Solar Faculae
Bright Regions on Sun's Photosphere or Visible Surface
Faculae (singular facula) are in a way the opposite of sunspots. Sunspots are darker than normal spots on the Sun's visible surface, the photosphere. Faculae are brighter than normal regions on the photosphere.
Observing Faculae
Never observe the Sun with an unfiltered telescope! However with a properly filtered solar telescope, faculae are most easily observed near the edge of the Sun's visible disk, the limb. Emitting as much energy, the limb appears fainter than the center because of an effect called limb darkening. Limb darkening increases the contrast between the faculae and the surrounding photospheric surface. Hence a facula is more easily observed near the edge of the Sun, but faculae occur just as frequently away from the limb.
Cause of Faculae
The Sun's magnetic field causes both faculae and sunspots. Convection currents transfer energy from the Sun's interior to the surface by moving the hot atoms from the interior to the surface. Solar convection currents therefore cause both physical movement of the material and considerable turbulence. The convection currents cause granules on the Sun's photosphere, which give it a mottled appearance. The granules are areas where the convection current cells push material upward
Turbulence affects the Sun's magnetic field. Sunspots are areas where the Sun's concentrated magnetic field deflects hot material flowing upward from the interior producing a cooler region. Faculae form where the magnetic field concentrates hot material flowing from the interior producing a hot spot. Faculae always form in areas surrounding sunspots. Faculae sometimes form along the boundaries between granules. The boundaries between granules are lower in the Sun's photosphere than the granules, which are at the top of convection cells. Therefore these boundaries are brighter and hotter, forming faculae.
Faculae and Plages
The layer directly above the Sun's photosphere is the chromosphere. Faculae occur on the photosphere, but they can extend upwards into the chromosphere. When they do, the upward extensions of faculae into the chromosphere are called plages. Plages are brighter than normal regions in the Sun's chromosphere.
Faculae and Solar Variability
The Sun's total energy output, its luminosity, varies with the 11 year sunspot cycle. Sunspots tend to make the Sun's luminosity slightly less, while faculae tend to increase the luminosity. The effects do not however exactly cancel out.
The numbers of both sunspots and faculae are greater during sunspot maximum than during sunspot minimum. This cycle in the numbers of sunspots is 11 years long. Satellite measurements show that during sunspot maxima the Sun is very slightly more luminous than during sunspot minima. The brighter faculae have a slightly greater effect on the Sun's luminosity than the fainter sunspots.
Faculae, Sunspots, and Climate Change
The changes in the solar luminosity during the 11 year sunspot cycle are too rapid to cause long-term climate change. Normal year to year variations are greater. However there was an extended sunspot minimum during the 17th century. The extended Maunder minimum in sunspot activity coincided with the coldest portion of the Little Ice Age in the late 17th century. The Little Ice Age may have been caused by a small decrease in the Sun's energy output during a period with very few faculae or sunspots.
There are also other times in the past 1,000 years when changes in the solar luminosity may have affected Earth's climate. As part of the mechanism affecting the Sun's luminosity, faculae may significantly affect Earth's climate.
Further Reading
Golub, L. & Pasachoff, Jay M. Nearest Star The Surprising Science of Our Sun, Harvard, 2001.
Solar Faculae and Sunspots
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