The Earth Magnetism and its Changes

- SPECIAL PUBLICATION NO. X:Y

THE EARTH'S MAGNETISM AND ITS CHANGES

BY

PROF. s. CHAPMAN, F.R.S.

INDIAN AssociATION FOR THE CutnvA noN oF. SciENCE CALCUTTA

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PROCEEDINGS

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THE EARTH'S MAGNETISM AND ITS CHANGES*

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PROF. S. CHAPMAN

INTHODUCTION

1\lay I first of all express my appreciation of tlle honour done me by your Society in inviting me to deliver the Ripon Lectures

fut

1949· It is a great pieasure to me to visit India as a

scientifl~-~st

^of

y~ur

government, to take part in the Indian Science Congress and other gatherings .of Indian scientists, and to meet my fellow scientists in your Universities and research institutions, many of them already old friends by personal meetings abroad, or familiar by name through their distinguished contributions to kno·wledge.

Among the subjects on which I have some competence to address you, it seems to me that the one I have chosen is especially appropriate for considera- tion by a general and scientific audience in In:iia, and not least in this great city and port of Calcutta. The history of the growth of our knowledge of the earth's magnetism has many links with the history and scieutific endeavours of India ; and the fact of the directive influence of this magnetism on the compass bas been and is one of great importance in oceanic navigation, which has so profoundly influenced India's history in recent centuries, and on which the growth and prosperity of Calcutta so largely depend. Moreover, in recent years the work of the eminent physicists of this city has greatly enhanced our knowledge of solar and upper atmospheric phenomena that are closely associated with the changes in the earth's magnetism.

EARLY HISTORY OF MAGNE'riC SCIENCE

The attractive property of the lodestone or natural magnet was known to the ancient Greeks ; they were aware that the lodestone not only itself attracted iron, but could also impart its own attractive power to iron, by stroking it, so that it too became magnetic. For well over a millenittnl this was the only magnetic knowledge of which we have reliable evidence. But in the twelfth century of the Christian era accounts were given, by Guy de Provence in France, and by Alexander Neckam iu England, of the use of the magnetic compass by mariners; this ::;bowed that, perhaps many decades before, the directi'Vc property of the magnet had been discovered-that a magnetized rod or needle, if free to turn, will set itself along a particular direction, lyi.og (in

mo~t places) approximately north and south-and that this property had been seized' upon and practically applied for the benefit of seamen, to indicate to

*

Text nf Ripon Professorship I,ecture delivered in the Tnrlian Association for the Cultivation of Science, Calcutta, on 14. r. 49 and 15. 1. 4~·

ii S. Chapmart

them the direction upon the trackless ocean, under clouded skies, when the guidance of the heavenly bodies was denied to them. For a long time it was believed that the compass needle itself was controlled by the poles of the heavens, about which the celestial spht:re appears daily to revolve.

The thirteenth century suppiied another great landmark in magnetic history-a famous letter by a Frenci1 soldier-scientist, Petrus Peregrinus (Peter the Pilgrim), who in 1267 wrote, for the instruction of a neighbour, an account of his magnetic experiments. In some of these he used lodestones cut to the spherical form. He showed that every magnet has two poles, of which one seeks the north, and the other the south; and that whereas unlike poles attract, like poles repel each other- the first clear statement of magnetic repulsion, a property as potent as magnetic attraction, though, so far as our records indicate, it had escaped the notice of the Greek natural philosophers.

For many centuries it was thought that the magnetic compass pointed truly to the geographical north (or south), but experience and the growth of skill in measurement gradually led to the recognition that this is not so-in general, the compass deviates or declines from the geographical meridian. The angle between the northward meridian and the line from the south-seeking to the north-seeking pole of the needle is called the magnetic declination (or, by mariners, the magnetic 'Variation). Our earliest records of this discovery are not written records; they are marks made upon portable sundials used (in the fifteenth century and later) as time-keepers for travellers; these sundials, made notably at Nuremberg in Germany, were provided with a small mag- netic compass to enable the dial to be set in the meridian plane; and when it became known to the dial-makers that the compass needle did not itself lie along the meridian, they indicated by a mark the direction along which the needle should lie, when the gnomon was correctly oriented-thus giving the magnetic declination at Nuremberg at the time. Gradually also it was found that the magnetic declination is not everywhere the same, so that the mariner or land traveller, in order to infer the true north from the compass direct:on, must know the magnetic declination of the locality in which he is. · Map- makers began to mark the compass declination upon their charts, supplying another unwritten early record of know ledge of the magnetic direct ion.

