Small-scale dynamo simulations: Magnetic field amplification in exploding granules and the role of deep and shallow recirculation

TL;DR

This paper investigates how small-scale dynamo processes in the solar photosphere amplify magnetic fields during granule evolution, emphasizing the roles of shallow and deep recirculation in magnetic field organization and energy conversion.

Contribution

It provides new insights into the mechanisms of magnetic field amplification and organization in the solar photosphere, highlighting the importance of recirculation processes and magnetic Prandtl number effects.

Findings

Vertical magnetic fields are amplified from a few 10 G to over 800 G in downflow lanes.

Shallow recirculation transports turbulent magnetic fields into the photosphere within minutes.

Magnetic energy conversion depends strongly on the magnetic Prandtl number.

Abstract

We analyze recent high resolution photospheric small-scale dynamo simulations that were computed with the MURaM radiative MHD code. We focus the analysis on newly forming downflow lanes in exploding granules since they show how weakly magnetized regions in the photosphere (center of granules) evolve into strongly magnetized regions (downflow lanes). We find that newly formed downflow lanes exhibit initially mostly a laminar converging flow that amplifies the vertical magnetic field embedded in the granule from a few 10 G to field strengths exceeding 800 G. This results in extended magnetic sheets that have a length comparable to granular scales. Field amplification by turbulent shear happens first a few 100 km beneath the visible layers of the photosphere. Shallow recirculation transports the resulting turbulent field into the photosphere within minutes, after which the newly formed downflow lane shows a mix of strong magnetic sheets and turbulent field components. We stress in particular the role of shallow and deep recirculation for the organization and strength of magnetic field in the photosphere and discuss the photospheric and sub-photospheric energy conversion associated with the small-scale dynamo process. While the energy conversion through the Lorentz force depends only weakly on the saturation field strength (and therefore deep or shallow recirculation), it is strongly dependent on the magnetic Prandtl number. We discuss the potential of these findings for further constraining small-scale dynamo models through high resolution observations.