Astroplasmas Seminar

Next seminar 

Friday,  October 30, 12:30 pm, exclusively on Zoom but also in the Dome  Room

Title: Does MHD Turbulence Care About Its Prandtl Number? Plasmoids, Decay, and Tunable Numerical Dissipation

Speaker: Vinay Kumar (International Centre for Theoretical Sciences, India)

Abstract: The magnetic Prandtl number, the ratio of viscosity to resistivity, is rarely of order unity in astrophysical plasmas. Most MHD simulations, explicit or implicit, nonetheless run with it at or near unity. In this talk I ask when MHD turbulence actually cares about its dissipative coefficients, and whether we can control them even when they are purely numerical.

In the first part, I revisit how the plasmoid-mediated reconnection rate depends on the magnetic Prandtl number. The question is motivated by freely decaying magnetic fields in cosmic voids. Reconnection-controlled decay models assume that plasmoid reconnection is strongly suppressed at high Prandtl number. However, DNS of decaying turbulence find decay times independent of both viscosity and resistivity. I will present high-resolution simulations of coalescing magnetic islands that probe this tension deep in the plasmoid regime, and discuss what they imply for reconnection as the mechanism governing turbulent decay.

In the second part, I turn to implicit large eddy simulations (ILES), where the discretization itself sets the viscosity and resistivity. As a result, the effective magnetic Prandtl number is usually locked near unity. I will show how the scheme can be modified to change this, and ask whether large-scale turbulent statistics respond as they would to physical dissipation.