Enlightened mariners realised the need to measure tne magnetic declina- tion along the main sea routes and at the chief ports, by making astronomical observations (when the state of the sky permitted) to determine the true north, and comparing it with their compasses. Prominent among such pioneers was the Portugue,;e navigator Joao de Castro, Viceroy of the Portuguese settle- ments in India; on a voyage from Portugal to India in 1538-1541, round the Cape of Good Hope and up the east coast of Africa onwards to India, he made a series of 43 measures of declination, constituting what may be regarded as the first major contribution to the magnetic survey of the globe. His results were recorded in the log of his voyage, which remained enshrined in the Portuguese naval archives for centuries. lVIodern researchers have delved

Earth's Magnetism and its Changes iir

into these archives and into those .of the English, Spanish and Dutch Admiral- ties, to unearth their many treasures of early magnetic data, till then un- published and unremembered. These data proved, from intercomparison of measures made by different mariners at about the same time and place, to be accurate to about one degree : the early observers had the advantage, which was lost with the advent of steel ships, that their wooden vessels did not disturb the natural magnetic field of the earth and the direction of the compass.

THE GROWTH OF MAGNETIC KNOWLEDGE TN RECENT CENTURIES

The next great step in magnetic science was the discovery in 1576 of the magnetic dip, by Robert Nonnad of London, a maker of ships' instruments.

He found that a needle, perfectly balanced horizontally on its pivot before magnetization, did not remain horizontal when magnetized. On giving the needle freedom to turn in the vertical plane through the magnetic north (the compass direction), by r<:sting it on a horizontal axle at right angles to this plane, he found that the north end of the needle pointed downward at an angle (called the magnetic dip) of 7I⁰ to the horizontal. From this obser- vation he drew the conclusion that the control of the direction of the compass is exerted by a "point respective" within the earth, and not by the pole of the heaven.

In r6oo A.D. William Gilbert of Colchester, physician to Queen Elizabeth, published his book de Magnets, one of the first great modern experiment treatises on physical science. It contained a wealth of new discoveries concerning electrostatics and magnetism, and an extensive critical survey of all available earlier literature on magnetism. But its outstanding feature is summed up in the pregnant sentence "Magnus magnes iPse est globus terrestris "-the terrestrial globe is itself a great magnet. Gilbert based this conclusion on experiments with tiny pivoted magnetic needles placed at various points on a spherical lode-stone-a model magnetic earth. He showed that they dipped increasingly towards the sphere, when moved from the ' magnetic equator ' (the circle midway between the two poles of the sp_here), where they rested horizontally (parallel to the magnetic axis joining the two poles), to the poles where they stood upright on the sphere. He showed also that the controlling influence on the needles came from the whole body of the sphere, and not merely from any one point in it. His inferences were based on Norman's discovery, and extended and corrected N onnan 's own conclusion.

Not long afterwards, in 1635, William Gellibrand, Gresham Professor of Astronomy in London, announced that ·the compass direction is not constant at London, whereas it had always hitherto been supposed to remain the same at any one place. The compass direction, and later the dip and intensity of the earth's magnetic force, were thus found to undergo a slow

iv .S, Chapman

l

'secular' variation, now observed at London for nearly four centuries, and over a shorter time elsewhere. As Fig. I shows, the compass direction at

FIG I

The direction of the magnetic force at London since A. D. rsSJ

London moved we~twards from I I ·' East in xs8o to 24 o West in r8oo, since when it has been moving steadily eastward ; the dip increased to a maximum of over 74 o in I68o, and has since decreased to what has been in recent years the nearly constant value 67o. The diagram suggests that the magnetic direction may return after the lapse of about five centuries to its value in 1576, but only time. can confirm this conjecture. The secular course of the magnetic direction elsewhere does not accord with the snpposition of a cycle of five centuries or so iu the variation of the magnetic direction, nor is there any warrant for the hypothesis that the magnetic axis of the earth has any regular rotation round the geographical axis. The secular variation of the earth's magnetic field is not predictable from our present knowledge, and to keep up-to-date in our information as to the distribution of direction and intensity of the magnetic field over the earth, it is necessary to maintain magnetic observatories, and to make magnetic· surveys of the globe every fifteen or twenty years.

