During this programme, in which Professor C. W. Allen (Director of the University of London Observatory) took part, we were able to show parts of the 'continuous solar movie' assembled by observers at Sacramento Peak in the United States. This was the first time that any of the film had been shown in Britain, and it was certainly spectacular.
During the programme I said that solar minimum was expected in early 1964. This seems to have been borne out, though it is of course very difficult to decide upon the precise date of minimum. At any rate, it seems that solar activity should be on the increase again by the spring of 1965.
Latest images of the Sun taken with the NASA SOHO Satellite
Solar photography began many years ago. There can be few people who have not seen pictures of the Sun, showing the brilliant surface or photosphere together with the darker patches known as sunspots; such photographs are taken daily at professional observatories, and are also obtained by amateurs working with modest telescopes. It is much better to photograph a sunspot than to draw it, since the details are often so complex that they are extremely difficult to sketch accurately.
The Sun, a typical star, has an intensely hot surface (6,000 degrees C), while the spots are about 2,000 degrees cooler, appearing dark only by contrast. The precise cause of sunspots is not known, but their behaviour has been extensively studied, and it has been found that their numbers vary in a semi-regular cycle of about eleven years. This solar cycle, first recognized more than a century ago by the German amateur Heinrich Schwabe, is of fundamental importance, since it affects not only the spots but almost all the other features of the Sun.
At a spot-maximum, there may be many groups visible at the same time; this was the case in 1957-58, the time of the International Geophysical Year. Since then, activity has died down, and we are approaching the period of minimum activity. Large groups are now uncommon, and during August 1963 I recorded several days when the disk was completely clear. Minimum is expected in early 1964, so that the next maximum will not take place until 1969^ or 1970. We cannot be more definite, since the cycle is not constant; the interval between successive maxima may be as short as nine, or as long as thirteen and a half years. Moreover, some maxima are more active than others; that of 1957-58 was the most energetic since regular observations of the Sun began.
Spots are often associated with bright irregular patches known as faculae, which lie well above the photosphere, and may be regarded as luminous clouds hanging in the upper regions. They often appear in positions where a spot-group is about to break out, and persist for some time after the group has disappeared. Then, too, there are the granules, thought to be the tops of gas-currents which rise and fall; they are in constant motion, and each has a width of between 500 and 1,000 miles. There is nothing surprising in this constant turmoil; the surface of the Sun is never calm.
Visual observations can tell us little more, and photographs taken in ordinary or integrated light are of limited value in solar physics, spectacular though they may be. With ordinary telescopes it is impossible to study the Sun's surroundings, except during the rare moments of a total eclipse, when the photosphere is blotted out by the body of the Moon and the solar prominences flash into view, together with the glorious pearly corona. However, much more may be learned by means of instruments based upon the principle of the spectroscope. Spectroscopic analysis has shown that much of the Sun's atmosphere consists of hydrogen - which again is not surprising; hydrogen is much the most plentiful substance in the universe. According to Goldberg and Aller, the percentage number of hydrogen atoms in the solar atmosphere is 81 -8; helium comes next with 18-2, and oxygen third with 0-03, while the remaining elements are even less abundant.
When photographs of the Sun are taken in hydrogen light only, the picture is very different from that of an ordinary view. Bright and dark areas are seen; the distribution of the hydrogen may be studied, and it is possible to detect the violent, short-lived phenomena known as solar flares, whose importance it is difficult to overestimate. The photograph given in this book, taken in H-alpha light, shows these features clearly.
Flares are occasionally seen in integrated light. The first case was that of 1859, when two British amateurs, Carrington and Hodgson, saw a pronounced flare by means of an ordinary telescope; one of the most recent was that of 23 March 1958, when, at 10.05 G.M.T., Waldmeier was fortunate enough to observe a brilliant flare near a sunspot on the Sun's limb. Yet such opportunities are so rare that were it not for spectroscopic equipment our knowledge of flares would be virtually nil.
The true origin and nature of flares is still something of a mystery, but present evidence seems to point to their being storms in the chromosphere, of an electrical nature, the hydrogen atoms being made to glow brilliantly by electrical excitation. They spread through large areas of the chromosphere horizontally, i.e. parallel with the Sun's surface, but there is relatively little vertical movement. They emit charged particles as well as short-wave radiation, and they produce marked magnetic storms on Earth, as well as affecting radio communication. It is probable that emissions from flares will form a major hazard to future space- travellers, and this is one reason why energetic studies of them are now being carried out.
A flare generally takes less than five minutes to form, and has a total lifetime of less than half an hour. It would therefore be instructive to take 'movie' films of flares, so that their development and decay could be studied. This idea is not new; it was first suggested thirty years ago, but until the invention of the special device known as the monochromatic filter, by the French astronomer Bernard Lyot, it was difficult to follow up. The principle of the Lyot filter is complex, but, in brief, it cuts out all light except that of hydrogen, so that when it is used in conjunction with a telescope the observer sees the hydrogen light only. (It is possible to select another element, such as calcium, but on the whole the hydrogen studies are the most significant.)
