книги из ГПНТБ / Pushkov N.V. Quiet Sun

magnetic declination and two (horizontal and ver­ tical) components of the total magnetic-field in­ tensity.

Magnetograms (which is the name given to traces of the magnetic pulse of our planet) note every single variation. There are days when they exhibit clear-cut regular variations with a period equal to the solar day. Such days are termed mag­ netically quiet days. Variations like these are par­ ticularly characteristic of the lower latitudes and are most frequent when the sun is at lowest activ­

ity.

During the years of maximum solar activity, diurnal amplitudes of differences between great­ est and least values in diurnal variations come out 60 per cent greater than at times when the sun is quiet. Generally speaking, the magnetic field changes more in the summer than in winter and more in the daytime than at night. On mag­ netically quiet days, all these variations are more or less regular and orderly.

But there are times when the pulse of the earth jumps out of its regular variations and becomes rapid and irregular. This is a magnetic disturb­ ance. If the disturbance is considerable and the magnetogram displays large zigzags, it may be called a magnetic storm.

Storms in the magnetic field of the earth are most frequent when the sun is close to maximum activity, and are strongest in the higher latitudes, particularly in the Arctic and Antarctic that have frequent auroral displays. Still, even in the polar regions and even in the most intense magnetic storm the magnitude of magnetic disturbance nev­ er exceeds several per cent of the constant mag­

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netic field. Thus, despite a certain change of moods, the magnetic temperament of this planet may be taken as rather balanced.

In scale, magnetic storms are divided into world-wide that embrace the whole planet and persist for a number of days, and polar, which occur only in the higher and medium latitudes and in the course of a few hours.

There is yet another phenomenon that has at­ tracted attention since it was first detected at the end of last century when new and more sensitive instruments were designed. It consists of rapid and rather regular variations of the magnetic field with periods of a few minutes or less. They go by the name of magnetic micropulsations and occur at all times, no matter what the mood of the magnetic field, quiet or excited. Some are re­ corded simultaneously even by observatories sep­ arated by large distances, others (the most in­ tense ones) are real giant pulsations that appear only in comparatively limited regions near the auroral zone.

Where does all this regular and irregular ebb­ ing and flowing in the magnetic sphere stem from? Mathematical methods of analysis have come to the aid of geophysicists. They suggest that most of the diurnal variations and magnetic storms are generated by external sources lying outside the solid body of the earth, and only about one-third originate within our planet.

As for variations of an internal origin, there is no doubt that they are produced by currents in­ duced in the upper electrically conducting layers of the earth by variations in its exterior magnet­ ic field.

Diurnal changes in the magnetic field are ac­ counted for most realistically by the “atmospheric dynamo” hypothesis advanced at the end of last century by the English investigator Arthur Schus­ ter and develooed in more detail by Sidney Chap­ man, also of England. According to this theory, the quiescent diurnal variations are due to elec­ tric currents in the ionosnhere, which is an at­ mospheric laver roughly 60 km above the earth’s surface. It differs from the lower-lying layers in that it contains large quantities of charged par­ ticles, ions and electrons. Quiescent diurnal var­ iations occur when the ionosphere moves in the magnetic field of the earth and responds (like the world ocean) to the tidal forces of the sun and moon and to temperature variations. In a word, then, the magnetic field of the earth is a “perma­ nent magnet”, the atmosphere is the “armature” and its conducting layers the “winding” of a gi­ gantic “dynamo”, in the middle of which we live.

Ionization of the upper lavers of the atmo­ sphere (the formation here of large quantities of ions and electrons) is due to the action of the sun’s ultraviolet and X-radiations. The ionization and, hence, electric conductivity of the upper atmo­ sphere over every spot varies with the time of day, the season of the year, and the cycle of so­ lar activity. Also connected with this factor are the diurnal variations of the magnetic field men­ tioned above.