In the seventeenth century observations of the magnetic deClination gradually increased in number, and covered an ever greater area, as also, more slowly, did those of the magnetic dip. The first ocean voyage for the specific scientific purpose of measuring the declinalio~ was made by Edmond Halley in the north and south Atlantic shortly before 1700, in which year he published his results in the form of the first magnetic chart. This showed the isogonic lines along each of which the declination has the same value (indicated beside the line) ; from these lines, by interpolation, the magnetic declination at any point can be found. Halley's chart w<.1s an instant success, and two or three years later he published a more extensive chart showing also the isogonic iines over the Indian and (in part) Pa\.:dic oceans, using observations collected from eastern voyages ; part of this map, including India, is shown in Fig. 2. For comparison a recent world isogonic chart is shown in Fis-. 3·

Earth's Magnetism and its Changes

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The magnetic declination around fndia in ^IiOO A. D. (Halley)

FIG. 3 Isogonic lines for 19Z2

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Vi S. Chapman

Though such charts at"e in the foi·m most serviceable to mariners and others for the purpose of reading off the declination at any place, they give no direct visual representation of the distribution of compass direction over the earth. This is best seen from the magnetic meridian lines, as 5hown in Fig. 4, which at each point have the direction of the compass there. The

FIG. 4

Lines of horizontal magnetic force, and lines of equal inclination, for the Northern hemisphere, epoch 1830 (After Airy [G 9])

meridians diverge from the south pole . of magnetic dip, where the north- seeking end of a freely poised magnet points vertically upward, and converge to the north magnetic dip pole, where the dip (downward) is go^0• The diagram shows also the isoclinic lines, or lines of equal dip. · The line of no dip is called the magnetic equator. The two dip poles are not antipodal;

the line joining them misses the earth's centre by several hundred miles;

owing to slight regional irregularities of the earth's field in the polar regions, they differ somewhat from the magnetic axis poles; these are the ends of the axis parallel to the direction of uniform magnetization agreeing most closely with the earth's actual field. This magnetic axis is inclined at I I⁰

to the geographical axis.

In the eighteenth century Coulomb proved that the attractions and repulsions between magnetic poles vary as the inverse square of the distance between them, and later Poisson developed the theory of the field of

Earth's Magnetism and ils Changes Vii

magnetised bodies in general. For a uniformly magnetised sphere this theory gave the simple formula tan I= 2 cot 8 for the d1p I at a place at angular distance

e

from the magnetic pole ; it also indicated that the magnetic intensity increases twofold in going from the magnetic equator to the magnetic poles on such a sphere.

T H H B R G I N N I N G 0 F ?\'[ A G N H T I C 0 B S H R V A '1' 0 R I R S

Gauss, in 1832, first showed how to measure the magnetic intensity F at any point on the earth, and after that time it became possible to draw also isodynamic lines, or lines of equal F ; the twofold increase of F from magnetic equator to magnetic pole, indicated by Poisson's theory for a uniformiy magnetised sphere, was approximately verified for our terrestrial globe. Gauss also applied a mathematical (spherical harmonic) analysis to the availabie data for the earth's field, and so was able to estabiish rigorously the c-onclusion reached by Gilbert over 2uo years before, that the surface magnetic field of the earth originates from within: later analyses have gradually reduced the estimated fraction that may possibly be of external origin, to less than one per cent; they also indicate that, contrary to what was thought for a time, there is no reliable evidence for the presence of any part of tbe fie:d attributable to eiectric currents crossing the earth's surface, apart from the very minnte currents associated with atmospheric electricity.

Gauss was also a pioneer in the establishment and equipment of magnetic observatories, of which those at Giittingen and Gre.::nwich were among the earliest. Some years later magnetic observatories were established at Bombay and Trivandrum in India, and at Batavia in Java-observatories which have earned a most honourable place in magnetic history for their important observations, and for the reductions and interpretations of their data. The great two-volume discussion by Moos of 40 years' Bombay magnetic data is one of the classics of terrestrial magnetism.

