Contributed Talk - Splinter Magnetic

Thursday, 24 September 2020, 16:40   (virtual room H)

Small-scale dynamo action at small magnetic Prandtl numbers in the Sun

Warnecke, J. (1), Käpylä, M. (2,1), Gent, F. (2) & Rheinhardt, M. (2)
(1) Max Planck Institute for Solar System Research, (2) Aalto University

The Sun exhibits magnetic fields on various scales ranging from the size of the star all the way down to the smallest observable scales and probably below. The small-scale magnetic field can be generated by a small-scale dynamo instability (SSD). The very low magnetic Prandtl number (Pm) in the Sun, however, challenges the existence of the SSD, as previous numerical studies have shown that the critical magnetic Reynolds number (Rm) for its excitation increases with decreasing Pm. We present here a numerical study investigating the SSD at lower Pm than studied previously, using numerical simulations of forced turbulence with fluid Reynolds number (Re) up to 40.000 and a resolution of 4096^3 grid points. We found that the critical Rm first increases as function of decreasing Pm, but then starts decreasing again: SSD can still be excited with a Pm of 0.005 and Rm of around 180. The increase and later decrease of the critical Rm for low Prandtl numbers in our simulations can be explained by the bottleneck effect, an effect where kinetic power spectrum has an local maximum between the inertial and dissipative scale. Our study predicts that the SSD in the Sun should be easier to excite than previously thought.