Shining a Light on the Sun’s Magnetic Field
Electrical power blackouts. Flares that knock out GPS signals and interrupt air traffic control communications. Fast-moving particles that can endanger astronauts and Earth-orbiting spacecraft. We know magnetic fields develop within our Sun and reconnect in its atmosphere—unleashing flares that can cause damage to electrical systems on Earth and in space. Scientists are working to figure out just how these magnetic fields evolve during a magnetic cycle to better predict when and where flares will occur.
To help make more accurate predictions of solar magnetism, researchers at Northwestern University ran simulations of the Sun and other stars on the Pleiades cluster at the NASA Advanced Supercomputing (NAS) facility and made some discoveries that help answer questions about how these magnetic fields develop and travel to the surface—leading to clues about how they influence the Sun’s atmosphere and Earth’s magnetic shield.
A Better Understanding of How Magnetic Fields are Built
Using observational data from NASA’s Solar Dynamics Observatory (SDO), Solar and Heliospheric Observatory (SOHO), and the National Solar Observatory, these scientists are using asteroseismology techniques to study the sound wave frequencies of the surface of stars, in order to follow and measure those waves and other internal properties in the stars’ interiors—similar to how seismology data is used to measure earthquake activity and properties in Earth’s interior. Their simulations provide a better understanding of how magnetic fields are built and how the flows in the stars’ interiors generate waves that flow to the surface.
“What’s exciting is that with NASA funding and compute resources, we are afforded the opportunity to explore a large range of fluid dynamics flow problems that are related to astrophysics as well as geophysics and experimental fluid dynamics conducted here on Earth,” said Kyle Augustson, a research associate at Northwestern.
HECC resources have underpinned all the results of this project. Using these resources is very easy and has allowed us to produce better results more quickly.
With help from NAS code optimization experts Johnny Chang and Evan Dodge, Augustson and Principal Investigator Daniel Lecoanet were able to run the open-source Dedalus code on Pleiades to solve general partial differential equations using spectral methods. Dedalus is used to simulate a wide variety of physical and mathematical systems. Some of the systems considered by Lecoanet’s group involve the ways convectively generated waves propagate in stars; the stability of magnetic field configurations in a stably stratified plasma; and the assessment of the magneto-rotational instability in the solar near-surface shear layer. The simulations enabled the scientists to compare some of their results to real-world observations of the Sun and other stars.
Convection in the Sun occurs in the outer 30% of its radius, whereas it has a comparatively quiescent deeper interior. The nature of the magnetic field in this deep solar interior, and how it connects to the fields observed at the solar surface, are important questions in heliophysics. Using Dedalus to simulate the stability of the Prendergast magnetic field—a magnetohydrodynamics solution commonly used to study the Sun’s interior—the researchers found that the field is unstable in the presence of diffusion. This means that the widely accepted hypothesis that this magnetic field is stable is not correct—it is unstable over a long timescale.
“From a heliophysics standpoint, the idea of magnetic equilibria—meaning a magnetic field that doesn’t evolve, as it is in balance with the other forces acting in the system—implies that a magnetic field could be hiding deep in the Sun’s stable interior. What we’ve shown is that one such equilibrium—the Prendergast configuration—does not work and that an alternative should be sought for time-dependent simulations,” Augustson explained.
A New Take on the Stellar Dynamo
Simulation results for the magnetic field in the convective zone of the Sun led the Northwestern team to form a new take on the stellar dynamo theory, which describes the process through which a rotating, convecting, and electrically conducting fluid can maintain a magnetic field over astronomical time scales.
“Our new path looks at the magneto-rotational instability, which operates by exchanging the energy in the differential rotation to the magnetic field. This mechanism is potentially at work in the Sun’s near-surface shear layer, the outer region of the convection zone,” said Augustson.
The Northwestern team’s alternate theory of instability in the near-surface shear layer is key to making more accurate predictions of solar magnetism, provides a reliable model to use in future studies of magnetic fields in the Sun and other stars, and guides the interpretation of solar and stellar observations—all in support of the agency’s goals to understand how the universe works and help protect our technology and astronauts in space.
—Jill Dunbar, NASA/Ames Research Center
