SUN STORMS AND THE EARTH: THE AURORA POLARIS AND THE SPACE AROUND THE EARTH

Sigma Xi, The Scientific Research Society

SUN STORMS AND THE EARTH: THE AURORA POLARIS AND THE SPACE AROUND THE EARTHAuthor(s): SYDNEY CHAPMANSource: American Scientist, Vol. 49, No. 3 (SEPTEMBER 1961), pp. 282A, 249-284Published by: Sigma Xi, The Scientific Research SocietyStable URL: http://www.jstor.org/stable/27827851 .

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Courtesy of Dr. V. P. Hessler, University of Alaska Fig. 3. Multiple arc, College, Alaska. In many cases, several arcs stretch across

the sky in the geomagnetic east-west direction.



AMERICAN SCIENTIST AUTUMN SEPTEMBER 1961

SUN STORMS AND THE EARTH : THE AURORA POLARIS AND THE SPACE

AROUND THE EARTH* By SYDNEY CHAPMAN

TN RECENT years I have spent some months each winter in Alaska. - -

There, on many nights, at the State University near Fairbanks, we have opportunity to see the beautiful spectacle of the northern lights or aurora borealis. Many superlatives can justly be applied to it?grand, majestic, marvellous, mysterious, infinitely various and changing. It continues to arouse wonder, delight, and awe, however objectively studied to probe its nature and origin.

By and large it is seen only by a minority of mankind. Millions in

Asia, Africa, and tropical America never see it. Only a few times each

century does it come within their range. Then it goes unrecognized and unrecorded by almost all those who there chance to behold it.

It is less local, more complex, than such other beauties of the sky as

rainbows, dawn and sunset glows, the flashing lightning. Like them it is linked with the sun, but more mysteriously. They are phenomena of the lower atmosphere, the region of weather. The aurora is planetary. It is caused by hot ionized gas shot out from stormy regions on the sun. It is shaped on a grand scale in the extensive "radiation belts" that en

circle the earth. It has a great range of brightness, much variety of color and a multiplicity of forms. Sometimes it appears over a far larger part of the earth than usual. Then it betokens the occurrence of other worldwide phenomena, such as magnetic and ionospheric and cosmic

ray storms, of which our senses give us no direct inkling. Throughout the aeons during which man evolved, these complex events were entirely hidden from him, and had no impact whatever on his life, except such as

the aurora might produce. Finally in the nineteenth century, with the invention of telegraphy, they began to interfere with his affairs. In our

century they arouse still greater practical concern, through their influ * A Sigma Xi-RESA National Lecture, 1960-61. This research has been supported

variously by the Universities of Alaska, Colorado (High Altitude Observatory), and

Michigan (Institute of Science and Technology), The National Bureau of Stand

ards, the Air Force Geophysical Research Directorate, and the National Science Foundation.

249



250 AMERICAN SCIENTIST

Courtesy of Dr. V. P. Hessler, University of Alaska

Fig. 1. Detail of a looped homogeneous arc, College, Alaska.



sun storms and the earth 251

Courtesy of Dr. V. P. Hessler, University of Alaska

Fig. 2. Active rayed arc, College, Alaska. At an active stage, parts of an arc are

often folded or curled.




ence on radio propagation, and most recently because of possible as

sociated dangers to travelers in interplanetary space.

The Aurora in High Latitudes

Here is a description of the aurora borealis or northern lights as seen from places in high altitudes, as, for example, from the campus of the

University of Alaska (65? N). Inevitably my description falls far short of adequacy.

After darkness has fallen, a faint arc of light may sooner or later be seen low on the north horizon, or centered somewhat to the east of north.

Gradually it rises in the sky, and grows in brightness. The stars shine

through it with undiminished brilliance. It may be quite regular in

form; but sometimes it is looped (Fig. 1). As it mounts in the sky, its

ends, on the horizon, advance to the east and west. Its light is a trans

parent white when faint, and commonly pale yellow-green when bright? rather like the tender color of a young plant that germinates in the dark. The breadth of the arc is perhaps thrice that of a rainbow. The lower

edge is generally more definite than the upper. The motion upward toward the zenith may be so slow that the scene is one of repose. As the arc rises, another may appear beyond it, and follow its rise. At times

four, five, or even more arcs may thus appear. They rise together, and some of them may cross the zenith and pass onwards into the southern half of the sky. Those in front may even disappear below the southern

horizon; or they may retreat again to the north, or the whole sky may become clear of the aurora for a time.