There are now about roo magnetic observatories distributed over the globe, the northern hemisphere having the preponderent share of them.

They continuously record the variations of the magnetic elements-usually the declination, and the horizontal and vertical components of the intensity.

Besides providing our most accurate information as to the secular mugnetic variation, they are indispensable for the reduction of magnetic survey measurements to a common standard and epoch.

The magnetic observatories provide a mass of data rather embarrassing in its volume and detail. In the course of a century many workers have gradualiy devised suitable means of study of these data. The experience thus.

gained enables the much younger science of ionospherics-the study of the ionised regions of the upper atmosphere of the earth by means of radio techniques-to progress more rapidly in the geophysical interpretation of its data (of even greater richness and volume), than would otherwise have

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S. Chapmari

bl!en possible. At many observatories the main types of magnetic change have been elucidated ; besides the secular variation there are several distinct types of transient variation. Chief among those is the daily variation periodic in a solar day ; and there is also a swaller daily variation, generally of about one-fifteenth the intensity, periodic in a lunar day. These regular variations change with the season and wax and wane in amplitude, approxi- mately in parallel with the II-year cycle of spots on the sun ; they also undergo moderate: changes fmm day to day. One exceptionally important early investigation of the lunar daily magnetic variation was that made by Broun from the declination records at Trivandrum. In addition to these regu1ar periodic changes, there are frequent irregular disturbances of the earth's magnetic field ; when speciaily intense these are called magnetic storms. As in the case of the regular daily variations, these have been studied in great detail at a number of magnetic ob3ervatories, for example at Bombay.

After investigating the regular magnetic variations, and the nature of magnetic disturbances, as they affect the records of several individual magnetic observatories, the next step is to synthesize the results to obtain a world view of these phenomena. The study of the geographical distribution of these variations may be termed the morPhology of the solar and lunar daily magnetic variations and magnetic disturbance. It is a complex subject because it deals with the changing distribution, over a sphere, of a "vector field ", that is of a magnetic jorce that has both intensity and dir~ction.

Despite the difficult nature of the task, much progress has been made in obtaining comprehensive graphical representations which summarize and synthesize an immense mass of detailed data.

THE 1\IAIN GF,OMAGNETlC FIELD

'!'he first approximation to the earth's magnetic field is the field of a uniformly magnetized sphere, concentric with the earth, and of equal or smaller radius, uniformly magnetized along a direction inclined at ^{I l0} to the geographical axis; the e;rth 's diameter along this direction is c.alled the geomagnetic axis, and its ends, which are at 78°.5 N, 6g⁰W, and 78°.5 S,

III⁰E, are called the geomagnetic; axis poles. The strength of this, the main part of the earth's magnetic field, is 0.31 at the geomagnetic equator (the circle midway between the axis poles), aud o.62 at the axis poles (measured in the m<~.gnetic unit called the gauss, and denoted by r).

The actual field of the earth differs, however, from this s;mple regular field, so that the dip Poles, \Yhere the field is vertical, do not coincide \\"ith the axis poles; their positions in 1945 were at 71°1\', g6°W and 73°3, 156°E, not antipodal. Likewjse, the magnetic equato1·, where the dip I is zero, differs from the geomagnetic equator, and is not a great circle. The hori- zontal intensity is not constant along the magJ)etic equator, being notably greater (o.4r) at roo⁰W than at about 3o⁰E, where it is about

o.2sr .

The

Earth's Magnetism and its Changes ix

magnetic equator nearly touches the southern tip of India, and the dip increases from approximately zero there to about 30° in the north of India;

a rough average value of the total intensity over India is 0-4 gauss.

These departures of the earth's field from that of a uniformly (obliquely) magnetized sphere imply the existence of considerable " regional anomalies,"

as can be seen, for the horizonlai component of the field, iu F·ig. 5. These include a field which, over most of Asia, converges towards a centre at about 40°N, I Io⁰E; another large regional anomaly extends over the south Atlantic ocean, and there are yet others in the South Indian and southeast Pacific oceans.

