Small-scale dynamo Simulations

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Wednesday, December 13, 2017

Magnetic field amplification in exploding granules and the role of deep and shallow recirculation

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 the most weakly magnetized regions in the photosphere (center of granules) evolve into the most 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 initially a few 10 G to field strengths of up to 1 kG on a time scale of about 2 minutes.

Magnetic field amplification in exploding granules image
Evolution of bolometric intensity (left), vertical velocity (middle), and vertical magnetic field (right) in an exploding granule over a time span of about 400 seconds (top to bottom). The initial magnetic field amplification by horizontally converging flows (top three snapshots) leads to strong magnetic sheets in the newly forming downflow lane. Turbulent magnetic fields appears at later stages as consequence of shallow recirculation at the edge of granules.

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. In all the analyzed cases we find that the turbulent field appears only on one side of the downflow lane, which suggests asymmetric horizontal vorticity that is also evident in the emergent intensity at the edges of the nearby granule. We discuss the potential of these findings for further constraining small-scale dynamo models through high resolution observations. We stress in particular the role of shallow and deep recirculation for the organization and strength of magnetic field in the photosphere. We estimate that the total energy conversion of the small-scale dynamo accounts to about a third of the solar luminosity when integrated over the uppermost 1.5 Mm of the convection zone.

Publication Name: Astrophysical Journal