книги из ГПНТБ / Pushkov N.V. Quiet Sun
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frequent visitors near 15° to 20° and by the end of the cycle, they are still closer to the forbidden equator.
There is no real dividing line between two neighbouring cycles: for a year or two, the old and new cycles rather overlap, and spots of the outgoing cycle occur along with those taking over, but each one knows its place. The oldtimers bunch up closer to the equator, while the new ones congregate in the higher latitudes.
Spots do not represent the only activity of the sun. It is simply that they are the most obvious formations and were discovered first.
Sunspots have permanent satellites, so-called faculae, which are the brightest regions of the photosphere. These bright moving filaments orig inate just a little before the accompanying spot and persist after the spot is gone. Long-living faculae sometimes last for about a year, while a spot rarely lives for more than four to five 27-day rotations of the sun. And the area occupied by a facula is on the average about four times that of a spot.
In general, faculae may be considered the crea tions both of the photosphere and the chromo sphere. At times and quite unexpectedly, a por tion of a facula is lighted up very brightly and we have what is called a solar flare (chromo spheric flare). In 10 to 15 minutes it reaches peak brightness and then gradually decays. Some scien tists believe that these flares are caused by nuclear decay, something in the nature of an explosion of a hydrogen bomb. Flare intensity is measured in area as Class 1 to Class 3+. You can imagine the scale of this phenomenon if the explosion of a
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Class 3 Hare occupies over three thousand million square kilometres!
Other manifestations of solar activity—promi nences—belong both to the chromosphere and the corona. These are perhaps the most magnificent events on the sun. Prominences are gigantic splashes of flame, blazing clouds of a kind of erup tive origin. They project out of the sun’s surface moving at speeds of many hundreds of kilometres a second. These fiery tongues can extend out into space for hundreds of thousands of kilometres. At times, if the speed exceeds 620 km/s, they can completely tear away from the sun and go off into interplanetary space.
Prominences consist of rarefied clouds with temperatures of several tens of thousands of de grees Celsius. They take on the weirdest forms on the background of the limb of the solar disk and call to mind spouting jets of water, arcs, trees and clouds.
There are not only rising prominences, but also descending ones when the solar matter moves downwards from the corona to the photosphere. This most often occurs over sunspots. Promi nences have a variety of lifetimes. The most quies cent prominences persist for several solar rota tions, while eruptive prominences disappear in just a short time.
The cycle of solar activity is accompanied by changes not only in the photosphere and the chromosphere, but in the corona of the sun as well. During maximum solar activity, we can observe multitudes of long coronal lines (rays) that are more or less uniformly distributed over the solar disk. During years of minimum activity,
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they shrink closer to the sun’s equator. The coro na is denser during maximum than during the period of minimum.
The various manifestations of solar activity ob served in different regions of the sun are always interrelated in some way. It may be that the pri mal cause of these changes lies in the changes in the magnetic fields of the photosphere and chromosphere of the sun.
This is how events very often develop. A day or two after a local magnetic field appears in some region of the sun, faculae are seen to come to life in the vicinity; these are followed by the first signs of the sun’s malaise—spots. The area of the faculae and spots increases. One or two weeks later, one can expect solar flares. Their lifetime is ordinarily drawing to a close as the sun completes one rotation on its axis. But in a short while a second phase of activity sets in: the faculae become smaller in number and then long quiescent filaments appear on the surface of the sun. These are prominences that project on the solar surface. Another few rotations of the sun and the visually observable events come to an end. But sensitive instruments show that in this place a special kind of magnetic field with a sin gle pole persists for quite some time.
As early as last century it was noticed that magnetic storms have a strong tendency to recur roughly every 27 days. But this is precisely the time that it takes the sun to rotate once on its axis. There was one case when magnetic storms recurred every 27 days during 17 solar rotations. Quite obviously, having perceived this property, scientists are able to predict magnetic disturb-
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During the minimum of solar activity of 19531954, scientists correlated these recurrent storms with the appearance of single-pole magnetic fields during the relatively short time from April 1953 to January 1954 and found a rather definite relationship between them. However this interval of observations is too small for an exacting spe cialist to draw any definitive conclusions. At the present time, all hopes are being pinned on studies of the results of the International Quiet Sun Year.
Almost 200 years ago scientists first turned to the sun in attempts to explain the origin of mag netic storms and the polar lights (auroras). Even at that time they maintained, though they lacked proof, that the sun might be sending to the earth some kind of minute charged particles, corpus cles, that generate magnetic disturbances and the auroras.
