книги из ГПНТБ / Pushkov N.V. Quiet Sun

Now if it were possible to find a regular re­ lationship between an increasing or decreasing flux of primary cosmic rays and sidereal time, this would mean that they have a preferential direc­ tion of arrival on the earth. This would help to determine the origin of the mysterious galactic wanderers.

Unfortunately, so far no one has been able to establish any such sidereal-diurnal variations of cosmic radiation. Ten years ago the observational facilities were still imperfect and the network of stations and observatories was too sparse. During the 1GY a proper network was set up, but the sun was “on the warpath” and the excessive ac­ tivity was too much for such delicate experiments. During the IQSY, when the sun had calmed down, attempts were made in many countries to trace sidereal-diurnal variations.

Cosmic-ray intensity patterns “like” to recur every 27 days. Studies by a number of workers have shown that this tendency is sometimes ob­ served during certain periods, particularly during maximum solar activity, but it is not stable. The amplitude of 27-day variations during minimum solar activity is close to zero, while at maximum it lies between 0.1 per cent and 0.5 per cent.

“Hunters” of cosmic rays at over a hundred stations in thirty countries (including 15 Ameri­ can, 14 Soviet, 8 Argentine, 7 French, 7 Austra­ lian, and other stations) are now equipped with refined apparatus. Complicated neutron monitors, ionization chambers reminiscent of H. G. Wells’ martian tripods, batteries of Geiger counters combined into whole telescopes—such were the facilities delivered during the IGY to the far

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it possible to measure particles with energies about 1014 eV and determine the distribution of primary protons within the energy range from 1010 to 1014 eV. On the earth, this equipment has yielded results only in studies of particles of ener­ gy a hundred times less.

We may summarize by saying that the mini­ mum of solar activity coincided with the maxi­ mum activity of those studying it. These mes­ sengers from deep space will not be able to keep their secrets for long.

The invisible belts of the earth

Everyone knows of the rings of Saturn, but only recently did we learn that our own earth is not devoid of a certain cosmic embellishment. True, the rings, called radiation belts, are not visible in any way, but instruments clearly feel the presence of particles there.

Today nobody considers the earth as consisting only of solid and liquid spheres. No, the litho­ sphere and the hydrosphere are customarily sup­ plemented by the atmosphere, or the gaseous mantle of the planet. It is clear that the atmo­ sphere is an unalienable part of the earth, for, among other things, it rotates with the earth.

The magnetosphere is just as integral a part of the earth as the other three familiar spheres. Anyone who wants to determine how big our earth is, will have to figure out where the mag­ netic sphere ends, for it is within this sphere that all events of earth-sun relations take place.

By all rights, the “end” of the earth should be

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that limit in space where the earth’s magnetic field no longer diminishes with distance from the surface, for here it is no longer the earth’s field but the interplanetary magnetic field, which does not depend on distance from the earth.

So the true boundaries of our big home, if taken as a real whole from cellar to roof, pass at a distance exceeding some twelve earth radii.

During the International Geophysical Year no one would be surprised to hear that the earth is under constant bombardment of cosmic rays. Bal­ loons and rockets sent up shortly after World War II showed that at heights of 40 to 50 kilometres above the earth’s surface substantial changes occur in the intensity of cosmic radiation, yet there does not seem to be any change at all above this level.

But what about still greater distances from the earth? Here the second and third Soviet Sput­ niks carried cosmic-ray counters out to distances exceeding a thousand kilometres. Here it was found that the intensity of cosmic radiation be­ gins to increase with distance. The satellites en­ countered whole swarms of particles when they approached the zone of auroral displays. That was the pattern observed by both Soviet and American satellites.

Analyzing the results of the first satellite meas­ urements of radiation, Soviet scientists headed by S. N. Vernov, Corresponding Member of the U.S.S.R. Academy of Sciences and American specialists working under Professor James A. Van Allen came to the conclusion that we are dealing here with a radically new phenomenon: radiation that the force lines of the magnetic field capture

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and do not allow to reach the earth’s sur­ face.

Before, scientists had allowed for the possibil­ ity of a temporary capture of charged particles by the magnetic field of the earth, as witness the experiments of Birkeland dealing with the mo­ tion of charged particles in a magnetic field, or the theoretical calculations of Stormer and the prominent Swedish astrophysicist Alfven. But at the time that was only a hypothesis. What is more, it was believed that the capture occurred at considerable distances from the earth, the space in the immediate vicinity being a zone prohib­ ited to particles (true, with the exception of the region of polar auroras).

In contrast, today on the basis of a whole se­ ries of experiments a theory has emerged stating that the whole magnetic field of the earth is an enormous trap for particles coming from the dis­ tant expanses of space. It became quite clear that with this gigantic magnetic net, the earth just sweeps up the little “butterflies” that flit in from the depths of the universe.

It was natural to presume that the distribution of the particles, the radiation in the space about the earth, should depend on the shape of the earth’s magnetic lines of force. To verify this and to figure out the dimensions and structure of the radiation zone, it was necessary to measure the radiation out to appreciable distances from the earth.

The first measurements of this type were car­ ried out by the American space probe Pioneer 3 on December 6, 1958. It was fired moonwards, but due to some malfunction returned from a

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distance of about 107,000 kilometres. In the proc­ ess, Pioneer 3 went through the magnetosphere twice (out and back). Its return trip did not go over the same route, but both trajectories passed close to the magnetic equator, and the Geiger counters on board showed variations in the num­ ber of charged particles along the way.

