Sunspots and Related Phenomena

A sufficient number of sunspot phenomena have now been recovered from ancient chronicles, that the known sunspot records now extend back to at least 1 a.d. (Clark and Stephenson 1978) and possibly to 165 b.c. (Wittmann and Xu 1987). Mythical depictions of a crow in the Sun from Han dynasty tombs likely stem from observations of sunspots (Xu 1987), a suggestion supported by the circumstance that the word wu, often used in the annals prior to 28 b.c., means "crow" as well as "black" (Needham 1981, p. 211). A rubbing from a stone engraving of the family of the Han emperor Wu Ti shows an archer shooting crows that threaten drought by burning up the Earth. This is cited as an example of a

"drought myth" by Fairbank, Reischauer, and Grieg (1989, p. 67). Between the 1st century b.c. and 1638 a.d., the annals, mainly the Wen Hsien Thung Khao from the 13th century and the Thu Shu Chi Chheng imperial encyclopedia dating from 1726, list 112 descriptions. Wittmann and Xu (1987) provide 235 such descriptions from 165 b.c. to 1684 a.d. The crow-in-the-Sun image is part of the mythology deriving from the Chou and early Han dynasties.

The association of the crow and the Sun is found in other groups as well. The possible identification of the crow and the Sun in Celtic mythology is discussed in §6.2.8. The "bird of Apollo" in Hellenistic Greek belief was a crow. Stories told about Raven in myths of North American Indians contain motifs that seem to apply to the Sun. What is known about sunspots does not conflict with such associations.

Sunspots are regions of cooler temperatures in the photosphere of the Sun; they appear dark by contrast to the higher temperature of the surrounding solar atmosphere. They were telescopically discovered as true solar phenomena by Galileo (1613), who first traced their passage across the face of the Sun and thereby found the rotation of the Sun. Occasionally, very large spots appear on the Sun; sometimes they can be seen without optical aid. One of the largest seen in modern times occurred in April 1947 and was several arc-minutes across, clearly resolvable by the naked eye. There are many anecdotal accounts of such sightings when the solar disk is near or at the horizon (for example, A.J. Wesselink mentioned one such occasion to EFM). A systematic naked eye investigation correlated with telescopi-cally determined sunspot numbers and areas would be useful to calibrate pretelescopic observations of sunspot numbers. The sunspots are basic indicators of solar activity; prominences and solar flares on the Sun and terrestrial aurorae are other indicators (see Figure 5.14).

Figure 5.14. Solar phenomena, August 31,1988, observed with the RAO's heliostat: (a) A white light image of the Sun with sunspots. (b) The Sun in the red light of Ha, 6563 A, showing

Figure 5.14. Solar phenomena, August 31,1988, observed with the RAO's heliostat: (a) A white light image of the Sun with sunspots. (b) The Sun in the red light of Ha, 6563 A, showing

prominences projected onto the disk. (c) Ha image from October, showing prominences on the limb. Photos courtesy Frederick M. Babott.

These phenomena wax and wane with an 11-year cyclic-ity (the sunspot cycle24). It has long been suspected that there is a connection between solar activity and weather patterns on Earth, but the complexity of Earth's weather patterns has precluded the determination of a reproducible relationship. Recently, some progress appears to have been made in decoupling other effects from those involving solar activity (cf. Kerr 1988). The connection, although difficult to understand in a cause-effect way, has attracted attention recently because of the Sporer-Maunder or Maunder minimum in solar activity during the period 1645-1715 (during the reign, ironically or perhaps appropriately enough, of the "Sun King," Louis XIV), when Europe suffered a "little ice age." This was a time when Europe underwent unusually severe winters. Supporting evidence that the spots were in fact few in number, and not merely unreported or under-reported, is to be found in the appearance of the solar corona observed during the solar eclipses of the period. It had little structure or extent—typical of coronae during solar minima. The records of auroral sightings also support the reality of the Maunder minimum. However, it would still be worthwhile to perform the calibrating experiment mentioned earlier.

In a typical 11-year cycle, the numbers of sunspots, flares, prominences, and so on, increase until reaching a maximum, typically, between two and five years after the preceding minimum. Although solar flares occur frequently, white-light flares, i.e., those that emit continuum radiation as opposed to those that emit light only in certain spectral lines (like the Balmer lines of hydrogen or the H and K lines of ionized calcium, for example), are rare. Such flares brighten a small portion of the solar disk and have limited lifetimes of half an hour or more; they are followed by auroral activity, changes in magnetic indications, and other effects, when the charged particles reach the Earth about 50 hours later.

In the chronicle, the "Kai Yuan Zhan Jing," descriptions of "flame-like gas" in, or "jumping in," the Sun are described (Xu 1987; 1990), which are considered to be the comments of diligent observers of solar phenomena (Wang and Siscoe 1980). These observations could describe solar prominences, which, when viewed through a telescope, give the appearance of dark, highly elongated regions when on the disk (cf. Figure 5.14c) but when seen off the limb, may appear as bright hoops or flame-like extensions off the disk. Only very large-scale features, viewed near sunrise/sunset, would be visible to the naked eye. Chromospheric and coronal features are best viewed, of course, during the total phase of a solar eclipse.

24 Strictly speaking, the solar activity cycle is a 22-year one;after 11y, the solar magnetic field polarity changes;this reversal causes leading spots in "bipolar groups" to have the polarity that following spots had in the previous 11y cycle. The magnetic polarity of leading spots in the Northern Hemisphere is opposite to that in the Southern Hemisphere, and this too reverses in alternate 11y cycles. From time to time longer cyclicities are suggested as being present in solar activity history. See §5.8 for further discussion of solar variability.

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