The Inconstant Sun1

(ProQuest Information and Learning: ... denotes formulae omitted.) SOLAR INCONSTANCY is a subject well suited to the APS, for it bears on material considered by all its classes, not least Class V. Astrophysicists, evolutionary biologists, anatomists, archaeologists, and historians illuminate the problem; but any action arising from their findings is likely to be politically charged, especially when acceptance of solar variability is seen to undermine the case for control of manmade greenhouse gases. And, as Eddy (1976) has observed, it is a question more for social than for physical science why we should think that the Sun, unlike other stars, is perfect, if imperfect then consistent, and if inconsistent then regular in its behaviour. The title of this paper evidently alludes to the solar constant, a measure of the radiant energy impinging upon the top of the Earth's atmosphere. When such a value was first sought, it was perforce measured at the bottom of the atmosphere. In the late nineteenth century S. P. Langley voiced the suspicion that the constant was no such thing, but the matter could not be resolved until the development of space probes and artificial satellites. They revealed various small but measurable variations. My aim is to show that we cannot hope to understand this variability until we extend the Sun's history back well beyond the few years spanned by instrumental observations. To the astrophysicist even an imperfect chronicle of solar variability is useful, as it bears on the Sun's dynamics, internal structure, and magnetic activity, and thus on stellar evolution in general (Weiss 1980). It is a twoway street: without information furnished by other stars there would be no main sequence for the Sun to inhabit, and there are numerous variable stars which display periodic fluctuations in luminosity that, as in the Sun, appear to reflect differential rotation and fluctuating levels of activity. The same applies to space exploration, where, even if the prediction of solar weather is in its infancy, a knowledge of the potential hazards is essential to sensible planning. Skylab was forced down in 1979 by increased drag resulting from heating and expansion of gases in the Earth's upper atmosphere associated with solar turbulence, as was the Solar Maximum Mission in 1989. Flares and coronal mass ejections create magnetic storms that may lead to radio blackouts and widespread computer failure, as in the Toronto stock exchange in 1989, and the associated radiation especially in the X-ray range endangers astronauts and also airline passengers on polar routes. Such solar storms tend to be most numerous when the 11-year solar cycle discussed below nears its peak, but the violent flares of October 2003, near solar minimum (Lopez et al. 2004), show that the assumption is fragile. On Earth it is the UV portion of the solar spectrum (100-400 nm) that offers the greatest threat to human health. UVB radiation (280-315 nm), which makes up a mere 1% of the total, can result in skin cancers and other chronic skin changes, the suppression of immune responses, and cancer of the conjunctiva and cataracts (WHO 1994). Direct damage to proteins can occur at wavelengths within the UVA range (315-400 nm). The ozone layer blocks all UVC (100-280 nm) and 90% of UVB radiation from reaching the Earth's surface, although any such effect varies from place to place and is subject to seasonal fluctuations as well as any change in the integrity of the ozone shield. One estimate puts the consequence of a 10% decrease in stratospheric ozone at a worldwide increase of cataract cases by 1.6 million. The contribution of solar variability to climatic change, the most self-evident-or at any rate topical-motive for pursuing the subject, remains curiously unclear. Part of the blame must lie with the widespread reliance on the matching of time-series even when the causal link is not self-evident; witness the enthusiasm that greeted the demonstration by Friis-Christensen and Lassen (1991) that temperature anomalies in the Northern Hemisphere since 1860 had fluctuated in sympathy with variations in the length of the sunspot cycle. …

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