Abstracts Prepared by Workshop Speakers - The Effects of Solar Variability on Earth's Climate - page 38

Recent Findings on Dimming of the 17th Century Sun
Peter Foukal, Heliophysics, Inc.
Sunspots and faculae modulate solar convective heat flow and total solar irradiance (TSI) over the
11- year activity cycle. The amplitude of this modulation attains 0.09 percent in annual means during the
largest recorded solar cycle that peaked around 1957. The rising envelope of this modulation, caused by
the increase in spots and faculae observed since the beginning of regular collection of sunspot data around
1700, sets a lower limit of about 0.04 percent to the 11-year smoothed TSI increase over the intervening
2.5 centuries.
Additional solar dimming might have occurred during the 17th century if the area covered by the
small-diameter, bright photospheric magnetic flux tubes of the quiet Sun decreased during the extended
Maunder Minimum of solar activity between about 1645 and 1715. Their contribution sets the zero level
of TSI at activity minima. Evidence for their disappearance and for a resulting additional 0.2 percent
dimming during the Maunder Minimum was put forward, based on stellar photometry. 1 That stellar
photometry evidence was retracted in 2002, but this does not necessarily mean that the dimming did not
occur.
New findings from solar photometry indicate that the TSI-effectiveness per unit area of these
small flux tubes increases with decreasing cross sectional area. So the progressive removal of ever-
smaller flux tubes with declining solar activity during an extended minimum would dim the Sun more
than expected from standard irradiance models. Such models linearly extrapolate the irradiance
contribution of larger active region faculae, to the smaller flux tubes of the quiet Sun.
These findings 2 make it more likely that the 17th century Sun might have dimmed by a
climatologically significant (~ 0.2 percent) amount without requiring complete disappearance of
photospheric magnetism. Such partial disappearance would agree better with radioisotope evidence that a
weakened 11-year cycle persisted throughout the Maunder Minimum. Improved observations are under
way to accurately measure the uncertain TSI contribution of the quiet Sun flux tubes down to sizes barely
resolvable with the largest solar telescopes. 3
Also, it remains to be seen whether 17th century photospheric magnetism weakened below the
level observed during normal 11-year activity minima. 4 During the extended activity minimum of 2008-
2009, the main indices such as F10.7 and Mg II dipped several percent below their preceding 11-year
minima. These anomalous dips during a minimum only about 1 year longer than normal, suggest that
magnetism during a minimum extending for 70 years would have decayed well below quiet Sun levels.
However, examination of this important conclusion using, for example, improved 10 Be radioisotope
evidence is desirable. This work has been supported by NASA LWS grants NNX09AP96G and
NNX10AC09G.
1 J. Lean, A. Skumanich, and O. White, Estimating the Sun's radiative output during the Maunder Minimum,
Geophysical Research Letters 19(15):1591, 1992.
2 P. Foukal, A. Ortiz, and R. Schnerr, Dimming of the 17th Century Sun, The Astrophysical Journal Letters
733:L38, 2011.
3 R. Schnerr and H. Spruit, The brightness of magnetic field concentrations in the quiet Sun, Astronomy and
Astrophysics 532:A136, 2011.
4 C. Schrijver, W. Livingston, T. Woods, and R. Mewaldt, The minimal solar activity in 2008-2009 and its
implications for long-term climate modeling, Geophysical Research Letters 38: L06701, 2011.