During this phase the aurora is seen as a soft diffuse band of light, or more than one: grand, majestic in extent and gradual motion. Though it seems almost in repose, a closer look may discern small changes going on, changes of form, or faint waves of light progressing along the band or bands.

This may be all that appears on some nights. But on others the aurora enters after a while on a new and distinctly different phase, much more active and varied. The transition from the quiet to the active phase may be speedy, even sudden. The band becomes thinner, rays appear in it, it begins to fold and also to become corrugated in finer pleats. It becomes a rayed band (Fig. 2) of irregular changing form, like a great curtain of

drapery in the sky. Often it is not solitary but multiple (Fig. 3, Frontis

piece). Its color may remain yellow-green, but often a purplish-red border appears along the lower edge, perhaps intermittently. Sometimes red of another shade may show along its whole height. This, however, is

more often seen when the aurora appears in lower latitudes, outside its usual range, rather than in the polar regions. Vivid green or violet or blue colors sometimes appear. At times the rays seem to be darting down, like spears shot from above. Sometimes there seems to be an upward



SUN STORMS AND THE EARTH 253 motion along the rays, or motion to the east or west along the band. The curtains may sweep rapidly across the sky as if they were the sport of breezes in the high air; or they may vanish and reappear, in the same place or elsewhere. This grand display may continue for many minutes or even hours, incessantly changing in form, location, color and in tensity; or intermissions may occur, when the sky has little or no aurora. At times the observer may look up into a great auroral fold nearly

overhead, when the rays in its different parts will seem to converge, forming what is called a corona or crown. Often such a corona rapidly fluctuates in form, and its rays may flash and flare on all sides, or roll around the center.

At the end of an outstanding display the aurora may assume fan tastic forms, no longer in connected curtains and bands. There may be a widespread collection of small curtains, stretching over a large part of the sky, which brighten and fade, or, as it is said, pulsate. Finally the sky may be covered by soft billowy clouds, not unlike a mackerel

sky with rather large "scales"; but these "scales" and patches appear and disappear, with periods of not many seconds. At last the sky becomes

altogether clear, with no more aurora. But later the whole sequence may begin anew, and continue till dawn pales the soft auroral light.

Auroral Catalogs and Auroral Visibility

Approach to an understanding of the nature and origin of the aurora must be based on much quantitative knowledge about it. This is not

easy to obtain. However, the first fruitful researches on the aurora were rather simple. Scattered records of its appearance were collected?records of dates, times, places, and descriptions. Elias Loomis, professor of natural philosophy at Yale University, was one collector of such records. He knew that as one goes northward from New England the aurora is seen more and more often. But he found evidence that this northward increase of frequency of visibility does not continue to the pole. Beyond a certain latitude, which is different in different longitudes, the aurora is seen less often. His data enabled him in 1860 to provide the first general

map of what is called the auroral zone. It was rather crude, but it cor

rectly showed one major fact: the zone, which is distinctly oval, is not centered on the geographical pole. Its center, indeed, is about ten degrees away from this pole.

Fritz, an Austrian professor of physics, made a much larger collection of auroral records. His catalog, published in 1873, included observations

dating from before the Christian era. Using this extensive material, he drew lines of equal frequency of auroral visibility. These lines he called isochasms. The isochasm of maximum frequency lies in Loomis's broad auroral zone, and is itself often called the auroral zone. Within the polar cap it enclosed, Fritz drew another oval, along which the aurora is seen




Fig. 4. Isochasms (lines of equal frequency of auroral visibility) over North America.

equally often to the south and north. Inside this oval the aurora appears with less frequency, and most often to the south. Outside the maximum

isochasm it is seen usually to the north, and with a frequency that falls off rapidly with increasing distance from the zone. The two outermost

isochasms (Fig. 4) show where the aurora, on the average over many years, is seen only once a year, or once in ten years. The once-a-year line crosses

the North American continent from California to Florida. The once-a

decade line passes over Mexico and Cuba. During the International

Geophysical Year (IGY), 1)^ years long, three auroras were seen from

Mexico?thus at the rate of two per year. But the IGY was a period of



SUN STORMS AND THE EARTH 255

exceptional solar activity. Many years go by with no aurora in sight of Mexico.

