The Inconstant

Solar Radiation Varies Over the 11-Year Activity Cycle

Day after day the Sun rises and sets in an endless cycle, an apparently unchanging ball of heat and light, whose radiation makes life on Earth possible, tte total amount of this life-sustaining energy has been called the "solar constant," because no variations could be reliably detected from anywhere beneath the Earth's changing atmosphere. Indeed, only a few decades ago, most astronomers and climatologists insisted that the Sun shines steadily.

Yet, reliable as the Sun appears, it is an inconstant companion, tte Sun actually fades and brightens in step with the changing level of magnetic activity, and no portion of the Sun's radiation output is invariant. Indeed, the Sun's radiation is variable on all measurable time scales, and this inconstant behavior can be traced to the pervasive role played by magnetic fields in the solar atmosphere.

tte discovery of variations in the total solar radiation output, or solar constant, was somewhat unexpected. For instance, a group led by Charles Greeley Abbot (1872-1973), secretary of the Smithsonian Institution from 1919 to 1944, carried out daily measurements of the solar constant for decades, including expeditions to four continents from sea level to mountaintops. And despite Abbot's claim that the Sun is a variable star, his colleagues concluded that any fluctuations noticed by Abbot were due to varying atmospheric absorption of sunlight or to improperly calibrated instruments.

Definite evidence of the changing solar constant was not obtained until the early 1980s, with an exquisitely sensitive detector, created by Richard C. Willson (1937-) of the Jet Propulsion Laboratory in Pasadena, California. He measured the solar constant with his Active Cavity Radiometer Irradiance Monitor, or ACRIM for short, which was placed aboard NASA's Solar Maximum Mission, abbreviated SMM, satellite, launched in February 1980. tte solar constant, defined as the average amount of radiant solar energy per unit time per unit area reaching the top of our atmosphere at the Earth's mean distance from the Sun, was found to be remarkably constant, rarely changing by more than 0.2 percent, or one part in 500, with an average minimum value of about 1365.56 ± 0.01 watts of power per square meter.

But ACRIM determined the solar constant with an incredible precision of 0.01 percent or one part in 10,000, when averaged over one day, and at this level of accuracy, the Sun's total radiation output is almost always changing, at amounts of up to a few tenths of a percent and on all time scales from 1 second to 10 years. Similar variations were also detected by the Earth Radiation Budget (ERB) radiometer aboard the Nimbus 7 satellite, launched in October 1978, with an even longer record, but with less precision.

Hugh S. Hudson (1939- ), a solar physicist at the University of California at San Diego, teamed up with Willson to help him identify the irradiance variations with well-known features on the Sun. tte largest, relatively brief, downward excursions correspond to, and are explained by, the rotation of a large group of sunspots across the face of the Sun. tte concentrated magnetism in sunspots acts as a valve, blocking the energy outflow, which is why sunspots are dark and cool, producing reductions in the solar constant of up to 0.3 percent that last a few days.

Bright patches on the visible solar surface cause the increased luminosity, ttey are called faculae, from the Latin for "little torches." On the short-term scale of minutes to days, sunspot blocking dominates facular brightening; on the long-term scale of months and years, faculae are dominant because they have significantly longer lifetimes than sunspots and cover a larger fraction of the solar disk.

tte decade-long variations in the Sun's total radiation output are the result of a competition between dark sunspots, in which radiation is depleted, and bright faculae, which are sources of enhanced radiation, with the faculae winning out. As magnetic activity increases, they both appear more often, but the excess radiance from bright faculae is greater than the loss from sunspots. ttis was demonstrated by Peter V. Foukal (1945- ), of Cambridge Research and Instrumentation, Inc., in Massachusetts, and Judith Lean (1953- ) of the Naval Research Laboratory in Washington, DC. ttey showed that there is a good correlation between the long term, solar constant variations and facular emission from the entire solar disk, once the effects of sunspot dimming have been removed.

More than five instruments on different spacecraft have made precise measurements of the solar constant for more than three decades, including the second Active Cavity Radiometer Irradiance Monitor, or ACRIMII, on the Upper Atmosphere Research Satellite, abbreviated UARS and launched in 1991, and the Variability of IRradiance and Gravity Oscillations, or VIRGO, instrument on the SOlar and Heliospheric Observatory, or SOHO for short and launched in 1995. ttey indicate that during recent epochs of high solar activity, near maxima in the 11-year solar cycle, the mean levels of the solar constant increase by about 0.7 percent, relative to solar cycle minimum levels (Fig. 9.9).

FIG. 9.9 Variations in the solar constant Observations with very stable and precise detectors on several Earth-orbiting satellites from 1978 to 2005 show that the Sun's total radiation input to the Earth, termed the solar irradiance, is not a constant, but instead varies over time scales of days and years. Here the total irradiancejust outside our atmosphere, also called the solar constant, is given in units of watts per square meter, abbreviated W mr2, where one watt is equivalent to onejoule per second. The observations show that the Sun's output fluctuates during each 11-year sunspot cycle, changing by about 0.1 percent between maximums (1979, 1990 and 2001) and minimums (1987 and 1997) in magnetic activity. The average minimum value is 1365.560 ± 0.009 W m~2, and the cycle amplitudes are 0.934 ± 0.019, 0.897 ± 0.020 and 0.829 ± 0.017 W mr2 above the average minimum value. Temporary dips of up to 0.3 percent and a few days' duration are due to the presence of large sunspots on the visible hemisphere. The larger number of sunspots near the peak in the 11-year cycle is accompanied by a rise in magnetic activity that creates an increase in luminous output that exceeds the cooling effects of sunspots. The capital letters given at the top are acronyms for the different radiometers. (Courtesy of Claus Fröhlich.)

tte entire spectrum of the Sun's radiation is modulated by solar activity. Overwhelmingly the greatest part of solar radiation is emitted in the visible part of the spectrum where the 11-year variations are a relatively modest 0.1 percent. In contrast, radiation at the short-wavelength, invisible parts of the solar spectrum change significantly during the solar cycle, though contributing only a tiny fraction of the Sun's total radiation, tte ultraviolet radiation is at least ten times more variable than visible radiation, and the Sun's X-ray emission changes even more, by at least a factor of one hundred.

tte Earth's upper atmosphere acts as a sponge, soaking up the unseen ultraviolet and X-ray radiation. At times of high solar activity, the Sun pumps out much more of the invisible rays, the air absorbs more of them, and our upper atmosphere heats up; when solar activity diminishes the high-altitude air absorbs less and cools down.

tte X-rays are absorbed at high altitudes in the terrestrial atmosphere, where the global mean temperature can double between the minimum and maximum of solar activity (Fig. 9.10). tte Sun's variable ultraviolet radiation modulates the vital ozone log Electron Number Density (number m 3)

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