At the 1958 meeting of the International Astronomical Union, the scheme for making a 'continuous solar movie' was discussed by four solar physicists, headed by Henry J. Smith of the Sacramento Peak Observatory in the United States. It was pointed out that if one picture of the Sun were taken each minute, and the successive frames projected at sixteen per second, the acceleration would be 960 times, so that a fortnight's observations could be condensed into a twenty-minute film. This speeding-up would be ideally suited to showing the development of active centres on the Sun, and for studies of flares. It would, of course, involve using hydrogen light only, but with the Lyot filters readily available this would present no problems.
No single observatory could hope to carry out such a programme. For one thing, clouds would have to be taken into account; for another, there would be gaps during the periods when the Sun was below the horizon. For the movie to be truly continuous, photographs from observatories scattered all over the world would have to be used. Fortunately, such photographs might be expected to be available in view of the establishment of 'solar patrols' for the International Geophysical Year programmes. Finally, Smith and his colleagues selected the fifteen-day interval between July 6 and July 20, 1959, which represented one of the high peaks of the solar maximum, and was particularly well observed. Photographs were obtained from observatories in Britain, France, Japan, Russia, Australia, Ireland, and South Africa, as well as the United States; there were more than 20,000 of them, and the originals varied widely in contrast and density, so that it was by no means easy to make a homogeneous integrated copy. An immense amount of labour was required, and the compilation took over a year even after the photographs had been collected and checked. A few gaps in the record remained, but on the whole the result proved remarkably satisfactory.
During the period covered by the film, records indicated that there were twenty-nine major flares, the most significant of which was that of July 16. This was well recorded from Lockheed Observatory, in America, and was important because it was found to emit cosmic rays with surprising energy. Many smaller flares were also shown, together with associated phenomena of all kinds. The original purpose of the movie film was to provide a pictorial record, but its success shows that the principle may well have important scientific applications, and it will certainly be repeated in the future - not immediately, since the Sun is 'quiet' and flares are relatively uncommon, but certainly at the time of the next maximum.
Mention has been made of the possible space-travel hazard due to emissions from flares. Once an astronaut is above the protection of the Earth's atmosphere, he is exposed to radiations of all sorts, and it used to be thought possible that the danger from radiation would prove a fatal obstacle to interplanetary or orbital flight. This has not turned out to be the case, and none of the pioneer astronauts has suffered harm from this cause. On the other hand, all journeys so far have been of limited duration, and when considering longer trips, lasting for many days, we must not be overconfident. If a violent flare broke out on the Sun, the resulting emissions might have serious consequences for a space-man.
The trouble is that flares cannot be predicted with any degree of certainty, and when they appear they do so very rapidly. It is therefore highly desirable to gain a better idea of the moments when flares are likely to break out - so that astronauts can avoid such times, and, conversely, instrumented vehicles are ready to record the emissions and send back the information. The continuous movie technique may prove helpful, though it is too early to make any hard and fast .slafemfjats, It roust he stressed, too, that flares are by no means absent even when the Sun is at its calmest, as it will be in 1964, and outbreaks may occur at any moment. All we can say is that flares, like spots, are much more frequent at solar maxima. It may be that prolonged flights beyond the atmosphere will have to be restricted during the years when the Sun is at its peak, but unless the problems set by the flares can be interpreted, interplanetary travel will always be a somewhat hazardous business even apart from the obvious dangers. On the other hand, further research may show that the flare emissions will not have serious consequences after all. We can only wait and see.
The Sun may be nothing more than an insignificant star in the Galaxy, but to us it is the most important body in the universe; without it, we could not survive for a moment. The more we learn, the more we find that solar activity influences terrestrial phenomena. A case in point concerns the density of the Earth's upper atmosphere. At sixty miles, the atmospheric density seems to remain almost constant, but at 200 or 300 miles there are pronounced fluctuations, and the extreme upper regions have been contracting ever since the last sunspot maximum. The Russian vehicle Sputnik III, sent up on May 15, 1958, gave proof of this. Its initial height ranged between 135 and 1,167 miles, so that it was appreciably affected by atmospheric resistance, and was not expected to complete more than 8,000 circuits before descending into the lower air and burning away. Actually it went round the Earth 10,037 times, and did not come to the end of its career until April 6,1960. The discrepancy was extremely puzzling at the time, but the answer has now been found. The expected lifetime was worked out according to data obtained when the Sun was at its most active; later on, while the Sputnik was in orbit, solar activity became less. Reduction in upper-atmosphere density was the reason why Sputnik III lasted for more than 2,000 extra circuits.
The solar cycle has equally marked effects upon the Van Allen radiation zones which surround the Earth, but we must remember that these zones are of recent discovery; they were detected only around the time of the last sunspot maximum, and we have not yet been able to follow their changes throughout a complete solar cycle. Fortunately, the co-operative programmes organized for the International Geophysical year proved to be so fruitful that they are being extended, and the observations to be made during the aptly named 'International Quiet Sun Year' are expected to add greatly to our knowledge of solar physics.
We have much to learn. We can watch the sunspots with our telescopes; we can use Lyot filters and other instruments to study the Sun at selected wavelengths; we can obtain detailed photographs, and even movie films - but there are many problems which still remain to be solved, so that every possible technique must be brought into operation. Remember, too, that in finding out more about the Sun, we are also improving our knowledge of the millions of other suns contained in our Galaxy.
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