Simulations of quiet Sun magnetism: On the role of deep and shallow recirculation in small-scale dynamo simulations
Abstract
Observations suggest that small-scale magnetic field in the solar photosphere is mostly independent from the strength of nearby network field as well as independent of the solar cycle. This supports the view that the origin of small-scale magnetism is due to a small-scale dynamo that operates independently from the large-scale dynamo responsible for the solar cycle. The saturation field strength and structure of the resulting magnetic field in the photosphere depends critically on the contributions from deep and shallow recirculation within the strongly stratified convection zone. 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 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. This results in extended magnetic sheets that have a length comparable to granular scales. These sheets are a consequence of deep recirculation. 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. Furthermore, deep recirculation leads to a magnetic flux imbalance on larger scales that can maintain a quiet Sun (mixed polarity) magnetic network solely through small-scale dynamo action. We discuss the potential of these findings for further constraining small-scale dynamo models through high resolution observations.
- Publication:
-
2018 Triennial Earth-Sun Summit (TESS)
- Pub Date:
- May 2018
- Bibcode:
- 2018tess.conf11505R