книги из ГПНТБ / Pushkov N.V. Quiet Sun

Distribution of total magnetic-field intensity over earth’s surface in 1945 (in oersteds)

magnetic-field intensity here is roughly three times that at the magnetic poles of the earth.

The north (actually, the south) pole of the field of homogeneous magnetization (it is called the geomagnetic pole) is located near the Cana­ dian polar station Alert at a point with co-ordi­ nates 80.1° north latitude and 82.7° west longi­ tude in the Arctic Archipelago of Canada, on the ice-covered island of Ellesmere, and the south (really, the north) pole lies in the region of the Soviet Antarctic station Vostok with co-ordi­ nates 76.3° south latitude and 121.2° east longitude.

If the magnetic field of the earth were only

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a field of homogeneous magnetization, the geo­ magnetic poles would also be the magnetic poles; that is, places where a magnetic needle freely rotating about its centre of gravity would settle in a vertical position.

But since magnetic anomalies interfere, the magnetic and geomagnetic poles do not coincide. By a 1962 determination, the north magnetic pole lies on Bathurst Island, near the Canadian polar station of Resolute Bay. Its co-ordinates are 75° north latitude and 100° west longitude. The south pole, also determined in 1962, lies near Commonwealth Bay (Antarctica) at 67° south latitude, 142° east longitude not far from the French scientific observatory of Dumont D’Ur­ ville.

I n c i d e n t a l l y , b y t h e t i m e t h i s b o o k g e t s t o i t s

r e a d e r s t h e p o l e s m a y n o l o n g e r b e t h e r e b e c a u s e t h e m a g n e t i c p o l e s d e p e r i d o n s e c u l a r v a r i a t i o n s

m a g n e t i c f i e l d .

The position of the geomagnetic poles is deter­ mined by a mathematical analysis of the distribu­ tion of the magnetic field on the earth’s surface according to a method developed and first rea­ lized in 1838 by the great German mathematician Gauss. But to determine the magnetic poles, one requires direct measurements of the magnetic field in their vicinity.

A t t e m p t s t o d e t e r m i n e t h e l o c a t i o n o f t h e m a g ­

n e t i c p o l e o n t h e b a s i s o f o b s e r v a t i o n s o f d e c l i n a -

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t w n a t w i d e l y s e p a r a t e d p l a c e s i n m e d i u m l a t i t u d e s . B u t t h e o n l y t h i n g t h e y c o u l d a g r e e u p o n w a s t h a t i t i s l o c a t e d t o t h e n o r t h o f t h e A r c t i c C i r c l e b e t w e e n G r e e n l a n d a n d A l a s k a . E x p l o r e r s s p e n t d e c a d e s t r y i n g t o r e a c h t h i s r e m a r k a b l e s p o t , b u t

a l l a t t e m p t s f a i l e d .

In the first quarter of the nineteenth century, the Englishman William Parry and the American John Franklin, while searching for a north-west sea passage along the northern shores of Ameri­ ca, carried out magnetic measurements and spared no effort to reach the coveted point. On one of these expeditions, Franklin and his com­ rades perished.

Though the magnetic pole was not reached, their observations helped to narrow down the re­ gion of the mysterious meeting point of the mag­ netic meridians. And Professor W. H. Barlow, a theoretician who had never been in this area, was able to determine the position of the magnetic pole on the basis of fresh data obtained from sites close to the pole. Fie placed it close to where it was found several years later by the Scottish po­ lar explorer James Clark Ross.

Participating in a sleigh expedition in search of a north-west passage, with great difficulty he reached Cape Adelaide region on the coast of Boothia Peninsula, which juts way out into the waters of the Arctic Ocean on the Canadian coast.

He carried out magnetic observations here on June 2, 1831, and finaly found the spot or, rather, the region where the north magnetic pole was then situated and where the inclinator needle stood up vertically. Needle inclinators were so imperfect in those days that one could not speak

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of an exact determination of the position of the magnetic pole. Ross felt “hurt” that a place so important to science should have been left un­ marked by nature and so commemorated it with a small pile of rocks.

The next scientist to visit these uninviting wastes and try to determine the site of the north magnetic pole was the great Norwegian polar ex­ plorer Roald Amundsen. In 1903-1906 he and six companions in a small ship sailed westwards from Greenland to Alaska.