Besides these great and widespread departures from a simple sphericai- magnet field, there are very numerous anomalies of a more local nature ; the greatest of these is near Kursk in Russia. It is over ¹⁵⁰ miles long, but, where most intense, quite narrow-little over a mile wide; the verticai intensity is everywhere above normal, and ranges up to I .g gauss. It is due to a magnetic ore containing up to 40 per cent of iron, extending nearly up to the earth's surface. Another great local anomaly is at Kiruna in northern Sweden, due to magnetic ore which is of great commercial vaiue; there the vertical intensity rises to 3.6 gauss.

Many of these local anomalies are mysterious in that the magnetization of the rocks which produce the anomaly is opposite to that which the earth's present magnetic field would tend to induce. It is known that many subs- tances, such as lava and pottery, become magnetized by the earth's field

FIG. 5

I,ines of equal horizontal intensity, 1922

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Chapman

during the process of cooling, and some of these substances seem able to retain thereafter, over long periods, the magnetism thus acquired, despite subsequent changes in the field. In South Africa and clsewh~re there are systems of volcanic dykes of early geologica: age (e.g., paleozoic) which radiate from a centre of volcanic activity, and may extend outwards 100 miles or more; they consist of thin sheds of igneous rock, often no mote than Ioo yards wide, that has welled upwards through cracks made in existing strata ; and their vertical magnetization has the " \vrong " sense. One explanation of this is that 200 million years ago, when they were formed, the earth's magnetization was in a sense opposite to the present sense ; but this hypothesis cannot easily be accepted, though no alternative one has as yet been established.

Observation of local magnetic anomalies affords one of the cheapest and simplest ways

of

searching for hidden deposits that are commercially valuable -such as oil, gold, iron and so on ; but naturally there are some kinds of deposit that cannot be detected in this way.

The earth's magnetic field is now kno\vn with fair accuracy and in much detail over most of the earth's surface. This knowledge can be used to infer very approximately the distribution of the field above the surface outside the earth, at any height ; the distribution of the main "smoothed " :field over any external concentrir sphere is approximately the same as over the surface, except that the intensity is reduced proportionately to the inverse cube of the distance from the earth's centre ; the anomalous part of the field decreases upwards much more rapidly, so that its relative importance becomes st~adily less ; thus the further we go outwards from the earth's surface, the more nearly does the field agree with that of a uniformly magne- tized sphere. The main fact of the inverse-cube law of decrease has been confirmed up to heights of the order roo miles by radio and rocket measure- ments of the magnetic field in the ionosphere, in the outer layer of the earth's atmosphere.

THE 0 R I G I N 0 F THE E A R T H' S l\I A G N E 'l' I SM.

The distribution of the magnetic field within the earth is much less certainly known. The question is closely bound up with the long-standing mystery of the o1igin or cause of the earth's magnetism. Very many answers to this question have been proposed, but the matter remains in doubt. It has generally been considered that the approximate alignment of the earth's magnetic axis with the geographical axis is not merely a coincidence, and that the earth is a magnet because of its rotation. Numerous ways in which the rotation could produce the magnetism have been suggested, but most of them have proved on examination to be quite inadequate to explain the existing strength of the field.

New interest was added to the question in 1913 \Vhen the great American astronomer Hale, who haq discovered strong magnetic fields in sunspots,

Earth's Magnettsm and its Changes xi

announced the detection of a general magnetic field of the sun. He gave the polar intensity as so gauss, and stated that tht: direction of the field bore the same relation to the rotation of the sun as obtains between the magnetism and the rotation of the earth.

l\lnch more recently, H. W. Babcock has discovered strong magnetic fields in certain stars.

Consideration of the relation between the fields and the rotation and angular momentum of the earth and sun, and later of Babcock's first-measur~d

magnetic star, led Blackett to propose that all three \.'ases were manifes- tation of a hitherto unrecognized fundamental property of rotating matter, perceptible o-nly in bodies of great size and mass ; he found that the ratio of the magnetic moment to the angular momentum could be expressed in simple terms involving the constant of gravitation and the speed of light ; the ratio was somewhat uncertain, however, both for the sun and magnetic star.