The following year, similar measurements were taken three times, in January and September by the Soviet automatic interplanetary stations Lu­ na 1 and Luna 2 and in March by the American space probe Pioneer 4. The results obtained were all very much alike. It turned out that enormous regions of circumterrestrial space, practically the whole magnetosphere of the earth (that is, to the extremes that our magnetic field extends) is filled with charged particles of a range of energies that have been captured and are contained by the earth’s magnetic field.

The discovery of this captured-particle region, which goes by the name of “radiation zone”, or “radiation belt”, is one of the most interesting and most important discoveries of the International Geophysical Year. From the very start it was clear that this belt is directly related to the auro­ ras and to magnetic disturbances and that the level of radiation and the shape of the zone may be dependent on the activity of our sun.

Measurements were first made with counters that permitted determining the radiation inten­ sity within comparatively narrow ranges of par­ ticle energies. These measurements pointed to the existence of two zones of radiation, two belt res­ ervoirs of particles circling the earth.

When the counter readings of Pioneer 3 were

plotted, the curve quite unexpectedly turned out to be double-humped. Out to distances of ap­ proximately 10,000 to 12,000 km from the centre of the earth the curve rose sharply indicating an increase in quantity of particles. This was fol­ lowed by a sudden decline (a trough in the curve).

23.000 km, the number of particles went into a smooth gradual decline. On the return leg of the journey, all these events were reversed.

The only thing to conjecture was that each of the humps in the curve denoted an individual region of radiation. The one closer to the earth was called the inner radiation belt, the more distant one, the outer belt. Investigators were first inclined to think that the inner radiation belt was made up of positively charged particles (high-energy protons), and the outer, of highenergy electrons.

However, subsequent experiments demonstrat­ ed that electrons and protons are present in both regions. Later, it became clear—although this might have been assumed earlier—that the lowenergy particles could be at greater distances from the earth than the more energetic particles because they could be held in by a weaker mag­ netic field.

Today, scientists incline to the view that the radiation zone that stretches out to the very lim­ its of the magnetosphere is an integral whole. However, the inner portion differs drastically from the outer portion, so we shall discuss them separately.

The inner zone has the shape of a belt sym-

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ю

How radiation is captured. Cross-hatched portions indi­ cate inner and outer radiation belts. Labels indicate number of particles counted per second. Data are based on meas­ urements of satellite Explorer 4 and space probe Pionneer 3

metric about the magnetic equator but somewhat shifted relative to the geographic equator. Its

The belt circles the earth at an angle (is asym­ metric) because the magnetic axis of the earth does not coincide with the axis of rotation. It is displaced several hundred kilometres towards the eastern hemisphere and the swarms of particles obedient to geomagnetism pass lower down in our hemisphere than in the western hemisphere.

The radiation belt is rather wide, extending from the geomagnetic equator northwards and southwards to 45° and embracing a swath over

Central and South America, nearly the whole of Africa, the southern part of Asia, Australia and Oceania. At its thickest, in the plane of the equa­ tor, this radiation belt measures several thousand kilometres.

The inner radiation belt is always in a rather calm mood. During a whole year the number of particles inhabiting this region does not change by more than a factor of two or three in either direction. And the location of the belt in space hardly at all varies with time.

This is apparently the result of its compara­ tively indifferent attitude towards solar activity and magnetic activity. True, a case was observed when a burst of cosmic rays of solar origin boost­ ed the particle intensity somewhat. Apparently, this does not occur so often, and the burst exert­ ed its main effect on the most distant part of the inner belt.

A few words about the composition of the inner belt of the earth. Photographic plates that trav­ elled by rocket to 1,200 kilometres returned with autographs of protons of energy from several tens of millions to hundreds of millions of elec­ tron volts. Another rocket related that at 1,000 kilometres oqt it encountered electrons of ener­ gy only about 150,000 eV.

The next problem is of course to find out where these particles come from that fill up the enormous belt-like reservoir. We have to admit that opinions differ: not much time has passed since the discovery of this phenomenon and very little information has been accumulated so far. At present the most widespread view is this.

Cosmic rays are believed to be responsible for

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the inner radiation zone. Primary particles com­ ing to the earth from the depths of the universe bombard our atmosphere. Collisions with atoms generate secondary particles which include quite

field and splash out in ail directions. Some move towards the earth, others, away into space.

Now neutrons have short lifetimes, on the average of about 13 minutes. They decay into protons and fast electrons, both of which are charged and cannot therefore ignore terrestrial magnetism. The magnetic field captures them and delivers them to the inner radiation belt of the earth, where they stay trapped for quite some time.

We now turn to the outer radiation belt. From the very first measurements it was evident that there is always a “no-man’s land” between the

of two half-moons concave to each other. The

within the zones of auroral displays.

This belt is farthest from the earth over the equator. Here the lower fringe lies at a distance of 12,000 km from the surface. Over the Arctic and Antarctic, the narrow bent horns drop to 250-500 km above the earth.

Now the outer boundary of the outer belt is quite a different matter. It is not easy to deter­ mine because we are dealing with captured ra­ diation and its sphere of influence should van­

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