книги из ГПНТБ / Pushkov N.V. Quiet Sun

ish where the magnetic field ceases to capture and hold particles. But this region coincides with the boundaries of the magnetosphere. It is ob­ vious that the boundary line varies with the energy of the particles and will pass at different distances from the earth.

In the first measurements, when counters kept

km. But later, when charged-particle traps were employed first by Soviet and then American in­ vestigators to measure the less energetic parti­ cles, the boundary kept moving away from the earth. At one time it was even believed that there was a third zone made up of low-energy particles and located beyond the outer zone, a

The trouble is that it is all the time doing busi­ ness with the sun and the magnetic field.

The satellites Injun 1 and Injun 3, orbited at about 1,000 km, passed close to the poles; with a special device they oriented by the mag­ netic lines of force. They found that in the outer zone there is a considerable diurnal variability in the position of captured particles of different energy. Thus, the northern boundary of elec­ trons with energies exceeding 280 keV was, at local noon, confined to the line of force passing through the magnetic equator at a distance of eight earth radii, while at local midnight, the distance was twenty earth radii. This may be due to the fact that the magnetic field is upset by gusts of solar wind.

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What particles inhabit the outer belt of ra­ diation? In contrast to the inner belt, there are very few protons here. Its principal inhabitants are electrons, and their numbers can increase or decrease tenfold in a single day.

The greatest electron intensity is observed in the plane of the magnetic equator at a distance of about 22,000 km from the centre of the earth. Electrons of the outer belt have energies roughly within the range from 40,000 to 5,000,000 eV, so that there is quite some variety here. Most nu­

There are so few protons in the outer belt that they were not found right off. Only in 1961 were protons detected by the Explorer 12 satel­ lite. So far we have encountered protons with energies only between 100,000 and 4,500,000 eV, but it is suspected that there are protons with higher and lower energies, but not such as in the inner zone.

To summarize, then, the size of this radiation zone is enormous. On the sun-lighted side, it can extend out from eight to thirteen earth radii, on the night side, as far as we can conjecture, out to more than twenty-two earth radii. With the exception of cosmic rays that penetrate the magnetosphere, all the other radiation captured and contained by the magnetic field for long periods forms just as integral a part of our plan­ et as does the solid portion and the gaseous mantle. But no other sphere of the earth expe­ riences such large fluctuations as does the mag­ netosphere in a single day.

Scientists now feel convinced that the long night tail of the magnetosphere with all the par­ ticles therein does not participate in the daily rotation of the earth. It is very likely that only a small portion of the tail (out to eight or ten earth radii) that goes to make up the more or less stable nucleus of the magnetosphere takes part in the earth’s rotation. The remainder slips over the nucleus and maintains the same posi­ tion relative to the sun.

But how do charged particles behave that get picked up by the magnetosphere? One thing is sure, they have no easy life of it. Electrons and protons are constantly hurtling from one end

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of the enormous magnetic trap to the other with tremendous velocities, different particles having different velocities.

We do not know how the low-energy parti­ cles of the solar wind can penetrate into the depths of the magnetosphere and acquire high energies. So far there are only a variety of sup­ positions. But we do know that once they have been taken prisoner by the magnetosphere, they are constrained to move and obey the deflecting action of the earth’s magnetic field.

If the direction of motion of a particle coin­ cides with the direction of the line of force, it simply slips along it. But if, as is more often the case, the particle enters the field at some angle to the force line, it gyrates down the line of force.

The radius of a spiral traced out by some par­ ticle is directly proportional to its mass (this makes the diameter of a proton-turn 1,840 times that of the diameter of turn of an electron) and is inversely proportional to the intensity of the earth’s magnetic field in the region that it is confined to. Which means that the higher the latitude the particle enters, the narrower the loop that it makes round the line of force. What is more, electrons loop round it in one direction, and protons in the opposite, and the tiny mag­ netic field set up by each within the spiral re­ duces the total magnetic field of the earth at the given point.

The magnetic field increases as it approaches the magnetic poles, thus reducing the radius of the spiral traversed by the visiting particle. At the same time the angle between the line of

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force and the direction of motion of the particle increases. Finally, the particle revolves at a right angle to the line of force. Then something like reflection occurs: the direction of translatory mo­ tion is reversed, and the particle spins down the spiral along the same line of force to the other end of the earth.

And so on, numberless times, the particle swings down the line of force covering in a second or so the enormous arc from one point of reflection to the other and back again. That would go on indefinitely if all events took place in an absolute vacuum.

But the upper atmosphere is far from an ab­ solute vacuum. Scattered about in profusion are atoms and molecules of gases, and their num­ bers increase as one approaches the earth. The newcomers collide with particles of the air, lose energy by giving up their charge to neutral par­ ticles and perish by absorption in the atmo­ sphere. These collisions are particularly frequent in the bent horn tips of the radiation zone that bring the particles close to the earth’s surface where they enter the denser layers of the at­

far south of our planet. On occasion, nature im­ mortalizes them with resplendent auroral dis­ plays over the icy wastes of the Arctic and the Antarctic. Other events take place at the same time; the upper atmosphere heats up and radio waves are absorbed at an accelerated rate. But that will be described in a later chapter.

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Flights along lines of force are not the only motions of charged particles that are captured by the magnetosphere. Such particles are con­ stantly being shifted about; they drift round the earth along magnetic parallels. This is the reason.

The magnetic field of the earth diminishes with altitude. The trajectories of particles spiral­ ing along a line of force are more gradual in the upper part of the spiral and steeper in the lower part. That is what makes a particle shift in a direction perpendicular to the field during each traverse: protons slip westwards and elec­ trons eastwards. This sets up a ring electric cur­ rent round the earth in a westerly direction. A particle with energy about a million electron volts shifts round the earth making a complete circuit once every 24 hours.