Vestine improved Fritz's map of isochasms, using post-1872 data. He did not alter the outer lines, but he corrected those of higher fre quency, and added isochasms within the auroral zone.

Auroras in the Southern Hemisphere

The aurora appears also in the southern hemisphere. Captain Cook, who was among the first of those who recorded it, called it the aurora australis or southern lights. The complete phenomenon, northern and

southern, is comprised under the names aurora polaris or polar lights. But the aurora is sometimes seen from the tropics, when it might be called the aurora tropicalis. Owing to the sparsity of land and population in the higher southern latitudes, the aurora there is much less seen and recorded than in the northern hemisphere. The records are still too

scanty to enable a map of the southern isochasms to be based on them. Even the southern auroral zone is not yet perfectly known.

Auroras and the Earth1 s Magnetic Field

There is an intimate connection between the aurora and the magnetic field of the earth. This field roughly resembles that of a small magnet or magnetic dipole, at or near the center of the earth. The direction of the

magnet is inclined at 11? to the geographic axis, about which the earth rotates. The diameter along the direction of the dipole is called the mag netic axis of the earth (or geomagnetic axis), and its ends are called the

(geomagnetic) axis poles. If the earth's magnetism were of purely dipole character, the magnetic needle would dip vertically, at these poles, and the compass would lose all direction. The points where this happens are called magnetic or dip poles. On the earth they differ from the axis

poles, because regional irregularities modify the field. The northern axis

pole is in the northwest corner of Greenland, at 78?.5 N, 69? W. The northern dip pole is at 70?.8 N, 96? W, hence about 1150 kilometers from the axis pole. The center of the northern auroral zone is very close to this axis pole, but the shape of the zone is affected by the regional ir

regularities of the field. Hence the zone is oval; not circular. Another connection between the aurora and the geomagnetic field

is that the auroral rays lie along the direction of the magnetic force. In auroral latitudes these lines are nearly vertical, but diverge from it

by 10? to 15?. Hence the rayed bands and curtains are slightly inclined to the vertical, and the quiet or diffuse arcs have the same slope. Also the arcs, bands, and draperies tend to lie along the parallels of geomag netic latitude, namely, the circles centered on the axis poles.

The southern auroral zone is likewise centered approximately on the southern axis pole, but its shape differs notably from that of the northern




zone. This is because the regional irregularities of the magnetic field in the southern hemisphere differ from those in the northern. One token of this is the position of the southern magnetic dip pole, at 71?.2 S, 150?.8 E. It is nearly 1500 miles from the antipodes of the northern dip pole. It is now possible to draw the curve of the maximum isochasm from certain theoretical considerations, knowing one point on it. This has been done with success for the northern maximum isochasm. From our

knowledge of the earth's magnetic field it is also possible to infer the course of the magnetic lines of force that extend outward in space. Their ends on the earth's surface are called "conjugate" points. Vestine and

Sibley have shown that the points conjugate to those on the arctic maximum isochasm lie on a curve that agrees closely with the southern maximum isochasm inferred from observation (Fig. 5).

Fig. 5. The lines of maximum frequency of auroral appearance, or center lines of the auroral zones. Arctic (right), antarctic (left). Broken lines, observed; full

lines, calculated (E. H. Vestine and W. L. Sibley). Their true locations are not yet known with certainty.

