Slow Appearance of Sunspots Challenges Theory

Sunspot groups (active regions) are the most prominent manifestation of the 11 year solar magnetic cycle. Understanding how sunspots form and evolve provides crucial constraints on models of the solar cycle and is also central to understanding and predicting the impacts of the varying solar activity in terms of solar flares and coronal mass ejections. Sunspots form when bundles of magnetic field originating in the solar interior pierce the visible surface (photosphere) of the sun, but little is known about the structure and strength of such magnetic field prior to the appearance in the photosphere. Since sunspots are very strong magnetic field concentrations in the photosphere (about 10,000 times the strength of the earth’s magnetic field), the prevailing models assumed that magnetic field connected to active regions is also strong in the solar interior. This assumption has been put to a test in a new study, which was published July 13 in the journal Science Advances.

Flux emergence simulation image
Example of a flux emergence simulation performed by HAO scientist Matthias Rempel. The top panels show intensity (left) and the vertical component of the magnetic field (right), the bottom panels the vertical flow velocity (left, upflows red, downflows blue) as well as the subsurface field strength (right) on a vertical slice through the center of the simulation domain. Flux emergence simulations were compared to observational constraints in order to infer the maximum permissible sub-photospheric flux emergence velocity. Computing time was provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center under projects s1325 and s1326.

HAO along with researchers from the Max Planck Institute for Solar System Research, The University of Göttingen, and NorthWest Research Associates have now shown that magnetic flux concentrations forming active regions on the sun emerge at a rate much slower than predicted by the prevailing current model.

In their study they compared satellite observations from the NASA Solar Dynamics Observatory (SDO) and computer simulations performed by HAO scientist Matthias Rempel. It was found that typical emergence velocities cannot exceed 150 m/s in a depth of 20,000 km beneath the solar photosphere, which is a velocity comparable to convective motions. This shows that convection in the solar interior plays a crucial role even for active region scale flux emergence. This finding suggests that sunspots originate from a magnetic field in the solar interior that is sufficiently weak to be mostly passive with respect to convective motions. Understanding the details of sunspot evolution requires a proper characterization of the large-scale convection patterns in which they evolve.