In May 1904 they reached the region of King William Island by dog-sled and established that the magnetic pole had “moved” some sixty-five kilometres from the spot indicated by Ross. Amundsen did not coniine himself to a brief visit; 250 kilometres away at Petersen Bay, where he had to halt, he set up a magnetic observatory Gjoa Haven, where the wintering party recorded magnetic variations during 19 months.

This first long-term recording of variations of the magnetic field near the magnetic pole brought to light that the declination can change here dur­ ing magnetic storms by hundreds of degrees. This is not because magnetic storms are particularly strong in the area but that the horizontal compo­ nent which ordinarily holds the compass needle in the direction of the magnetic meridian is close to zero.

That is why the magnetic poles of the earth can, during diurnal variations of the exterior magnetic field, move tens of kilometres and even “run off” hundreds of kilometres during mag­ netic storms. From this it is clear that any deter­ mination of the positions of the magnetic poles

to within minutes or, all the more so, seconds of latitude and longitude is physically meaning­ less. The magnetic poles of the earth are constant­ ly in motion.

But time passed and with the demand for in­ creasingly exact magnetic determinations came improved instruments for scientific observations and transportation facilities for the geophysicists. A virtual assault was made on the Far North. Frenchmen, Canadians, Americans, Danes, En­ glishmen and again Canadians pushed by ship and dog-sled and by aerosleigh and skis to many near-inaccessible regions. Magnetic measurements were conducted over vast areas from Greenland to Alaska.

Soviet scientists also carried out a large number of magnetic measurements in the Arctic.

In this work of determining the magnetic pole in the Central Basin of the Arctic Ocean, a very important role was played by the Soviet “North Pole” drifting station. For the first time in his­ tory, in May 1937, four Soviet polar explorers were dropped onto an ice-floe near the point of the geographic North Pole. For nine months they fought the rigours of Arctic nature and studied one of the largest blank spots on the map of the world. During these 274 heroic days the expedi­ tion collected far more scientific material than any polar expedition of the past.

Of particular significance were the observations of the earth’s magnetic pole conducted during the drift by the young magnetologist E. K. Fyodorov. These observations were conducted over a range of 2,500 km of drift and greatly refined scientific conceptions about the state of the geomagnetic

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field near the geographic pole of the earth and towards the magnetic pole.

Since that time the setting up of drifting sta­ tions for studying magnetic, meteorological, hy­ drological and other geophysical phenomena has become a tradition. The example of the Soviet Union was followed by the United States, which during the IGY organized the drifting stations “Alfa” and “Bravo”. During the IOSY, the So­ viet Union had two drifting stations in the middle of the Arctic, “North Pole 12” and “North Pole 13”. On land too, we find more and more air-sup­ plied “jumping” expeditions like the Soviet an­ nual expeditions “Sever” (North) and the Cana­ dian explorations of the Arctic by aircraft, which were begun in the late 1940s by the Dominion Observatory at Ottawa.

Finally, an attack was made on the other blank spot of the globe, the Antarctic. One of the first to attempt a determination of the south magnetic pole was that indefatigable polar explorer of the 19th century James Clark Ross. Ten years after the discovery of the north magnetic pole he came

Robert Scott, the outstanding English polar ex­ plorer who later heroically perished on a wind­ swept plateau of ice, was the first to reach the vicinity of the south magnetic pole. In 1903 he determined the position of this remarkable point with co-ordinates 72°45' south latitude, 156°22' east longitude.

Several other polar explorers made subsequent determinations of the position of the south mag­

netic pole. Among them were the remarkable observations of the geophysicist Prof. David, a member of the British Antarctic expedition head­ ed by the famous polar explorer Ernest Shackleton. In January 1909 David “caught” the magnet­ ic pole 2,500 km from the geographic pole, deep within the Oates Coast at 72° south latitude, 155° east longitude.

One of the most recent determinations right at the pole itself was made in 1952 by a French po­ lar expedition that found the pole at about 68°40'

reaches of the Mertz Glacier (68° south latitude, 144° east longitude). Thus, in 45 years it ran a distance of over 600 km.