Bullard pointed out that if lllackett's hypothesis were true, so that matter ordinarily considered non-magnetic did in fact contribute to the earth's field, the variation of this field with depth, in the (approximately non- magnetic) outer crust of the earth, should be different from the inverse-cube reiation hitherto assumed to exist there. This remark led Blackett to organize a series of researches on the depth-variation of the geomagnetic field, by taking measurements in aeep mines at points where t~aturallocalmagnetic

disturbances, and distnrbance due to iron or steel mine-structures, were absent or small. The consequences of Blackett's hypothesis did not differ from those of the classical theory in the case of the vertical magnetic force (near the earth's surface), but they did differ as regards the horizontal force;

they indicated a downward decrease lfor some distance) instead of the hitherto assumed increase.

At first the mines magnetic measurements gave some support to Blackett'lS hypothesis, but continned research on these lines has not confirmed the earlier results.

Moreov.::r, the astronomical support for the hypothesis has also practically disappeared. For many years after Hale's announcement of the general solar magnetic field, a cloud of doubt and uncertainty hung over the matter ; after the second world war a young German astronomer, Thiessen, reported new measurements of the sun's field, agreeing substantialiy with those of Hale;

but a year o;· two later he made new measures which disclosed no such :field, and most recently (since these lectures were given in Calcutta) he finds that the field, if present at all has a polar intensity of the order of one gauss, and has the sense opposite to that originally announced by Hale and later con-

firmed by himself.

The only inference to be drawn from these various announcements is that the observers are seeking to measure _..c;omething too small for their

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S. Chapman

methods to determine with any

cert~inty,

or else that their measures are valid and that lhe sun is a magnetically variable star.

The latter possibility is made more credible by a further discovery, by Babcock, of a strongly magnetic star which changes the polarity of its magne- tization every nine days. It appears most unlikely that this can be associated with a concurrent change in the sense of rotation, and the observation there- fore strikingly contradicts Blackett's hypothesis.

Bullard has lately developed a promising theory, entirely on dassical lines, to account for some of the outstanding features of the secular variation of the earth's magnetic field, by inductive effect of large-scale eddies in the sup[Josed convective motions within the earth's liquid core; in the presence of the main geomagnetic field this wotion will induce electromotive forces that will impel electric currents in that part of the core; the magnetic field of these currents, as the convectiw motions and electric currents vary, will he observed at the earth's surface as a secular magnetic variation.

Still mort! recently Bullard has extended his analysis with the aim of explaining the general magnetic field itself, as maintained by currents induced by dynamo action, in conjunction with the field, by large scale convection in the core. These researches seem to have much promise; an interesting consequence associated with them is the possibility that deep inside the earth the lines of magnetic force do not lie in meridian planes through tht: magnetic axis (as they do outside the earth): his theory st1ggests that the deep-lying magnetic field has a considerable component that is normal to these meridian planes.

'!' H H R F. G U L A R :'II A G N E '1' I C V A R I A T I 0 N S

The secular magnetic varia~ion, though extremely rapid compared with most long-term changes proceeding from the depths of the earth. is slow according to our every-day judgments about rates of change. The earth's magnetic field undergoes also much more rapid variations, partly regular, partly irregular. These are of several different kinds, and though they have been much studied for at least a century and a h·alf, there is almost certainly much more to discover about them.

The principal regular magnetic variations have a daily period, or rather two daily periods, one relating to the sun and the other jointly to the sun and moon. The solar daily magnetic variation is often, for brevity, referred to by the symbol S, to which the suffix q is often adde_d to indicate that the solar daily variation considered is that which is seen in its purest form on magneti- cally quiet days. The lunar (or more correctly luni-solar) daily magnetic variation is similariy dcnotc:d by the symbol L.

These variations Sg and L are not absolutely regular-they certainly change somewhat from day to day, besides undergoing a fairly regular (and substantial) seasonal change. They have mostly been studied in the form of

•

Earth's Magnetism and its Changes

xiii

averages from many days in the same calendar month or season, sometimes combined from several years' data.

Each magnetic dement is affected by these daiiy changes, in distinctive ways which depend on the latitude of the station concerned, and to a iess extent (in general) on the longitude-because to a first approximation, in middle and low latitudes, the same variations occur all round auy circle of latitude, at corresponding local times-indicating that the variations are determined by the situation of the station on the eat th as seen from the sun (or moon).

