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geomagnetic field uncertainties. Muscheler stated that further research would be required to understand
these differences.
Muscheler pointed out that there is good agreement between 10 Be and 14 C on the scale of the 11-,
88-, and 207-year solar cycles and that those signals can be clearly seen. On the other hand, there is no
evidence of sustained periods on the order of 1,000 years of low solar activity in either the 10 Be or the 14 C
record. This can be said with some confidence for the records going back over the past 10,000 years;
however, characterization any farther back than that is more complicated because of the influence of
climate change during the last ice age on the 10 Be record.
In response to a question from the audience on the “climate/cosmic ray hypothesis” (i.e., that
cosmic rays decreased over the last half of the 20th century and that this decrease is linked to the climate
change of the past 30 years), Muscheler stated that proxy data indicate that the cosmic-ray flux actually
decreased early in the 20th century, but recently the level has been steady and high. Based on the
proposed link between increased GCR flux and cloudiness, one might have expected that the late 20th
century would be cooler than the early 20th century—a state that was not observed.
Another audience member pointed out that it is necessary to be careful about the scale of the solar
activity minima; minima on the scale of the heliosphere are not appropriately grouped with those on the
scale of a hundred kilometers. The relationship between the large- and the small-scale field of the Sun is
not known. Muscheler agreed that in his radionuclide data, only the solar modulation of GCRs can be
clearly seen.
Solar Grand Minima Inferred from Observations of Sun-like Stars
Dan Lubin, Scripps Institution of Oceanography, University of California, San Diego
Dan Lubin discussed how the behavior of Sun-like stars can provide insight into the Sun's
activity and how solar forcing may change in the future. The frequency of grand minima (Maunder
Minimum-like occurrences) is difficult to extract from the geophysical proxy record. In a sample of
solar-type stars, the fraction of very inactive stars is analogous to the fraction of the Sun's lifetime spent
in a Maunder Minimum-like state. Early estimates of grand minimum frequency in solar-type stars
ranged from 10 to 30 percent, 15 implying that the Sun's influence could be overpowering. It was later
determined using much more accurate distance data from the European Space Agency's Hipparcos Space
Astrometry mission that these studies included many stars that evolved off the main sequence and are no
longer burning hydrogen like the Sun. 16,17 More recent studies, using the Hipparchos data and accounting
for the metallicity of the star, place the estimate in the range of less than 3 percent for the fraction of the
Sun's lifetime spent in a Maunder Minimum-like state of low activity. 18 The deduced frequency of
occurrence of a Maunder Minimum state is sensitive to the choice of metallicity threshold and the
definition of level corresponding to “inactive.”
Lubin pointed out that the early pre-Hipparcos estimates of Maunder Minimum analog frequency
gave estimates that are too large. Instantaneous activity measurements of the hydrogen and potassium
spectral lines (HK) are suggestive but not conclusive for identifying Maunder Minimum analog
candidates; the result depends strongly on the chosen inactive threshold. Very low activity may be seen
with an old star nearing the end of its main sequence lifetime. However, the historical Maunder
Minimum most likely did involve very low HK activity and weak cycling compared with the present-day
15 S. Baliunas, and R. Jastrow, Evidence for long-term brightness changes of solar-type stars, Nature 348:520-
523, 1990.
16 J.T. Wright, Do we know of any Maunder Minimum stars? The Astrophysics, Journal 128:1273, 2004.
17 P.G. Judge, and S.H. Saar, The outer solar atmosphere during the Maunder Minimum: A stellar perspective,
The Astrophysics Journal 663:643, 2007.
18 D. Lubin, D. Tytler, and D. Kirkman, Frequency of Maunder minimum events in solar-type stars inferred
from activity and metallicity observations, The Astrophysical Journal Letters 747: L32, 2012.
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