Sun’s activity level more easy to pick

Press, 2 August 1979, Page 12

Sun’s activity level more easy to pick

The activity of the Sun is manifest in a regular cycle of waxing and waning of sunspots, solar flares, radio-source outbursts, and other phenomena. So confident have astronomers become of the regularity of this cycle that it has been projected both forwards and backwards in time. Only the amplitude of the cycle remains unpredictable. It was this unpredictable aspect that mislead space researches in thinking that Skylab, the American space laboratory, would remain in orbit much longer than it did. They misjudged the level of activity of the current solar cycle. . Has the Sun always been so regular in its behaviour? Apparently not. It is known that between 1645 and 1700 there was a protracted minimum in solar activity. This interval is known as the Maunder Minimum. There is conclusive evidence that not only were there very few sunspots visible at that time but there were also fewer aurorae. The evidence is also supported by the appearance of the solar corona during eclipses and by radio-carbon measurements. The question can then be asked as to whether the 11-year cycle of sunspot activity is normal, or whether the Sun is irregular and the intervals in which the regular cycle operates abnormal. The only way of testing which view is correct is to study records dating back to long before the invention of the telescope. Sunspots, with areas greater than 500 millionths of the Sun’s surface, can be observed with the naked eye. This is possible by making observations through a semitransparent object, such as jade, or when a dusty or misty atmosphere reduces the glare of the Sun. European civilisations did not observe sunspots because it was considered to be heresy to see any blemishes on the “perfect” Sun. Other’ civilisations, which did not have the same restraints, regularly made observations with a standardised procedure. It

is thus possible to correlate records made by Arabian, Chinese, Japanese, and Korean civilisations, which provide observations covering 2000 years. Indirect evidence of solar activity, or the lack of it, comes from several sources. One of these is the frequency of aurorae. These are caused by the bombardment of the upper atmosphere by charged solar particles, which spiral along the Earth’s magnetic field into the polar regions. The particles are carried by the solar wind as a burst of plasma ejected from an active region of the Sun. Normally these active regions are also associated with sunspots. Aurorae are thus evidence of solar activity being much more frequent when the Sun is active, or during the two years after sunspot maxima, than at times of sunspot minima. Oriental records show there were less auroral displays during the Maunder minimum and also in the years 1400 to 1510, known as the Sporer Minimum. During total solar eclipses the outer atmosphere of the Sun is visible as a delicate pearly-white corona surrounding the eclipsed ' Sun. The corona varies in shape from almost spherical with long coronal streamers at sunspot maximum to an unsymmetrical and fainter appearance at sunspot minimum. The early records during eclipses provide supporting evidence that the Sun was inactive during the Maunder minimum. Radio-carbon measurements provide additional indirect evidence in support of both the Maunder and Sporer minimums. The ratio of carbon-14 to normal carbon-12 stored as carbon dioxide in plant growth, depends on the flux of galactic cosmic rays interacting with the Earth’s atmosphere. This flux is inversely -related to the strength of the solar magnetic field. The stronger field at sunspot

maxima prevents the cosmic rays from reaching the Earth so that the abundance of carbon-14 is lower. During a prolonged solar minimum there’is an increase in the ratio of carbon-14 carbon-12 ratio. This is best measured and dated in tree rings which can carry the record back for 5000 years. When all this evidence is brought together it appears that there were probably four periods during the last 2000 years when the Sun did not follow the 11-year cycle of activity. Most of these periods lasted for about a century. In the seventh century there was a prolonged minimum of activity, whereas in the twelfth century there was an extensive maximum. This was followed in the late fifteenth and early eighteenth centuries by the Sporer and Manunder Minimum. These last two coincided with the severe temperature drops in the “Little Ice Age.” Is then the output of the sun not constant? It certainly does change as far as what reaches the Earth is concerned because of variations in the Earth’s orbit. The ellipticity of that orbit varies over a period of about 100,000 years, and there is precession of the orbital longitude of perihelion in a period of 22,000 years. Additionally the inclination of the orbit varies in a period of 41,000 years. These three periods appear to match the periods of long term climatic change. However, it has been suggested that the over-all rise in sunspot activity since the Maunder Minimum is caused by an increase in the solar constant and thus an indirect cause of the warmer climatic trend since 1700. It is only since 1920 that the solar constant has been measured. The result indicates an increase of about 0.4 per cent per century which is sufficient to account for the small

over-all increase in the Earth’s surface temperature that has been recorded during the period in which peak solar activity has increased. There is thus a suggestion that variations in the solar constant are related to solar activity and that this in turn is related to climatic changes. With our present knowledge it is impossible to project ahead to predict what climatic changes will result from variations of solar activity in the future. The position is complicated too by man-made pollution of the atmosphere. Past evidence indicates that over the past 7500 years there have been at least eighteen departures from what we thought was normal solar activity. These suggest a period of about 2500 years with individual maxima and minima occuring at mean periods of 400 years, each lasting for 100 years. These appear to fit variations in climate. If this position is proved to be true then the present solar activity represents a transition from a series of minima to a series of maxima centred on the year 2800. That means there is a possibility of a decrease from the present maximum occurring over the next 50 years. It is quite clear, however, that solar activity does bring variations in the solar constant. It is equally clear that until we understand better both the solar and man’s input into the climatic system we cannot predict the future climatic changes. During August no bright planet is favourably placed for viewing. Mercury, Venus, and Jupiter are all too close to the Sun to be seen, while Saturn is only briefly visible after twilight ends during the first half of the month. Thereafter it sets during evening twilight. Mars, in the morning sky, rises about an hour before dawn and has yet to move into a good viewing position.

Astronomy EM. Bateson