Solar faculae

Definition

The word facula (plural: faculae) derives from the Latin word for torch. Faculae are bright spots on the Sun’s visible surface or photosphere. Faculae might be thought of as the opposite of sunspots. Sunspots are dark areas on the Sun’s surface caused by the solar deflecting energy coming up from within the Sun. Faculae are bright areas on the Sun’s surface that are also caused by the solar magnetic field. However, for faculae, the magnetic field concentrates rather than deflects energy from the interior. Because both and faculae result from solar magnetic activity, the numbers of both increase and decrease together as they follow the Sun’s activity cycle.

Faculae are most easily observed near the “edge” of the Sun. A phenomenon called limb darkening causes the edge of the Sun’s disk to appear fainter from Earth, even though it emits just as much energy. Limb darkening increases the contrast between faculae and the Sun’s surface, so faculae show up more clearly near the edge of the Sun.

Faculae can extend upward from the Sun’s photosphere into the Sun’s chromosphere, the layer directly above the photosphere. These extensions of faculae into the chromosphere are called plages. Both faculae and plages are always found around sunspots or sunspot groups. They can also occur on unspotted regions of the Sun’s surface.

Near the Sun’s surface, energy is transferred from the interior by convection currents similar to those that heat a room containing a radiator on one side but no fan. These convection currents make the Sun’s surface very turbulent, and the turbulence causes granules, which are higher regions at the top of convection-current cells. Faculae form in the lower regions along the boundaries between granules.

Significance for Climate Change

Bright faculae and dark sunspots both increase and decrease in time with the Sun’s eleven-year sunspot cycle. Accurate satellite measurements over the course of these sunspot cycles show that during maximum sunspot activity, the Sun emits about 0.15 percent more energy than during minimum sunspot activity. Thus, the total extra energy emitted by faculae is slightly greater than the total energy blocked by sunspots. The effect of the eleven-year sunspot cycles is too small and rapid noticeably to affect Earth’s climate; however, there are also longer, less regular, and poorly understood cycles in the Sun’s activity.

During the Maunder Minimum, from about 1645 to about 1715, there were very few sunspots and correspondingly few faculae. If the observation that, during the eleven-year cycle, the Sun is fainter during sunspot minimum holds for longer cycles, then the Sun should also have emitted less energy during the Maunder Minimum. Accurate instruments for measuring the Sun’s total energy output did not exist in the seventeenth century, so this hypothesis could not be tested directly. However, it is known that this time period was the coldest portion of the Little Ice Age. Thus, circumstantial evidence indicates that the Sun did emit less total energy during the Maunder Minimum.

During the period from about 1000 to about 1200, the Medieval Grand Maximum, the Sun had many more sunspots and faculae than normal. If the hypothesis that the Sun emits more energy during periods of high sunspot activity is correct, then the Sun should have been more luminous than normal during these centuries. Climate research shows that, excepting the latter portion of the twentieth century, this period was the warmest of the last one thousand years. For example, a Viking colony flourished on Greenland during this period, but it was abandoned when the climate cooled again.

Climate studies combined with solar-activity studies over the last millennium show that during extended sunspot maxima Earth’s climate is warmer and during extended sunspot minima Earth’s climate is cooler. Faculae play an important role in these climate changes. As the bright areas on the Sun’s surface, they contribute to the Sun’s increased energy output during periods of high solar activity. Their lack also contributes to decreased energy output during periods of lower solar activity.

Many of the eleven-year solar-activity cycles during the late twentieth century had higher levels of maximum solar activity, so John Eddy and a few other scientists have suggested that the Sun might be entering a period of increased activity. The increased solar activity to date is probably not sufficient to have caused the current global warming. However, it might have contributed to the trend on a small scale. Understanding faculae, sunspots, and their relation to the total solar energy output is important to completely understanding long-term climate changes on Earth.

Bibliography

Eddy, J. A. “The Maunder Minimum.” Science 192 (1976): 1189-1192.

"Faculae and Sunspots at Solar Maximum and Solar Minimum." Scientific Visualization Studio, NASA, 18 Jan. 2022, svs.gsfc.nasa.gov/4892. Accessed 26 Dec. 2024.

Foukal, P., C. Fröhlich, H. Spruit, and T. M. L. Wigley. “Variations in Solar Luminosity and Their Effect on Earth’s Climate.” Nature 443 (2006): 161-166.

Golub, Leon, and Jay M. Pasachoff. Nearest Star: The Surprising Science of Our Sun. Cambridge, Mass.: Harvard University Press, 2001.

Hoyt, Douglas V., and Kenneth H. Schatten. The Role of the Sun in Climate Change. New York: Oxford University Press, 1997.

Soon, Willie Wei-Hock, and Steven H. Yaskell. The Maunder Minimum and the Variable Sun-Earth Connection. River Edge, N.J.: World Scientific, 2003.