It is interesting to note that the movements of the north and south magnetic poles are not co­ ordinated. This suggests that what we have is not the motion of an imaginary magnetic axis, but local secular variations in the regions of the magnetic poles.

v a r i a t i o n s ? M a n y s c i e n t i s t s , s i n c e t h e t i m e o f G i l ­

At one time scientists were inclined to think that magnetism is the property of all large bodies in rotation. During the past 15 or so years other possibilities have been studied to account for the internal magnetic field of the earth by taking into consideration peculiarities of the internal struc­

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ture of the planet, which to a large extent are rather obscure.

It is presumed that the inner solid core of the earth is in a special metallic state and for this reason possesses a high electrical conductivity de­ spite the great temperature in the interior. The core rotates at a speed somewhat different from that of its fluid shell. The result is that eddy-type motions are set up within it. Apparently, the core of the earth acquired a certain initial magneti­ zation. Whether this is due to rotation or to the action of some exterior magnetic field is not clear. Because of eddy motions of the conducting masses of the shell of the core relative to the magnetic field of the core, electric currents are set up (like those in the windings of a dynamo), whose mag­ netic field builds up the initial magnetic field of the core.

This process of self-excitation continues until an equilibrium is established in which the energy lost due to dissipation of heat generated by the currents is balanced by the mechanical energy expended on generating eddies in the core’s shell.

An interesting thing is that space vehicles can play an important part in verifying such “innerterrestrial” laws as the theories of magnetism. The point is that artificial satellites and space probes have made geophysical experiments possible for the first time, and without experiments and com­ parisons no science can exist today.

Here is the way scientists reasoned. The earth has an inner core and a magnetic field. A variety of features suggest that Mars and Venus should also have inner cores, while the moon should be lacking (or almost lacking) one. The problem

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therefore was to find out whether they had mag­ netic fields.

The Soviet space probe Luna 2 was fired moonwards, and careful measurements by extremely sensitive apparatus at distances up to 50 km from the lunar surface indicated that the moon has no magnetic field, at least it was not perceptible.

So far attempts to measure the magnetic fields of Mars and Venus have been less successful. But sooner or later this will definitely be accom­ plished and then scientists will be in a better po­ sition to judge about our own magnetic field.

At the end of the eighteenth and the beginning of the nineteenth century, interest in studies of terrestrial magnetism was very great. On the ini­ tiative of Alexander Humboldt, the great Ger­ man naturalist, a network of magnetic observato­ ries was set up in Europe and, later, on the other continents. Here systematic observations of mag­ netic-field variations were conducted. Between 1836 and 1846, the whole network co-ordinated their scientific observations on the basis of a uni­ fied programme. This was the first case of inter­ national co-operation of scientists studying the earth and it went down in history under the name of the Magnetisches Verein. Russian observatories at St. Petersburg, Ekaterinburg (now Sverd­ lovsk), Barnaul and Nerchinsk also participated.

The work was arduous, for photographic re­ cordings were not yet known and the scientific workers had to spend hours on a stretch at their instruments taking down readings every 5 min­

utes.

This many-year effort was a great success; it was found that variations of the earth’s magnetic

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Lower curve (2) indicates variation of solar activity. The earth’s magnetic field (1) follows closely in step. True, in some cycles maximum magnetic activity can set in some­ what later than maximum solar activity

field are somehow connected with the variation in number and dimensions of sunspots. The Austrian scientist J. Lamont noted this relationship in the way the amplitude of diurnal variations of mag­ netic declination fluctuates with the solar cycle. The British magnetologist E. Sabine found the same relation from solar activity coinciding with the frequency and intensity of magnetic storms. Another English researcher, Brown, detected the tendency of magnetic storms to recur roughly every 27 days, which is approximately the time it takes the sun to complete one rotation on its axis if we make allowance for the earth’s mo­ tion about the sun. The discovery of a connec­ tion between solar phenomena and magnetic events stimulated the search for relationships between solar and other geophysical phenomena. That is how the great problem of sun-earth re­ lations, which today concerns many thousands of scientists, was born.

Today, all magnetic observatories of the world conduct round-the-clock photographic registra­ tion of variations of the magnetic field. Special instruments—magnetographs—also record the

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