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Table of ContentsClicking on any marked section on the list below brings up a file containing it and all unmarked sections immediately following it on the list. This list is repeated at the beginning of each file.
Chronology of Geomagnetism References: A-G References: H-P References: Q-Z Back to the index page
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12. Sunspots
The story of the Earth's magnetism is strongly tied to that of solar research, in several ways. Large magnetic storms, and observations of "northern lights" far south from their usual locations (i.e. of the "aurora borealis," now commonly known as "polar aurora"), were found to be associated with solar phenomena. And not only did the Sun have a magnetic field, but concentrated sources of that field--dark sunspots--were visible on its surface, quite unlike the sources of the Earth's field which are buried deep in the Earth's core. This led to valuable insights into how the geomagnetic field might be generated. Sunspots were first reported in 1609, independently by Galileo, Scheiner and Fabricius [Newton, 1958; Phillips, 1992], all of whom used the newly invented telescope. Sporadic reports of earlier observations also exist, because the unaided eye can see large sunspots when thick haze dims the Sun near the horizon. Unfortunately, sunspots practically disappeared for a 70-year period starting around 1645 ("the Maunder minimum"). During those years the interest of astronomers wandered elsewhere, and when spots again became frequent, they were not investigated systematically. The 11-year sunspot cycle was discovered accidentally by Heinrich Schwabe (1789-1875), a German pharmacist and amateur astronomer living in the town of Dessau [Meadows, 1970; Newton, 1958]. Schwabe was looking for a yet-unknown planet of the Sun, moving inside the orbit of Mercury. Such a planet (given by other searchers the tentative name of Vulcan) would be hard to spot (except during a total solar eclipse) because its position in the sky would always be close to that of the Sun, where daylight would obscure it. Schwabe hoped to observe it as a dark spot moving across the face of the Sun, and day after day, year after year, whenever the sky was clear, he observed the Sun and looked for it. To properly conduct such a search, Schwabe also had to identify and track sunspots, to make sure none was mistaken for a new planet. He did so from 1826 onwards, and by 1843 he noted a cyclical rise and fall in their number, as well as in the number of days when no spots were observed. He then published a table of his yearly totals, but until 1851 it attracted little notice, except from Rudolf Wolf, noted below. Then Alexander von Humboldt republished it (extended to 1850) in the third volume of his "Kosmos," and suddenly sunspots and their cyclic behavior became a hot scientific topic. Rudolf Wolf (1816-93) of Berne (later of Zürich) collected earlier observations, tracing sunspot cycles before Schwabe's time. He introduced the "Zürich sunspot number," an empirical criterion for the number of spots, taking into account the fact they usually occurred in tight groups. The length of the sunspot cycle turned out to vary, but the average value was near 11 years. Sir Edward Sabine (1852) found an association between the sunspot cycle and the occurrence of large magnetic storms, and Richard Carrington (1826-75), also in England, studied the rotation of sunspots around the Sun, noting that their period and other properties depended on latitude. In September 1859 Carrington (as well as R. Hodgson, another British observer) saw by chance a bright outburst of light in a group of large sunspots, lasting about five minutes [Meadows, 1970; Newton, 1958]. This was followed 17 hours later by a very powerful magnetic storm, strongly suggesting a connection, although Carrington cautiously commented "One swallow does not make a summer." But what were the sunspots themselves? We now believe that they appear darker because they are slightly cooler than the regions that surround them. Galileo speculated that they might be clouds floating in the Sun's atmosphere, blocking some of its light. Their most significant feature, however, was discovered only in 1908, by George Ellery Hale (1868-1938), leader among US astronomers, founder of great observatories and designer of novel instruments [Wright, 1966]. One of his inventions (in 1892) was the spectroheliograph, also devised independently in France by Deslandres (1853-1948), a spectrograph adapted to scan the Sun in a single spectral color, producing a photographic image. Whereas in white light the Sun presented (apart from its spots) a bland appearance, the spectroheliograph (e.g. tuned to the red H_ line of hydrogen) isolated light from higher layers in the Sun's atmosphere and revealed many new features. These included mottling of the surfaces, prominences arching high above the Sun (turning to dark linear features when passing in front of the Sun) and bright areas near sunspots. Hale also found that "solar flares," such as the one observed by Carrington and Hodgson, were much more frequently seen in H__light, and big ones indeed often preceded magnetic storms. In 1896 Pieter Zeeman discovered the "Zeeman effect" by which the characteristic colors ("spectral lines") of elements, when emitted by a gas located in a strong magnetic field, often split into two or more components of slightly different wavelength, with their separation depending on the intensity of the field. Using the Zeeman effect, Hale in 1908 showed that sunspots were in fact strongly magnetic, with a typical field intensity of 1500 gauss (0.15 Tesla). The spots generally appeared in pairs of opposite polarity, suggesting that field lines emerged from the Sun at one of the pair and re-entered at the other. One spot was usually ahead of the other in the direction in which the Sun rotated, and the magnetic polarity of the "leading" spot north of the equator, in any solar cycle, was always the same, and was opposite to the "leading" polarity south of the equator. In the following solar cycle both these polarities were always reversed, suggesting that the sunspot cycle was a magnetic phenomenon, with an average period near 22 years. Hale's method was greatly refined by Horace Babcock [Babcock, 1960, 1963; Phillips, 1992; Eddy, 1978], Robert Leighton and others. Using the polarization of Zeeman lines to construct a "solar magnetograph," they greatly increased the sensitivity of Hale's method, to the point where not only sunspot fields could be observed, but also a general dipole field of the Sun, of the order of 5 gauss. The existence of such a field had been suspected from a feature of the solar corona, the Sun's outer atmosphere, previously seen only during total eclipses of the Sun. At the poles the corona displayed rays or "plumes" in a pattern which reminded observers of magnetic field lines of a dipole, like the ones above the poles in Figure 6. The magnetograph also showed that this field reversed each 11-year solar cycle, typically 3 years after sunspot minimum. 13. The Dynamo Process on the Sun
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The difference in periods presumably reflects heat-driven flows in the Sun. If the Sun had started off with a dipole field, the differential rotation would have gradually wrapped field lines around it (Figure 9), in opposite directions north and south of the equator. |
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Next Stop: Magnetic Reversals and Plate Tectonics
Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: earthmag("at" symbol)phy6.org
Last updated 31 January 2003