Auroras and the Sunspot Cycle

Loomis and Fritz knew of the 11-year sunspot cycle, discovered by Schwabe in 1851. Sabine a year later had found that this cycle also ap pears in the transient variations of the earth's magnetism. This was a

great landmark in geophysics; weather and climate show no such close

relationship with the intrinsic changes on the sun. Much earlier, how

ever, an exchange of letters in 1741 between Graham in London and Celsius in Uppsala had revealed that there is a connection between the

appearance of auroras beyond their usual limits, and the occurrence of

magnetic disturbance. Thus a century ago it was natural to ask if the auroral records, like those of geomagnetism, show the influence of the

sunspot cycle. It was only after several decades that this was finally established. Even in the case of the magnetic changes, which are regularly




so

I 29456789 10 II

PHASE (YEARS)

Fig. 6. The frequency of auroras observed at Yerkes Observatory, University of Chicago, and of sunspot areas and magnetic activity, in relation to the sunspot cycle; averaged over four cycles, 1901-1944 (A. B. Meinel, B. J. Negaard and J. W.

Chamberlain).

recorded at numerous magnetic observatories, several years passed away before those who doubted Sabine's discovery became convinced. The auroral record is far less perfect. It is at the mercy of clouds; and

good statistical data depend on the assiduous work of observers who keep a regular watch for the aurora over many years. Such observers have

been rather few. One such auroral enthusiast was the great pioneer chemist John Dalton, the founder of the atomic theory. Another is Carl

W. Gartlein, of Cornell University. He was the leader of the visual au

roral observers in this country during the IGY, and is now in charge of




the section of the U.S.A. World IGY Data Center that stores and proc esses the visual data on the aurora.

In high latitudes the sunspots affect the vigor and type of the auroras seen there, rather than their frequency. But in lower latitudes the 11

year cycle is clearly manifest in the auroral statistics, as well as in those of magnetic disturbance (Fig. 6). The peak of auroral and magnetic activity lags by a year or two after the sunspot peak.

Fig. 7a. Base lines for simultaneous photography for determination of auroral heights}; C signifies Christiana, the former name of Oslo (C. Stornier).

The Height from Which the Aurora Shines

Returning to the story of early progress in our knowledge of the aurora, the next great step was to find its location. Many beauties of the sky are due to the reflection and refraction of sunlight. Some, like the rainbow, are partly subjective. The rainbow depends on the positions of the sun, the raindrops, and the observer. If we move when we see a rainbow in the spray of a waterfall or a garden sprinkler, we become conscious that it is our rainbow; it moves with us. The aurora is not like that. It is fully objective, as much so as the light from a lamp. It is a luminescence of




the air in a particular region. If seen from different places, its star back

ground is different. Noting this fact, Cavendish inferred the height of an aurora seen from two places in England in 1784. But our knowledge of the height and location of the aurora was first put on a firm basis by the

Norwegian mathematician and naturalist Stornier. From 1909 onwards he organized simultaneous photography of the aurora from Oslo and

elsewhere, with the aid of volunteer assistants with whom he had tele

phonic connection. Figure 7a shows the base-lines of his auroral survey. He continued this work almost to the end of his long life. The harvesting of the results involved much numerical computation. His efforts were

Fig. 7b. Heights of typical auroras, in the dark atmosphere and in the sunlit atmos

phere (C. Stornier).

well rewarded. He established beyond doubt some of the main facts con

cerning the location and form of these luminous regions of the air that we see as auroras. Figure 7b summarizes some of these facts: it shows that the shining air of auroras that lie in the shadow of the earth extends over heights mostly between 90 and 250 km above the earth. It also shows his surprising discovery that some auroras shine from the sunlit

air, though seen?to the west after sunset or to the east before sunrise? from places where darkness has fallen. These sunlit auroras may extend up to very great heights, of 800 km or even more. They may end on the shadow line, or extend down to the normal lower limits?sometimes with a break near the shadow line.

There is no reliable observation of an aurora appearing below 65 km. Thus auroras lie well above the weather region of our atmosphere. They are located in the ionosphere, where the density of the air is very small, at levels above 99.99% of the atmosphere.

