Astronomy Solar activity may alter climate

Press, 26 April 1989, Page 54

Astronomy Solar activity may alter climate

by

F.M. Bateson

The frequency and intensity of aurorae are visible evidence of solar activity, because such displays can be seen with the naked eye. They wax and wane. The 11-year sunspot cycle is most frequent during sunspot maxima.

Chinese and Korean astronomers have kept records of aurorae and sunspots for thousands of years. These records have been analysed by Xu Zhentao and Chen Bing, of the Purple Mountain Observatory in China, who found a mean cycle of solar activity before the seventeenth century was 10.62 years. They found a longer term variation in a period of 238 years corresponded to periods of low solar activity and a, still longer period of 800 years in solar activity.

V. F. Chistyakov, a Russian scientist, combined the same records with the thickness of annual tree rings in American saluoia trees from 1310 B.C. to 1914 A.D. He also measured the rings in other trees from various countries, as well as the thickness of the annual

silt layers in Saki Lake in Crimea. He claims that all show the effects of a solar cycle which varied during this interval from seven to 17 years.

He found that solar ac-_ tivity appeared to consist of alternative cycles of 95 and 115 years and so he considered the long term variation was equal to these two cycles or 210 years.

Combining these results with an analysis of carbon 14 deposits, it appears there were very deep depressions in solar activity in 3500 8.C.” 1000 B.C. and 1500 A.D. which correspond to what are called The Little Ice Ages.

The periods of 210, 238 and 800 years are, within the errors of the data, fractions of the 2500-year cycle. On this basis these researches made two predictions in 1986. One was that the present sunspot cycle and those which immediately followed it, would reach very high peaks of activity. The second was that the next little ice age would occur Howards the end of the

next century, around 3000.

It appears that the first of these predictions will prove to be correct. The last sunspot minimum occurred in September, 1986, and the rise to the coming maximum has been very fast. There is every indication that this will exceed the maximum

of 1957 and become the highest peak of solar activity this century.

French astronomers have recently shown that during the last little ice age the diameter of the Sun was about 0.2 per cent larger. Measurements of the sunspots recorded then showed that the Sun rotated more slowly with a more pronounced difference between the rotation periods of different latitudes. This is consistent with the view that the Sun is slightly variable and has a weak pulsation. Such changes would be sufficient to decrease the amount of solar radiation received by the Earth and account for the extreme cold of the littli ice ages. There* is little doubt

that the Sun has a marked effect on global climate. It is difficult to relate this to short solar cycles, because most researchers have sought correlations with the 11-year cycle, whereas the effects are probably long term, as Chinese and Russian scientists have -shown. Naturally short-term effects on the climate are also obscured by the way humans are altering the climate by pollution. The present strong solar activity was shown by the auroral display on March 13, which was seen as far north as Auckland. There and in the Bay of Plenty it was seen as a bright red glow along the south-west-ern horizon and reaching to an altitude of 15 deg.

It is rare for aurorae to be seen so far north, although there are a few records of displays being visible in the tropics. Aurorae are caused by the impact of charged particles .from the Sun on the upper atmosphere. These particles are sprayed out from solar flares, associated with active sunspot and carried by the solar ‘-wind.

They become trapped in the Earth’s magnetic field and oscillate along the lines of magnetic force between the hemispheres. Aurorae were frequent in 1988 and will continue to be so this year because the Sun is in a very active state. The most brilliant auroral displays are seen over the polar regions and can take many forms, classified into eight main types. The most beautiful are called draperies, resembling curtains of fine material, fluted and folded as they hang suspended in the sky. They appear to sway as if in a breeze and are coloured red, green or white.

The most common forms seen away from the polar regions are either a glow along the horizon or rays of light, varying in width, and radiating from magnetic south. The latter may appear slender or as broad as a searchlight beam, singly or in groups changing continuously and reaching 50 deg towards the zenith. The atoms and molecules which cause the shimmering aurorae to

glow are mainly oxygen and nitrogen. They become luminous at heights of from 100 km to 400 km through collisions with energetic particles which have leaked out of the magnetosphere. The green colour is caused by excited atoms of oxygen; blue is produced by ionised nitrogen molecules and red by atomic oxygen radiating at a different wave length to that which produces the green, colour. This winter we can expect to see more aurorae, especially in the far south of New Zealand. The solar flares which spray out the energetic particles are brilliant bursts of light occurring near an active sunspot group. These flares are more common near periods of maximum solar activity and rarely last more than a few hours. They emit streams of particles which reach the Earth in about 27 hours, carried by the solar wind at 1600 km a second. Their interaction with the Earth’s magnetic field causes aurorae and radio blackouts.