Though this scheme accounted for very many phenomena it remained a hypothesis until instru­ ment-carrying rockets made possible magnetic measurements at high altitudes. Scientists could then make direct “contact” with all these events,

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This is how the magnetic field near the geomagnetic equator diminishes with altitude. These rocket measurements were made in the vicinity of a local anomaly, so actual reduc­ tions (open circles) differ from the theoretically calculated values (solid line)

The first direct high-altitude measurements were obtained in the region of the geomagnetic equator. Theoretical calculations show that the earth’s magnetic field should diminish gradually with altitude, exhibiting no abrupt changes. How­ ever, rocket tests disclosed two breaks in this theoretically calculated decrease of field with in­ creasing altitude: in a zone 93 to 105 km up. The results were the same both in ascent and descent.

T h i s s h a r p j u m p i n m a g n e t i c - f i e l d v a l u e s i n d i ­

c a t e d a n a d d i t i o n a l m a g n e t i c f i e l d i n t h i s z o n e s e t

l a u n c h e d d u r i n g a n a u r o r a l d i s p l a y a n d i t t o o d e ­ t e c t e d t h e e x i s t e n c e o f c u r r e n t s i n t h e i o n o s p h e r e .

cess. In some cases such discontinuities in the magnetic field were observed at altitudes of 100 and 110 km, and once the rocket reached a height of 120 km but had not yet emerged from the current-carrying layer.

The most spectacular display in the magnetic arsenal of the earth are world-wide magnetic storms. Specialists are prone to say that during the hundred or so years that magnetic storms have been recorded, no two have been exactly alike. Try to classify them and construct a theory of such a phenomenon!

Still and all, many storms have a number of common peculiarities. Most of them commence imperceptibly and gradually, so that it is impos­ sible to state exactly when the magnetic weather went foul. On the contrary, the ones that com­ mence suddenly exhibit small but abrupt changes of the magnetic elements that are recorded at practically one and the same time at widely separated observatories.

Magnetic storms also terminate in a very pe­ culiar manner. After the principal fluctuations of the magnetic field cease, the field gradually set­ tles down into its normal state. The process is rather rapid at first but then slows down spread­ ingover many days. These post-disturbances are best studied during minimum solar activity when there is a long period of magnetic calm between storms.

Polar storms are another kind of disturbance in which the uneasy nature of the magnetic field is manifested. Unlike the world-wide storms, they embrace mainly the Arctic and Antarctic and adjacent regions, are of shorter duration, usually

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not more than a few hours, and are more intense on the night (unilluminated) side of the earth.

The year was 1918, Europe was still in the grips of World War One. But even during these

Observatory, the German scientist Adolf Schmidt came to the conclusion that during magnetic storms a ring of electric current can develop about the earth flowing constantly from east to west.

The magnetic field of a ring current of this kind should diminish the horizontal component of the magnetic field whenever the principal phase of a storm is in progress. At the end of the phase, the ring current falls off in strength and creates the effect of a gradual calming down, which is what we actually observe.

Schmidt did not say at what distance from the earth these currents circulate. And those scien­ tists that did could not come to any agreement. The Norwegian Carl Stormer considered that they could be a hundred earth radii from the sur­ face, while Sidney Chapman, just as much of an authority, maintained that the distance was only a few radii. The disparity is quite natural for we know too little about the immediate vicinity of the earth.

There is still no unified and generally accepted theory of magnetic disturbances, although many outstanding workers have attacked the problem. There are hypotheses galore, but they would take us too far afield. We shall only go into the basic ideas of two of them that not only have to do with explaining magnetic storms but are inti­ mately bound up with auroras and cosmic rays.

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The relationship, long since known, between magnetic disturbances and the auroras suggested that all magnetic-field variations and the North­ ern Lights are caused by charged particles ejected from active regions of the sun and cap­ tured by the field of the magnet on which we all live.

This problem was taken up at the same time but from different angles by two Norwegian scientists. Kristian Birkeland, an experimenter par excellence, recalled the childhood of geomagnet­ ism and, like Gilbert, reverted once again to the toy earth, the terrella sphere, this time not one of ironstone but in the form of a spherical elec­ tromagnet. He placed his model of the earth in a large vacuum chamber about the volume of a cubic metre and irradiated it with electrons. So as to be able to see the pathways of the rays, Birkeland coated the terrella with a phosphores­ cent material and in various places on it mount­ ed ingeniously designed phosphorescent screens.