In the absence of cloud, an observer at sea level can see all objects at 100 km height or more within a circle of diameter 2250 km. Thus any aurora is open to observation over a correspondingly great area. This area is probably still greater for auroras that come within sight of lower

latitudes, because they are likely to be higher in the atmosphere. An ob




Fig. 8a. All-sky auroral camera, developed and used at the Geophysical Insti tute, College, Alaska & other USA auroral stations (T. N. Davis and C. T.

Elvey).

server seeing an aurora in his northern sky cannot tell its geographical location, unless he knows its height. As an aurora can be seen by ob servers ranging over several degrees of latitude, it contributes to the statistics of visibility over this range. Fritz's map of isochasms was a

great contribution to auroral knowledge, but now we need a more objec tive map, one showing lines of equal frequency of location of auroras in different latitudes. Such lines may be called isoaurores. Such a map has not yet been prepared, but we are not likely to have to wait long for it. The IGY auroral program contributes to this objective, but its duration




ALL-SKY CAMERA PHOTOGRAPHS (DEC. 5-6,1958) COLLEGE, ALASKA

2359

Fig. 8b. A selection from the all-sky camera films of December 5/6, 1958. The selected films were chosen with different time spacing to illustrate some of the major auroral changes of that night.




was not enough to give average frequencies. The 1)4 years of the IGY were also not typical, because of the exceptional solar activity at that

time. The auroral watch should be maintained as fully as possible for at

least one sunspot cycle. The map of isoaurores will closely resemble

Fritz's map. The lines may indeed be identical. But they will be differ

ently numbered, because, except near the zones, the number of auroras

above a place is much less than the number seen from the place. Moreover

the isoauroral lines will not extend so far toward the equator as do the

isochasms; there will be no such line above Mexico and Cuba, because no

overhead aurora has ever been seen there.

Fig. 9. Synoptic map of the aurora of June 30, 1957, 19h before the IGY began (C. W. Gartlein).

The Geographical Location of Auroras

St?rmer's long-continued simultaneous photography of auroras from more than one station determined not only the height but also the plan position of the auroras. He found that they lie more nearly along the lines of geomagnetic latitude than along the geographic parallels of latitude. His photographs showed only a part of the auroral arcs visible in his sky. During the IGY the aurora was photographed more compre

hensively than ever before, by all-sky cameras. Gartlein was an early pioneer in their development. Improved all-sky cameras (Fig. 8a) were

devised for use during the IGY, when 90 were operated in the arctic, and 24 in the antarctic. They photographed the whole sky at regular




intervals, every minute at many stations, less often at a few. They give an auroral record that no visual observer could rival, especially when the

aurora extends over a large part of the sky, and is rapidly changing

(Fig. 8b). One question that they may or may not answer is whether the aurora

ever extends all round the auroral zone. The all-sky network did not

cover the whole arctic sky. In any case the cameras could operate simul

taneously all along the auroral zone only during mid-winter, the period of arctic darkness. But they have provided an immense mass of auroral

data that is now being actively studied. The Geophysical Institute of

the University of Alaska is charged with the care of the American World

IGY Data Center for all-sky camera auroral data.

The IGY opened very appropriately with a great magnetic storm, which was accompanied by an extensive auroral display. Dr. Gartlein's

observers and their colleagues in Canada, led by Dr. P. M. Millman of

Ottawa, were able to provide the data for Fig. 9. It is a map of that

night's auroral arc over this continent. It extended from coast to coast.

This is a fine example of what visual observers can contribute. The au

roral program was one of the few IGY fields in which the general public could and did greatly help. Seamen and airmen also took a part, espe

cially useful for auroras over the sea or above the clouds. For financial

reasons the all-sky camera network was almost completely confined to

the higher latitudes. Over the main belt of the earth reliance for auroral

data was placed solely on visual observers, except for one all-sky camera

in Japan. It recorded the three auroras that came within its range during the IGY.

