observed density-fluctuation power spectra and to refine constraints on effective viscosity and conduction in cluster plasmas.

Information Square Streamline Icon: https://streamlinehq.com

Background

In comparing observed, rescaled power spectra of density fluctuations from 16 clusters to passive-scalar spectra from direct numerical simulations (DNS), the analysis requires an assumption about the thermal Prandtl number, Pr, which controls the relative roles of viscosity and thermal conduction in shaping scalar cascades.

Because the intracluster medium (ICM) is a high-beta, weakly collisional plasma with anisotropic transport, the effective, macroscopic Prandtl number governing density fluctuations is not directly known. The authors therefore explored several values (Pr = 1.0, 0.25, 0.11) to bracket plausible regimes and derived viscosity-suppression limits under each case.

A precise determination of the effective Prandtl number for the ICM would directly sharpen viscosity constraints inferred from the spectral steepening (or lack thereof) at microscale-normalized wavenumbers, and improve physical modeling of transport processes in the bulk ICM.

References

We note that the Prandtl number of density fluctuations in the ICM is generally unknown, however it is expected to be less than unity \citep{kunz_plasma_2022}, thus we vary $Pr$ in this range.

— Merger-driven multi-scale ICM density perturbations: testing cosmological simulations and constraining plasma physics (2401.15179 - Heinrich et al., 26 Jan 2024) in Section 4.3 (Constraints on effective viscosity)