In numerous experiments he demonstrated that in all cases when magnetization of the terrella exceeded a certain level, dependent on the veloc­ ity of the cathode rays, they could bombard the terrella in only two zones: near the magnetic poles. The luminescence of the terrella due to bombardment with cathode rays was in the form of isolated spots in certain cases and spirals about the magnetic poles, in others.

Birkeland also found that the cathode rays could even circle the terrella a number of times before coming down to its surface. In certain cases they moved from one pole to the other. The favourite point of impact of the cathode-ray par-

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tides was the side opposite the sun, that is, away from the cathode ejecting the electrons (the night side of the terrella). That is how the chief peculiarities of auroras were reproduced in the laboratory.

A drawback in Birkeland’s experiments was that they did not enable one to judge about the trajectories of motion of the separate particles in the magnetic field of the terrella. Another Nor­ wegian, the physicist and mathematician Stormer, worked enthusiastically for many years over theoretical calculations of the trajectories of in­ dividual particles entering the magnetic field of the earth. All manner of orbits came out of his pencil point. There were open trajectories, nearly circular, lying in the plane of the magnetic

Stormer conjectured that it was these particles that build up during magnetic storms. Then the magnetic forces of the particles should combine at distances of a hundred earth radii, which is beyond the moon’s orbit, forming a belt of cur­ rents flowing from east to west.

Birkeland’s experiments, which were repeated by many investigators using more refined devices yielding very narrow electron beams, and the calculations of Stormer formed the basis of a theory of magnetic storms and polar auroras that bears their names. The experiments did not only agree with calculations, but predicted the exist­ ence of particle motions that were actually found only half a century later during the IGY, when

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Soviet and American scientists discovered belts of radiation surrounding the earth.

The Norwegian scientists maintained that the auroras are caused by luminescence of the air due to the influx of charged particles into the upper rarefied layers of the earth’s atmosphere, while magnetic storms result from the direct ac­ tion of the magnetic field of these moving par­ ticles.

But how are we to account for the great varia­ tions of the field which occur during magnetic storms? The point is that the particle flux must be very dense for this purpose. And the army of particles spewed out by the sun should “quar­ rel” en route, for particles of the same sign would set up repulsive forces and disperse.

The Birkeland-Stormer theory was not able to

it functions very well in accounting for high-ener­ gy particles of low density, such as, for example, cosmic rays moving in the magnetic field of the earth.

About thirty years ago, the English scientists Chapman and Ferraro advanced a theory of mag­ netic disturbances and auroras. They believed that the sun emits neutral streams of ionized gases—plasma—consisting of an equal number of charged particles of opposite sign. Measurements of the time lapse between a solar flare and the onset of a magnetic storm on the earth indicate that the mean velocity of cosmic particles ejected by the sun and presaging a storm should be about 1,500 km/s.

Imagine holding a water hose; the water is spouting out and you begin to rotate about your

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axis. The drops of water coming out of the hose continue in rectilinear motion but the stream as a whole begins to curve as you turn.

That is what happens to particles coming out of the sun, according to Chapman and Ferraro. The curvilinear corpuscular stream caused by the sun’s rotation reaches the earth and showers its magnetic field from the night side. In the process, electric currents should be induced on the side of the electric-conducting stream facing the earth; the magnetic field of the currents combines with that of the earth and enhances it in the space be­ tween the stream and the earth. Inside the stream, on the contrary, the field is weakened. The result is a sort of compression of the earth’s magnetic field. This is recorded in observatories as the first phase of a magnetic storm typifying a build-up of the horizontal component of the field.

The solar “forces” encounter resistance of the “earth-dwellers”—the magnetic force lines of our planet try not to allow particles to reach the earth. The stream of solar gas compresses the force lines of the field and begins to flow round the pocket of resistance, producing a sort of spher­ ical indentation in the cloud of sun-delivered plasma.

While the solar army is circling the earth, some of the invaders are taken prisoner: certain par­ ticles are captured by the force lines of the mag­ netic field and are forced to revolve round the earth forming a ring current. This whirling per­ sists even when the invasion is over and the earth emerges from the stream. But the intensity of the ring current gradually falls off with time.

The principal phase of a magnetic storm on

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