The Earth’s upper atmospheric winds are tied to the solar cycle. Above the equatorial zone the air temperature and wind direction change with a period of about 24 to 36 months, a variation called the Quasi-Biennial Oscillation (QBO). Karin Labitzke (Free University, Berlin) and Harry van Loon (National Center for Atmospheric Research) found that when these high-altitude winds come from the west, the upper-air temperature follows the 11-year solar cycle; when the QBO winds are from the east, the stratospheric temperatures anticorrelate with the cycle. Finally, Brian Tinsley. (University of Texas, Richardson) and his colleagues see a link between changes in solar magnetism and changes in the global electrical circuit of the Earth, with influences on cloud properties and thunderstorms, for example.
Cycle Of The Sun’s Magnetism
The observational record of sunspots begins around 1610 with systematic telescopic counts by Galileo, Christoph Scheiner, and others. In 1843, after 17 years of observing the Sun for evidence of the fictitious planet Vulcan, Samuel H. Schwabe noted roughly a 10-year periodicity in the number of sunspot groups and in the strings of days when no sunspots were seen. Then in 1908 George Ellery Hale at Mount Wilson Observatory found sunspots to have strong magnetic fields of up to several thousand gauss. Thus the historical record of sunspot numbers details the strength and extent of the Sun’s magnetic fields, with their 11-year cycle.
But strictly speaking, the period of the sunspot cycle is not 11 years; it varies from eight to 15 years. Other periods are also present, for instance the 80- to 90-year “Gleissberg Cycle” and a 200-year cycle. A period of roughly 2,200 years is suggested by records of solar magnetism from the radiocarbon abundance in bristlecone-pine tree rings that can be traced over several millenniums.
It wasn’t until the 1980s that satellites measured slight changes in the Sun’s total energy output, or irradiance, associated with the 11-year cycle. Although it may seem counterintuitive, near the height of the sunspot cycle, when the most dark spots are present, the Sun is brighter than at sunspot minimum. Careful study of the surface by several researchers showed that the dark spots are more than offset by larger bright areas of strong magnetism, or plages. Hence the sunspot cycle can be viewed as a curve of the Sun’s varying brightness, making our star a variable with a roughly 11-year period.
The best records of solar brightness changes, however, go back only 20 years, too short to determine whether these variations spur climate change. And the observed solar energy variations – 0.1 percent over a cycle – seem too small to be influential. But since the brightness changes correlate closely with changes in the Sun’s magnetic activity, and records of solar magnetic variability go back several millenniums, we may yet be able to learn something about the effects that long-term solar brightness changes may have on Earth.
Solar Magnetism Through Time
In 1890 Edward Maunder examined the historical records of sunspots and commented on a lull in the 11-year cycle from roughly 1640 to 1720, which had been noted first by Gustav Sporer, Jean Picard, and Gian Domenico Cassini. And William Herschel had written in 1801 that sunspot records from 1695 to 1700 showed that “no spot could be found on the sun.” Thus, the Sun’s magnetic variability has an additional complexity – the potential for diminished magnetic activity over the course of decades.
The long-term record of the Sun’s magnetic activity is deduced from radioisotopes such as carbon-14 and beryllium-10, which form as byproducts of energetic cosmic rays hitting the Earth’s upper atmosphere. The Sun’s magnetic field, carried out past Earth by the solar wind, deflects some of these cosmic rays. With high solar activity, and therefore strong magnetic fields, more cosmic rays are deflected and the formation of carbon-14 and beryllium-10 is inhibited. Some of these isotopes are incorporated in geologic records. They can be measured in the laboratory from tree rings and ice cores, yielding the history of solar magnetism. Both isotope records confirm that the sharp decrease of sunspots in the 17th century, the Maunder Minimum, indeed matched a very low level of solar magnetic activity.
Also, during the 17th century the Earth was globally about 1 [degree] C cooler than today. Especially hard hit was Northern Europe, where glaciers expanded and winters lengthened. In the last several thousand years similar periods of low solar magnetism have occurred every few centuries or so, and nearly all correspond to a cooling of the Earth by about a degree.
The Sun’s history is also marked by unusually high levels of magnetism. The last four sunspot cycles were among the most active in our 350-year observational record, but they were not as intense as the average level of solar activity in the 11th and 12th centuries as indicated by the isotope records. During that time the Earth seems to have been warmer than at present. Vineyards, for instance, grew in areas of Great Britain that cannot support them today. This is more evidence that changes in the Sun’s magnetism can alter terrestrial climate.
Studies of solar variability can be augmented with measurements of Sun-like stars – those close in mass, age, and magnetic activity to the Sun. We can thus watch a sample of many stars over a short time interval to deduce long-term information on one star, the Sun. But how do we observe the equivalent of sunspots – starspots – on stars too distant for their surfaces to be seen?
