Astronomy Sunspots wax and wane

Press, 29 November 1977, Page 29

Astronomy Sunspots wax and wane

Sunspots are .one form of solar activity and their numbers, and the areas they cover, vary in a continuous cycle. This cycle repeats itself in about 11 years. The last maximum was in early 1970. and a minimum took place in mid-1964. The sun has just ended another minimum phase, which commenced in 1975. This minimum has been a prolonged one, continuing into the first three months of this year. During much of this time no spots were visible for weeks at. a time. Now spots of the new cycle are appearing in increasing numbers. The increase from a sunspot minimum to the maximum is quite steep and normally averages 4.5 years, compared with the average decline from maximum to minimum of 6.5 years. If these averages persist throughout the new cycle the next solar maximum will occur in 1981. A reader has asked if I could explain how' counts of sunspots are made. He is puzzled by the obvious fact that counts with large instruments would differ from those made with small instruments on the same day. Many solar observers, using a wide variety of instruments, count both the number of individual spots and the number of groups of spots they see each day. The groups are given a weight r f ten and then added to t : m total number of spots, the result is called the relative sunspot number. Thus an observer who records three groups and twenty individual spots obtains the number 50. T<> co-ordinate observations made by different observers using different instruments under con-

ditions that vary, the daily relative numbers are reduced to a standard scale. The Zurich Observatory in Switzerland has observed sunspots for more than 100 years with an Bcm refractor. This is taken as the standard instrument. All .other observations are multiplied by a constant which is determined for each observer by comparison with the Zurich records. The final result is to give for each day a figure that represents the sunspot activity for that day. Means are formed from these daily figures for 27day periods, which is the time taken by the Sun to rotate on its axis. When these means are plotted on a graph it is easy to see how the activity waxes and wanes in a period of 11 years. However, it then becomes apparent that the Sun is in a much more active state at some maxima than at others. Some peaks are very high, with a figure of 190, whereas others reach a figure of only 40. Just why the Sun is more active at some times than at others is still a mystery. The variation at minima is much smaller, so that successive minima differ little from one another. At the times of greatest solar activity, spots, often very large, have been seen with the naked eye, especially if the Sun has been partially obscured by haze. Another sign of intense solar activity is the frequency of aurora and their visibility at low latitudes. Recently an Italian astronomer used the records of naked-eye spots and aurora to trace the solar cycle back to 482 B.C. He concluded that an average solar cycle of

11.13 years has remained relatively stable over the last 24 centuries. Sunspots vary in size from very tiny “pores” that can only be seen in very large instruments to large spots measuring some 150,000 kilometres across. The average size is around 10,000 kilometres. Normal spots appear dark. This is merely due to contrast with the surrounding bright surface of the Sun. The average temperature of a spot is 4000 degrees, which is sufficient to give off more light than the Full Moon. This temperature is 2000 degrees cooler than the surrounding surface, and thus accounts for the spots appearing dark. Spots can occur singly, or in pairs, but generally they form part of complicated groups. The jet black centre of the individual spots is called the umbra, and this is surrounded by a less dark ring, termed the penumbra. Groups are arranged in an approximate east-west direction and pass through different stages of development. This results in their growing in size and structure until large spots develop at the preceding and following end of the group. These large spots have opposite magnetic polarities. Later the group and its spots decrease in size and finally disappear, leaving the magnetic field to disappear last of all. The duration of spots varies from a day or so to some which last for several weeks. These latter can be carried around by the rotation of the Sun, disappearing at one limb only to reappear again at the opposite limb about 13 days later. Most sunspots occur within a belt within 35

degrees of either side of the solar equator. At the begining of a new sunspot cycle the spots appear farther from the equator than they do later in the cycle. Thus they steadily appear closer and closer to the equator, so that as the old cycle ends the spots are found within five degrees of the equator. About the same time the spots of the next cycle start appearing at much higher latitudes. Sunspots are associated with many other forms of solar activity, but they still provide a convenient method of recording the state of solar activity generally. This activity in turn has marked effects on our own atmosphere, and many people firmly believe that it plays a considerable part in the cyclic pattern of our weather. The chief interest during December is in the giant planet, Jupiter, which comes to opposition on December 23. It will be crossing the meridian about 1 a.m., well to the north of the zenith. Jupiter is close to the boundary separating the constellations Orion and Gemini, lying almost due north of the bright star Betelgeuse. Jupiter, with an opposition magnitude of -2.3, outshines all the surrounding stars. Mars can be found in Cancer, roughly half-way between the bright stars, Regulus, in Leo, and Pollux, in Gemini. Saturn, somewhat lower in the morning sky than Mars, can be located very close to Regulus. Both these planets are best seen about 3 a.m., by which time they have risen to a reasonable altitude in the north-east. Venus and Mercury are not favourably placed for viewing this month.