Modeling the Sun's Emerging Magnetic Field

Overview

Computational fluid dynamic simulations model the emergence of magnetic fields through the solar surface and the formation of pores and sunspots.

Project Details

Our goals are to understand the magnetohydrodynamics of solar surface convection, to clarify the role of local surface and deep convection zone dynamo action, and to provide synthetic data for validating local helioseismology and vector magnetogram inversions. We simulate a supergranule-scale portion of the surface layers of the solar convection zone and photosphere—48 and 96 megameters (Mm) wide by 20 Mm deep—with minimally structured, uniform, untwisted, horizontal magnetic fields, advected by inflows through the bottom of the computational domain to represent magnetic flux from a deep dynamo. Fast convective downdrafts pump magnetic flux downward, while slow convective upflows and buoyancy carry magnetic flux toward the surface. This forms a hierarchy of magnetic loops from the largest, supergranule-scales down to small granule-scales.

When Ω-shaped magnetic loops rise through the top boundary, they leave behind vertical flux concentrations, which sometimes form pores where the magnetic field inhibits convection and energy transport. As a result, the surface cools by radiating energy to space and becomes darker.

Results and Impact

This project supports NASA's goals of understanding the Sun and its interactions with the Earth and solar system and, in particular, what causes the Sun to vary. The primary cause of the Sun's short term variability is changes in its magnetic field. This project seeks to understand how the magnetic field in the quiet and plage (bright) regions of the solar surface varies due to the emergence of new magnetic flux that has been produced by dynamo action in both the surface and deeper layers of the convection zone.

Role of High-End Computing Resources

Convection is inherently three-dimensional, non-linear and non-local. As a result, most progress in understanding convection has come from analyzing numerical simulations. The computational challenge in modeling convection is to conduct analyses at a high enough Reynolds number to include interactions of very disparate spatial and temporal scales of motion. This requires simulations with very large, three-dimensional computational grids. We are currently using grids with 2,016 by 2,016 by 500 grid points. While we cannot hope to model the extremely large dynamic range and high Reynolds number that occur in the Sun, we can apply our limited numerical capabilities to provide insight into what phenomena actually occur. These calculations would be impossible without the resources of systems like the Pleiades supercomputer at NASA Ames Research Center.