In the late 19th century astronomers noted that the spectra of cool stars contain emission lines of singly ionized calcium. Hale and others successfully photographed the Sun in the calcium H and K lines (3968 and 3934 angstroms, respectively), which originate in the Sun’s chromosphere. During sunspot maximum, patches of bright calcium emission dapple the surface; at low activity levels this emission is sparse. Thus, without seeing the Sun’s surface, we can take the presence of bright H and K lines as surrogates of surface magnetic features, Since records of changes in these lines have proved to be good proxies for varying magnetic activity on the Sun, the same is presumably true for Sun-like stars.
Hale built the 60-inch telescope on Mount Wilson in part because he wanted to explore solarlike phenomena that he thought were discoverable on other stars. He wrote: “Thousands of stars, in the same stage of evolution as the sun, doubtless exhibit similar phenomena, which are hidden from us by distance. . . . In spite of the necessity, because of their feeble brightness, of basing our conclusions on spectra a few inches long, representing the combined light from all parts of the stellar disks, material progress could be made in this way.”
In the early 1930s Mount Wilson astronomer Seth Nicholson showed his chart of the 11-year solar cycle as seen in the calcium K line to Olin Wilson, a young astronomer newly arrived from Caltech. From the relatively large changes summed over the Sun’s surface, Wilson reasoned that the calcium emission from the plages changed by 20 percent or more during the cycle. Thus cycles on other stars might indeed be detectable in the varying calcium H and K lines.
Wilson obtained some photographic calcium H and K spectra of about two dozen dwarf stars in the 1930s, intending to reobserve them about a decade later. The follow-up Was delayed until after World War II, but in 1954 he reported no changes between the two sets of photographic spectra.
Undaunted, Wilson bided his time until more sensitive electronic detectors came along. In March 1966, he began a monthly survey of 91 stars on or near the lower main sequence, stars not radically different from the Sun. He used the coude scanning spectrograph of the 100-inch telescope on Mount Wilson, which was equipped with a photocell. The stars ranged from spectral type early F to early M and sported weak to strong H and K emission.
Wilson discovered three general classes of long-term variability in lower-main-sequence stars: cyclic variations similar to the Sun’s 11-year cycle; flat, or essentially no variations; and erratic variations, meaning substantial changes with no clear period. Roughly one-third of the stars fell in each group. Wilson’s vision was remarkable in betting that the chromospheric H and K lines would contribute to studies of solar and stellar magnetism.
In 1977, as Wilson’s retirement approached, Arthur Vaughan and George Preston, also at Mount Wilson, built a second-generation instrument to continue Olin Wilson’s program at the 60-inch telescope, where Hale had first envisioned such work 60 years earlier. Since 1980 the HK Project has made observations almost nightly of flux changes that reveal stellar rotation. Wilson’s program has been extended to giant stars, as well as to a large census of solar-neighborhood stars.
In addition to the use of H and K lines as benchmarks of surface magnetic activity, parallel observations are made of associated brightness changes. G. Wes Lockwood, Brian Skiff (Lowell Observatory), and Richard Radick (Sacramento Peak Observatory) obtained highly precise photometry of three dozen Mount Wilson stars. More recently, Greg Henry and Michael Busby (Tennessee State University) collaborated with us to construct 0.75-meter and 0.8-meter Automatic Photoelectric Telescopes (APTs) for cost-effective differential photometry of the entire list of stars. The APTs have achieved the astonishing precision of 100 to 200 millionths of a magnitude over a season. Such measurements allow the detection of brightness changes on Sun-like stars as small as 0.1 percent over a decade – comparable to the solar irradiance change. A comparison of the Sun’s calcium emission and irradiance to that of a solar-type star is shown above.
Like the Sun, solar-type stars brighten and dim as the calcium H and K emission lines trace their starspot cycles. However, the cycles do not repeat themselves exactly – the total brightness change over a cycle is proportional to the intensity of the star’s activity. This, in turn, suggests that the amount the Sun’s brightness varies will change as its activity increases or decreases from one cycle to another.
The observations of Sun-like stars in the Mount Wilson sample say that short cycles are intense. Sustained intervals of short cycles could result in more solar energy reaching the Earth – effecting a global warming. Conversely, intervals of long cycles and continued low magnetic activity should result in cooling:
Our most recent estimates suggest that the Sun changes in brightness by about a half percent between a phase similar to the Maunder Minimum and an active phase. Simulations of the Earth’s climate indicate that changes of several tenths of one percent in the Sun’s brightness sustained over several decades could cause temperature changes of 0.5 [degree] C or so on Earth. Thus solar-brightness variations can explain most of the past record of terrestrial global temperature fluctuations. Our understanding of the climate is far from complete, but one fact stands out: a varying Sun is one of the drivers of the Earth’s changing climate. Sallie Baliunas and Willie Soon are scientists at the Harvard-Smithsonian Center for Astrophysics and Mount Wilson Institute. The astronomer Olin Wilson is buried in the shade of the 100-inch telescope.