Mathematical analysis of these I ransient variations, both regular and irregular, as distributed over the earth, shows that their main cause is abo·ve the earth's surface_:_unlike the main field and its secular variation, whose origin is internaL There is, however, a part of each transient variation which is of internal origin, and the reason for this is simple. The main part of the

~arying magnetic field associated with each type of transient variation, produced above the surface, and observed at the surface, penetrates qJso below the surface, and its changes induce electric currents there, wherever the earth and its ocfans have suffieient electrical conductivity. The external and internal parts of the field can be separately determined from the obser- vations, and from the relation between them we can get information, obtain- able in no other way, about the electrical conducting properties of the.earth, deep down, far beyond the levels at which, in mines, these properties could be measured in situ. The salty oceans conduct electricity very well, and moist land conducts fairly well; dry earth and rocks are bad electrical conductors, but it is inferred from the transient magnetic variations that beiow ²⁰⁰ miles' depth the earth's electrical conductivity is considerably greater, and increases downwards.

There is reasonable certainty that the daily magnetic variations Sq and L are due to systems of electric currents flowing in the ionosphere-the layers of the earth's atmosphere, extending upwards from about so miles height, which assist world-wide radio communication by deflecting radio waves, so that they travel round the earth's curved surface, instead of losing themselves·

in outer space. The exact levels of these ionospheric Sq and L systems of electric current are not yet certainly known, but seem likely to be between so and 100 miles height.

Both these current systems are strongest over the sunht hemisphere, as is natural because there the ionization of the ionosphere is likewise strongest.

This ionization is due to absorption of solar radiation, and decays throughout the night, during which this radiation is cut off.

At the equinoxes the main features of these current systems are two circulations of currents (for Sq and also for L) in the northern and southern parts of the sunlit hemisphere; about 6o,ooo amperes flow in each of these circulations in the case of Sq, and about s,ooo amperes in the c~se of L. In northern summer the circulation in the northem hemisphere is increased, and

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S. Chapman

it also extends somewhat *ross the equator, invading the domain of the reduced southern circulati'on; in the southern summer these relations are reversed. The systems of current are stronger in sunspot maximum years than at the minimum of the eleven-year sunspot c·ycle. These changes arc parallel to those of the distribution of ionization and electric conductivity in the ionosphere.

These electric current systems are produced by dynamo action of the atmosphere moving in the presence of the earth's main magnetic field, in the same way as electric current is generated by the motion of the armature in an electric dynamo.

Such motions are produced by the sun, partly by its thermal and partly· by tidal action on the atmosphere. The moon's action is effectively only tidal.

These daily motions of the atmosphere due to the sun and moon ha\·e long been studied at ground level, and in many ways they must have the :;amc general character in the ionosphere, where, howe\'er, they are much advanced in amplitude and speed; this is clearly shown by radio observations of tlw ionized layers, which, for instance, have re\·ealed that the E layer undergoes a lunar tidal rise and fall, twice in each lunar day, with a range exceeding one mile.

The study of the daily magnetic variations has told us much, and has still much more to tell, about the variations of that part of the solar radiation that is intercepted high in the atmosphere, and can only be observed directly from very high flying rockets : and also about the motions and strncture of the

upper atmosphere.

GEOl\IAG!'\HTIC DISTURBANCE

Oftt:n the continuous records of the changes of the magnetic elements, while showing the presence of the regular solar daily variation, also show superimposed irregular magnetic changes. These are cailed magnetic disturbance or activity, or, whe11 specially intense, magnetic storms.

Magnetic storms are world-wide phenomena, and have many remarkable properties and associations, pahicularly \\·ith the sun. These are shown most clearly in the case of great magnetic storms, which always begin suddenly, and simultaneously all over the eat th, to within less than a minute.

Many, probably all, great magn~tic ^ston;1s ~re preceded by local erup- tions of briiliant light on the sun, to which are given the name solar flares;

usually they occur in the vicinity of sunspots.

The observation of a bright solar flare is generally accompanied by a mdio fade-out-a failure of communication on many radio transmissions, usually beginning suddenly at the commencement of the flare, and pl:rsisting throughout the life of the flare-generally from 20 minutes to an hour or so.

