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FIG. 9.10 Varying solar heating ofthe upper atmosphere During the Sun's 11-year activity cycle, the upper-atmosphere temperatures fluctuate by factors of two, and neutral (un-ionized atom) and electron densities by factors of ten. The bold lines (rightside) register maximum values and the less bold (left side) the minimum values. Enhanced magnetic activity on the Sun produces increased ultraviolet and X-ray radiation that heats the Earth's upper atmosphere and causes it to expand, resulting in higher temperatures and greater densities at a given altitude in our atmosphere. (Courtesy of Judith Lean.)

layer lower in the atmosphere. Only the least variable, visible portion of the solar spectrum penetrates through to the relatively placid lower atmosphere, but this sunlight might still vary enough to warm and cool the air.

Moreover, the 11-year cycle that clocks the rise and fall in magnetic activity on the Sun does not repeat with the same strength at maximum. It instead seems to be modulated over 100-year time intervals.

Solar Variability Over the Centuries

Despite diligent observations by European astronomers, very few dark spots were found on the Sun between 1645 and 1715, a 70-year period that included the reign of France's "Sun King" Louis XIV (1638-1715). tte German astronomer Gustav Sporer (1822-1895) called attention to the 70-year absence in 1887. An indirect consequence of the missing sunspots was reported much earlier, in 1733, by the French author Jean Jacques D'Ortous de Mairan (1678-1771), as a decrease in the number of auroras seen on Earth, but he was ridiculed for thinking that the northern lights could be related to increases in the number of sunspots.

E. Walter Maunder (1851-1928), at the Royal Greenwich Observatory in England, fully documented the dearth of sunspots using extensive historical records covering hundreds of years. His accounts, entitled A Prolonged Sunspot Minimum, were presented to the Royal Astronomical Society in 1890 and 1894, but they remained largely ignored until the 1970s when the American solar astronomer John A. "Jack" Eddy (1931- ) provided further evidence for the "Maunder Minimum", as he called it, using the growth rings of trees.

As the trees lay down their rings each year, they record the amount of atmospheric carbon dioxide captured in the process of photosynthesis, tte carbon intake comes in two varieties, or isotopes, stable carbon 12 and radioactive carbon 14, and the radioactive type tells us how active the Sun was at the time. Cosmic rays from outer space produce radioactive carbon 14 when they strike atoms in the air. Because the cosmic rays are deflected away from the Earth by the Sun's magnetic fields during high solar-activity levels, there is less radioactive carbon in the air when the Sun is very active, and more of it at times oflow activity on the Sun.

An analysis of the world's longest-lived trees, the bristle cone pines, suggests that the Sun's output has been turned low for several extended periods in the past millennia. Eddy used the technique to read the history of solar activity all the way back to the Bronze Age, and showed that the tree-ring data are supported by other evidence such as the ancient sightings of terrestrial auroras (Fig. 9.11). In an important paper entitled "tte Maunder Minimum", published in Science on 18 June 1976, Eddy concluded that the Sun has spent nearly a third of the past two thousand years in a relatively inactive state. He pinpointed several periods oflow activity with significantly more radioactive carbon 14, each about a century long, naming them the Maunder, Sporer and Wolf minima.

Two periods of missing sunspots, the Sporer minimum from 1460 to 1550 and the Maunder Minimum from 1645 to 1715, occurred during the Little Ice Age (1400-1850) when Europe and North America endured a period of extremely harsh winters and colder than normal summers. During that time, alpine glaciers expanded, and the river ttames in London and the canals of Venice regularly froze over. Owing to the apparent

FIG. 9.11 Long periods ofsolar inactivity Three independent indices demonstrate the existence of prolonged decreases in the level of solar activity. The observed annual mean sunspot numbers (scale at right) also follows the 11-year solar activity cycle after 1700. The curve extending from A.D. 1000 to 1900 is a proxy sunspot number index derived from measurements of carbon-14 in tree rings. Increased carbon-14 is plotted downward (scale at left-inside), so increased solar activity and larger proxy sunspot numbers correspond to reduced amounts of radiocarbon in the Earth's atmosphere. Open circles are an index of the occurrence of auroras in the Northern Hemisphere (scale at lefi-outside). The pronounced absence of sunspots from 1645 and 1715 is named for the English astronomer E. Walter Maunder (1851-1928), who fully documented it, and another noticeable lack of solar activity is named for the German astronomer Gustav Sporer (1822-1895) who previously called attention to the prolonged absence between 1645 and 1715. The third prolonged absence of sunspots is named for the Swiss astronomer Johann RudolfWolf (1816-1893) who investigated the connection of the 11-year sunspot cycle with geomagnetic activity and devised what is now known as the Wolf sunspot number. (Courtesy of John A. "Jack" Eddy.)

lack of solar activity, the Sun was probably noticeably dimmer, suggesting that it might be related to the colder temperatures during the Little Ice Age. But many uncertainties preclude a definitive understanding of the Sun's role in the Little Ice Age, and no one is certain what brought it about some 500 years ago.

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