To verify this supposition, at the end of last century the Norwegian physicist Kristian Birkeland carried out his famous experiments in ir radiating a magnetic model of the earth (terrella) with cathode rays (electrons). The terrella was a spherical electromagnet placed inside a large cathode-ray tube; the sun was the cathode that emitted streams of electrons. It was found that the electrons are deviated to the polar regions of the terrella and impinge on its night side.
Birkeland’s experiments were later corroborat ed by rigorous mathematical calculations of the motion of charged particles in the magnetic field of the terrella. The Norwegian mathematician Carl Stormer, who made the computations, later became a devoted investigator of polar auroras
Thence саше the Birkeland-Stormer theory of magnetic storms and the aurora.
This theory states that at definite instants, com paratively narrow but dense fluxes of charged particles of the same sign may be ejected from certain active regions of the sun. But due to mu tual electrostatic repulsion a dense flux of parti cles of one sign cannot persist during the time of flight of the particles from the sun to the earth. The Birkeland-Stormer theory could not account for this fact.
A new hypothesis was advanced in the 1920s which stated that solar fluxes consisted of an equal number of positively and negatively charged particles and that, hence, the stream or flux was, on the whole, neutral. Another two decades later it was suggested that the sun constantly emits streams of particles of both signs. These streams occasionally build up and generate mag netic storms. The idea was put forth that there is a constant solar wind, or corpuscular radiation of the sun. This is the way it developed.
Since remote antiquity people have been amazed, and more often frightened, by the appear ance of comets. Astronomers have long noticed that the tails of these visitors from deep space al ways point away from the sun, no matter what their direction of motion is.
This was first explained by the effects of light pressure which was discovered by the Russian physicist P. N. Lebedev. However, about 20 years ago the German scientist L. Bierman proved that it was impossible to account for such behaviour of a cometary tail by light pressure alone. He conjectured that the gases forming this tail are
whipped about by streams of solar particles and he computed the speed of this wind to be about 500 km/s.
The hypothesis of solar wind does not run counter to anything we know about the sun and circumterrestrial space. But it remained a hy pothesis until interplanetary probes proved just recently that between our planet and the sun there is a stream of corpuscular radiation emitted by the sun and travelling with velocities from 300 to 600 km/s.
The period of the IQSY was particularly con venient for a comprehensive study of the sun and the effects it exerts on our planet because com paratively few large-scale events occurred on the sun during this period, and those that did were isolated and could be traced throughout their life times, from birth to total decay.
During the IQSY, the sun was taken under particularly careful surveillance. Over a hundred observatories participated: 19 in the U.S.S.R, 17 in the U.S.A., 7 in France, 5 in Japan, and 4 each in the German Democratic Republic, India, the German Federal Republic, Canada, Italy, etc.
To keep the sun under round-the-clock sur veillance, and not let a single event on the sun escape notice, the different observatories dis tributed their observational times so that the sun was always under observation. This made it pos sible to study variations of solar activity in time and to correlate them with geophysical phenome na observed by ground stations and recorded in interplanetary space by satellites and space probes.
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These observations made it possible to predict solar activity and choose the best and safest times for space flights. Finally, they suggested to geo physicists times for more frequent observations during events of interest that might be expected.
The central event in solar studies during the IQSY was the joint investigation of active re gions of the sun which was conducted on the ini
tiative of Soviet |
geophysicists between April 1 |
and September |
30, 1965. Many first-class solar |
observatories in |
different countries participated |
in this work. The aim was to obtain diverse de tailed information about the origin, development and decay of active regions on the sun.
Apart from these special and co-ordinated (in time) observations, many observatories carried out their own individual studies in accord with the IQSY. Some made careful measurements of the magnetic fields of the sun in order to compile an exact magnetic map of the sun for every day of the IQSY. Others engaged in high-speed cin ematography of the chromosphere to obtain a film of the sun for the two-year period. Still other observatories sent instruments aloft in balloons, rockets, and satellites to photograph the solar corona outside the atmospheric mantle of the earth. Some observatories bounced radar signals off the sun’s surface. Others concentrated on the effects of the solar wind on cometary tails. Final ly, some investigators collected all their instru ments and devices and travelled thousands of kilometres to a small atoll in the Pacific Ocean (Manuae in the Cook Islands) where a total eclipse of the sun was observed for five minutes on May 30, 1965.
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