In the same interval, the Sq daily magnetic variation is temporarily enhanced over sunlit hemisphere of the earth. These two phenomena are both attribut- ed to the formation of extra ionization at an ionospheric level lower than

Earth's Magnetism and its Changes XV

us\lal, namely at about so miles' height-it is caused by the absorption of extra ultraviolet radiation from the flare, of a more penetrating kind than that which produces the ordinary ionised layers. _ 'fhese 'immediate' effects of a solar flare occur in some· degree wherever on the sun's disc the flare is situated-even where it is quite near the edge of the disc. They are not exactly contemporaneous with the flares because the visible and ultraviolet radiations by. which the flare is seen, and by which it affects the ionosphere, travel with the speed of light, which takes about ten minutes for its passage from the sun to the earth.

If the flare is near the edge of the sun's disc, it may produce no further effect upon the earth. But often, when it is both intense and not too far from the centre of the sun's disc, a great magnetic storm commences about a day later. The interval may range from 16 to 36 hours or so, and is general- ly interprt!ted as indicating the time taken for solar gas, ejected during the flare, to reach the earth. The corresponding speed is of the order of 10oos mile a second. Solar observers have observed the ejection of solar gas from the sun at the edge of the disc, with, a speed sufficient to carry the gas right away froni the sun's gravitational attraction, and travel onwards into space, sometimes with a speed· comparable with that inferred for the gas supposed to produce the magnetic storm. But such ejection from the 'edge' of the disc does not produce a storm-for this to happen, the ejection bas to include the₁ direction towards the earth, though the angle of scatter from the flare may be

a wide one.

The weaker magnetic storms cannot in general be associated with solar flares, but they show a property-not shared by the greater storms-that links them rather definitely with the sun. This is a tendency to recur after the lapse of a polar rotation: often there are multiple-sometimes inter- mittent-recurrences extending over several solar rotations. This is inter- preted as indicating the continued or intermit~ent emission of solar gas, in a stream with a more limited angle than that associateJ with flares, over one or more solar rotations ; as the sun rotates the direction of the stream changes, like that of water from a hose when the nozzle is rotated, as in a garden sprinkler : the magnetic storm occurs if the stream 'hits' the earth.

The recurrence of the stmm is not a certainty, because there may be some interruptions or slight changes of direction of the emission, so that the earth is not 'bit' at every rotation.

Magnetic storms are not perceptible by our senses-they are made known to us chiefly by the magnetic records; but they affect human life, because they produce, or are accompanied by, disturbances of telegraphic and radio communication, and they have even been known to affect electric power lines.

They are also accompanied by unusually intense and widespread displays of the aurora Polaris or Polar lights (known in the northern hemisphere as aurora borealis or northern lights, and in the southern hemisphere as aurora australis or southern lights). These lights commonly occur only around the

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S. Chapman

magnetic polar regtons; but during magnetic storms thef ~xtend more widely, and have even been .seen in India, I e.g., from Bombaf) in one very great magnetic storm, in t872. They show many connections with the earth's magnetism, and it is reasonably certain that their distribution is gov.:rn,M (in essentially the same way as is that of the cosmic rays) by the deflecting action ~£ the earth· s magnetic field upon the electrical particles in the solar gas, . which must be thought of as more or less c?mpletely ionised. But there are still many features of the aurora that as yet we cann.ot adequately explain.

The same is true of magnetic storms ; but it is likely that a magnetic storm is produced partly in the solar gas in the region .around the earth, beyond our atmosphere : and partly by strong systems of electric currents Jlowing in our atmosphere, along the auroral zones and across the polar caps encircled by them. The current in the zones may be as great as two million amperes.

A great magnetic srorm may last for a period of from half a day to two or three days, but after the irregular disturbance has died down there remains an effect of the storm that decays only gradually, over a period that may be . reckoned in days or even weeks. It seems not unlikely that this after·

effect is due to a ring of gas round the earth, somewhat resembling Saturn's rings, at a distance of a few earth-radii from the earth's centre; this ring carries an electric current that diminishes the horizontal component of the earth's surface magnetic field, and that slowly decays. During the decay there may be loss of gas from the ring to the polar regions, maintaining auroral displays at a low level of intensity after the solar stream of glls has ceased to pur on towards the earth.

There is much need for further research and discovery in this field, but the brief account I have given will, I hope, indicate some of the interesting possibilities and the future opportunities for students of the subject. May India continue to add, by its magnetic and solar observatories, to our know- ledge of the important facts of these problems and to their interpretation.

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