The Nature of the Auroral Light

The colors of the auroras that appear in high latitudes have already

been described. When auroras are seen in low latitudes they are com

monly diffuse and very red. Often, in the past, firemen have set forth to

subdue a supposed fire to the north, mistaking the auroral glow for the

light from a conflagration. The spectroscope was applied to the study of the auroral light many

decades ago. One of the most striking features of the spectrum (Fig. 10a)

is usually a narrow yellow-green line (5577 ?). It was long a puzzle to

spectroscopists. Finally it was identified as coming from neutral oxygen

atoms, by a forbidden transition between metastable states of long dura

tion. In recent years, through the use of more and more powerful spec

troscopes, the auroral spectrum has become known in great detail. All

the main lines and bands have been identified. Oxygen atoms and ni

trogen molecules are the particles that emit most of the light; they are

partly neutral, partly ionized. Thus the light comes mainly from the

gases that are the principal constituents of the air at low levels. However,




at auroral levels most of the oxygen is in the atomic form: it is dissociated

by sunlight. The atomic oxygen and nitrogen lines in the spectrum are

very narrow. Thus any Doppler widening of them by their random speeds is small, indicating that the emitting gas is at a low temperature. This is confirmed by studies of the structure of the spectral bands. A small part of the auroral light, however, comes from hydrogen atoms.

The air at auroral levels contains only a very small proportion of such atoms. The nature of the hydrogen lines in the auroral spectrum shows, indeed, that the hydrogen atoms that emit the light are not normal con

Fig. 10a. Spectrum of the great red high-altitude aurora of February 10, 1958. The slit of the spectroscope was in the geomagnetic meridian. The spectrum shows the variation of the auroral light from north to south. The exposure was one hour, 6 p.m. to 7 p.m. local time. The red oxygen light (6300, 6364) and the red hydrogen light (Ha, 6563) were unusually strong.

stituents of the air. When the aurora is viewed sideways, the hydrogen lines (Ha, H^) in its spectrum show what is interpreted as a considerable

Doppler broadening. It indicates random speeds of order ?400 km/sec along the line of sight. When the spectroscope views the aurora from

below, looking upwards along it, the hydrogen lines are displaced as well as broadened; the Doppler change indicates that the atoms are

descending with speeds up to 3000 km/sec (Fig. 10b). The permanent gases in the atmosphere, oxygen and nitrogen, are

believed to be excited to emit the auroral light by impact. Electron im

pact is the main action, but particles of atomic mass, such as protons, also take part. The entering particles are mainly the components of ionized hydrogen gas, which is believed to come from the sun. Some of the protons capture an electron as they descend through the air; thus

they become hydrogen atoms, and can reveal their presence by them selves emitting light, instead of only causing the air to do so. They emit their light while still descending, or when scattered in many directions near the end of their downward path.




Solar Streams and Clouds

Many indications point to the occasional ejection from the sun of some of its substance, which is mainly atomic hydrogen. Sometimes wisps of

partly neutral hydrogen at the edge or limb of the sun are visibly ejected, but this is somewhat rare. Even such masses of gas will become invisible, through becoming ionized by radiation, as they move outwards. Most of the gas ejected is not seen to leave, because it is already fully ionized, before it is impelled from the sun. Solar flares, intense brightenings of small areas of the sun, are believed to be regions of such ejection. The duration of a flare may be from about half an hour to a few hours. Intense

CORRECTED OBSERVED Ha PROFILES

cr FOR (01) 6364 Av?*

MAGNETIC HORIZON

MAGNETIC ZENITH

JL A

6562 8A

-m9 U ? v/ .

4000 3000 2000 1000 0

VELOCITY OF APPROACH

KM/SEC

Fig. 10b. Spectral line profiles of the Ha line of atomic hydrogen, in the spectra of auroras seen near the horizon (above) or near the magnetic zenith (below). As com

pared with the atomic oxygen line, whose width is shown above, the Ha line is broad ened by Doppler effect, and also (below) displaced, indicating descending motion.

ultraviolet light and X-rays accompany the visible light emitted; they produce extra ionization in our upper atmosphere, and thereby cause fadeouts of radio signals, and a minor magnetic disturbance over the sunlit hemisphere. About a day later a magnetic storm may break out, simultaneously, within a minute or so, all over the earth (Fig. 11). The

delay after the visible flare is interpreted as the travel time of the cloud of solar gas. Hence its mean speed toward the earth must be of order 1000 km or miles per second.