The foregoing is accounted for by a special theory. In 1958 an incident occurred providing experimental verification. In August of that year, a nuclear device was exploded over John­ ston Island, a coral island in the Pacific Ocean lying between the Hawaiian and Marshall islands. It was detonated at a height of about 70 km. The result was an unexpected (for it was a secret nuclear test) auroral display in the re­ gion of the Samoa archipelago over the Apia Observatory some 3,500 kilometres from the site of the explosion.

This typically polar phenomenon put in an appearance in the tropics not by accident at all. A close look at the magnetic map shows us that the point of explosion and the site of observa­ tion of the unusual aurora lie roughly on one

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and the same geomagnetic meridian symmetric about the geomagnetic equator: Johnston is north of the equator just about as much as Apia is south of it. They are conjugated by one and the same line of force of the earth’s magnetic field. Electrons generated by the explosive force rushed along these rails into the opposite hemi­ sphere triggering a tropical aurora and thus proving the foregoing theory about particle be­ haviour in the earth’s magnetic field.

Several other atomic explosions set off prior to the prohibition instituted by the Moscow Agree­ ment demonstrated that they set up about the earth an artificial belt of radiation in addition to the natural belts that were discovered during the International Geophysical Year. In July of 1962 the United States carried out Operation “Starfish” exploding a nuclear device in the up­ per atmosphere at a height of several hundreds

lower edge being only 350 km from the earth. The satellites Injun 1 and Injun 3 showed that half a year was needed for the number of elec­ trons injected by man into surrounding space to disperse by 15 per cent.

The electrons leaked into the atmosphere mainly from below and from the sides of the belt. Scientists believe that the particles producted by this explosion will continue to be detected, though in diminishing numbers, for a whole decade following the day of the deto­ nation.

Fortunately, none of the countries that have

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signed the Moscow Agreement on the prohibi­ tion of atomic explosions will carry out any ex­ periments of this kind. Atomic tests contaminate the atmosphere creating a danger to the health and lives of astronauts; they are also an obstacle to studies of captured radiation of natural ori­

gin.

The upper layers of the atmosphere in the polar areas are not the only spot where captured particles can leak out. Quite a few are lost in the region of the negative world magnetic anomaly in the South Atlantic. Particles moving along the lines of force emerging from this enor­ mous anomaly should be reflected at lower alti­ tudes where the air density is higher. Actually, however, they are absorbed here. Observations via the Soviet spaceships and satellite vehicles of the Cosmos series carried out at low (about 300 km) altitudes indicate that the intensity of the captured radiation over this anomaly is ap­ preciably greater than in other places on the same magnetic parallels.

The particle leakage from the radiation zone is particularly great during magnetic disturb­ ances. Although as yet there have been no simul­ taneous observations of how the magnetic field and the intensity of captured radiation vary during magnetic disturbances, the Explorer 6 sat­ ellite was able to find out that during a mag­ netic storm the electron density in the outer ra­ diation belt diminished by 75 per cent and the

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The proton density during the storm nearly dou­ bled as compared to normal.

When scientists had analyzed the measure­ ments made by the third Soviet satellite and its American colleague Injun, it turned out that within the auroral zone the stream of electrons with energies between 20 and 60 keV came out to ten million particles per second per square centimetre. Yet the calculations of some work­ ers show that even relatively weak auroras re­ quire electron streams tens of times more in­ tense. What this means is that just a few hours of a good auroral display can use up all the energy accumulated in the storehouse of the earth’s radiation belt. But actually not a night goes by without auroras shooting up in the skies over the Arctic and Antarctic. Where do these limitless supplies of charged particles come from?

As yet there is no final answer. It seems very likely that the sun is constantly sending parti­ cles into the radiation belt, high-energy particles that are captured by the earth’s magnetic field. It might very well be that these are particles of terrestrial origin that the magnetosphere has in some fashion accelerated to high energies.

During 1961 and 1962, satellites moving in elongated orbits followed developments beyond the earth’s magnetosphere that occurred after solar flares. It was found that Class 2 and Class 3 flares are attended by powerful fluxes of protons over a very wide range of energies: from units to several hundreds of millions of electron volts, the major portion and the greatest intensity of which involved lower-energy particles. Protons

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come to earth in two echelons, as it were: an “express” several hours after the flare, and the other—together with low-energy plasma—a day or two later. Satellites established that the sun’s chromosphere gives rise to a great many more flares than hitherto supposed; ground-based ob­ servatories simply did not record them. Appar­ ently, each of these flares is capable of pump­ ing supplies of protons into the outer radiation reservoir of our planet.

There may yet be another way of adding “solar warriors” to guard posts round the fringes of the earth. When speaking of the solar plasma, one ordinarily and tacitly assumes it to be completely ionized. But recently Chapman and Akasofu started with the assumption that the solar plasma is not completely ionized, that there are more or less substantial quantities of neutral hydrogen atoms present. Being neutral they (like neutrons) are not subject to the action of a magnetic field and can therefore penetrate the magnetosphere to great depths without exert­ ing any effects on its size.

They are then ionized in collisions in the magnetosphere and are accelerated and in this way participate in building up the current layer and the polar auroras. The authors of this hy­ pothesis say that the neutral hydrogen atoms in the solar plasma endow it with new properties and can account for the great diversity of mag­ netic disturbances.

The problem of the origin of high-energy elec­ trons is much more complicated. To solve it, scientists threw into the fray space probes and satellites put into eccentric orbits. They went

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