Sunspots Like Pinwheels
by R. Komm
Sunspots come in many sizes and shapes and are locations of strong magnetic fields that extend from below the solar surface through the solar atmosphere into the interplanetary medium. They are also the locations of eruptive phenomena, such as flares, that release a tremendous amount of energy and can disrupt technology on Earth. The resulting space weather creates not only the beautiful polar lights but also affects the accuracy of navigation systems (GPS) and threatens the safety of astronauts. Since twisted magnetic fields in large complex sunspots are the likely origin of energy for solar flares, it is of interest to understand the origin of this twist to improve space weather forecasting.
Some spots are deceptively simple like sunspot 11092, observed in August 2010, which was nearly round and not particularly large. It was thus surprising for NSO scientists Komm, Gosain, and Pevtsov, to see that its magnetic field was twisted in such a way as to resemble a pinwheel or hurricane that is rotating counter-clockwise (see Figure). High in the solar atmosphere, sunspot magnetic fields can be seen as mixture of darker structures that are usually twisted in all directions. A twist in the same direction happens only on rare occasions as in the case of sunspot 11092. While the orientation of the twist is clearly visible high in the atmosphere, what is the orientation of the twist at lower heights and below the solar surface? One might expect it to be the same but if not, it would be a clue towards solving the mystery of the causes of solar activity.
Measurements of the strength of the magnetic field along different directions in the lower solar atmosphere are now being made with the Vector Spectromagnetograph (VSM) instrument of the Synoptic Optical Long-term Investigations of the Sun (SOLIS) synoptic facility of NSO. Below the surface, the magnetic field can be estimated by using the motions of the solar material as a substitute. Currently, the only way to measure anything below the solar surface is with helioseismology, which studies changes in sound waves as they travel through the solar interior just as seismology is used in geophysics to study the Earth’s interior. These observations are obtained with the Global Oscillation Network Group (GONG) facility of NSO.
Using data from SOLIS and GONG, Komm and his colleagues found that the twisting in the lower solar atmosphere is in the same direction for sunspot 11092 as seen higher up. However, they found that below the surface the twisting is in the opposite direction. They investigated another sunspot with a clockwise whirl, and found the same result. As a control experiment, Komm and his colleagues analyzed six sunspots without a persistent whirl pattern. The orientation of the twist of the magnetic field at low heights turned out to be the same as that of the flows below the solar surface for four of the six regions. This suggests that opposite directions of twist above and below the solar surface is indeed a characteristic of active regions with whirls. But, could this be a coincidence due to the small sample size? NSO scientists plan to analyze many more active regions and find out. If the relationship turns out to be true, then there must be a thin region immediately below whirly sunspots over which the twist changes rapidly. This could result in an unstable zone that could trigger flares.
More details can be found in: Komm, R., Gosain, S., and Pevtsov, A.: Active Regions with Superpenumbral Whirls and Their Subsurface Kinetic Helicity, Solar Physics 289, p. 475-492, 2014.