BY DR EMILY BALDWIN
Posted: 19 June, 2009
Sunspots, dark markings on the Sun’s surface, are the manifestations of intense magnetic activity associated with solar flares and massive ejections of charged plasma that create geomagnetic storms on the Earth. Sunspots can also govern the variation in solar output, which in turn can influence weather and climate patterns on the Earth. Because they are formed by magnetic field lines, sunspots occur in pairs at the base of each end of the magnetic loop that formed them, and exhibit opposite polarities.
The interface between a sunspot's umbra (dark centre) and penumbra (lighter outer region) shows a complex structure with narrow, almost horizontal (lighter to white) filaments embedded in a background having a more vertical (darker to black) magnetic field. Farther out, extended patches of horizontal field dominate. Image: UCAR/Matthias Rempel/NCAR.
Scientists have been striving to understand the complexities of sunspots since they were discovered some 100 years ago. The new high-resolution simulations of sunspot pairs, created on the latest generation of supercomputer and using observational data of the Sun itself, will offer a window into the physical processes occurring inside our star and how it makes its presence known to the Earth.
“This is the first time we have a model of an entire sunspot,” says lead author of the paper published in this week’s Science Express Matthias Rempel. “If you want to understand all the drivers of Earth’s atmospheric system, you have to understand how sunspots emerge and evolve. Our simulations will advance research into the inner workings of the Sun as well as connections between solar output and Earth’s atmosphere.”
The models capture pairs of sunspots with opposite polarity and shows in unprecedented detail the dark central region – the umbra – with brighter umbral dots, and webs of elongated narrow filaments streaming out towards the peripheral penumbral regions. As well as the flow of mass, the models also record the convective flow and movement of energy that underlie the sunspots, and that are not otherwise directly detectable by instruments. These models suggest that the magnetic fields within sunspots need to be inclined in certain directions in order to create such complex structures, leading the authors to the conclusion that the structure is a consequence of convection in a magnetic field with varying properties.
Click to enlarge this first view of what goes on below the surface of sunspots. Lighter/ brighter colours indicate stronger magnetic field strength in this subsurface cross section of two sunspots. Image: UCAR/Matthias Rempel/NCAR.
Although the new models are far more detailed and realistic than previous simulations, the researchers caution that even their model does not accurately capture the lengths of the filaments in parts of the penumbra. Refining the model even further would require more computing power than is currently available – the model already makes 76 trillion calculations per second for a three dimensional simulated area measuring 50,000 by 100,000 across and 6,000 kilometres deep. Within the model, sunspot dynamics – including information on energy transfer, fluid dynamics and magnetic induction – are simulated at 1.8 billion points, with each point spaced about 16 to 32 kilometres apart.
“Advances in supercomputing power are enabling us to close in on some of the most fundamental processes of the Sun,” says Michael Knolker, director of NCAR’s High Altitude Observatory and a co-author of the paper. “With this breakthrough simulation, an overall comprehensive physical picture is emerging for everything that observers have associated with the appearance, formation, dynamics, and the decay of sunspots on the Sun’s surface.”