Streams of solar gas (Fig. 12) are believed to issue also from regions of the sun not marked by flares or other clear sign of disturbance, except




possibly by a local magnetic field on the sun. The emission may continue from the same region, perhaps with intermissions, for days or weeks, or even months. The continued outflow of such streams of gas is inferred from the tendency of magnetic storms to recur after an interval of about 27 days, the rotation period of the sun. The repetition is ascribed to the successive passages of the stream across the earth.

TORONTO H I

ZI-KA-WEI H.

BATAVIA H

CO LAB A H.

MELBOURNE H. \

SAN FERNANDO H.

VIENNA H.

KEW H

WILHELMSHAVEN H.

ST PETERSBURG H

STONYHURST H. I

LISBON H.

Fig. 11. Variations of the horizontal magnetic force during the great dual mag netic storm of 1884 June 24/25. The records from different parts of the world are shown on a common time base (but with different scales of force). The two storm commencements at 10:32 p.m. and 3:48 a.m. were almost simultaneous over the

globe. Local noon (AO and midnight (M) are indicated.

The sun's atmosphere extends throughout the interplanetary space between the sun and the earth, but its density is low. The solar clouds and streams can pass through it practically unhindered. The duration of a magnetic storm may be interpreted as the time during which the earth is immersed in the gas. This indicates that the solar stream or

cloud, when near the earth, is immense. The width, and also, for the clouds from flares, the length, along the direction of flow, is many thou sand times the diameter of the earth.




Protons traveling with a speed of 1000 km/sec have an energy of about 5 Kev (kilo-electron volts) ; for electrons it is only 3 electron volts. But the solar gas, being ionized and very electrically conducting, can

carry away from the sun tangled fragments of the sun's magnetic fields. Such magnetic fields embedded in the gas can imprison particles of much

higher energy, enough to entitle them to be called cosmic rays, or at least subcosmic rays. Thus, very energetic particles may travel from the sun with a speed much less than their individual speeds, and can be in volved in the complicated actions that follow the impact of such solar streams and clouds upon the earth.

BirkelanoVs Auroral Theory

More than sixty years ago, the many associations between auroras

and the earth's magnetism and its disturbances led Birkeland, a Norwe

Fig. 12. A stream of gas issuing radially from the rotating sun: drawn for a travel time of 36 hours from sun to earth. The stream overtakes the earth in its orbital

motion. The drawing is not to scale.

gian professor of physics, to ascribe the two phenomena to the impact of solar electrons upon the earth. By laboratory experiments he showed that a beam of electrons projected toward a magnetized sphere would be deflected so as to impinge on it in two zones, one round each pole.

Stornier, whose great observational contributions to auroral science have been mentioned, devoted much time and effort over several dec ades to the mathematical development of Birkeland's auroral theory. But his studies related almost exclusively to the motion of single par

ticles, influenced only by the earth's dipole magnetic field. These studies later proved to be applicable, with important results, to the cosmic rays that continually assail the earth, mostly from far beyond the solar sys tem. These rays were unknown when Stornier's work began. His calcu lations had but limited application to the aurora, however ; this must be caused by solar gas containing particles both positively and negatively charged. Their mutual influence is too great to be neglected.




The Earth7s Magnetic Shield; the DGF Magnetic Disturbance

If the gas coming from the sun consisted of whole atoms, each neutral because the positive charge on its nucleus is balanced by the negative charge of its one or more electrons, then the gas would travel straight onwards, and fall, like sunlight, on the half of the earth facing the sun.

This does not happen. The atoms, mostly of hydrogen, are broken up into charged particles, separate ions?mostly protons?and electrons.

On arrival near the earth these particles experience the deflecting force of the geomagnetic field. This influence becomes appreciable when the




gas is still far from the earth?at a distance of order 40,000 miles or ten earth radii.

A single proton, traveling alone, with the speed of 1000 km/sec al

ready mentioned, could approach the earth much closer than could a

single electron traveling alone with that speed. When protons and elec trons come together, in great numbers, sufficient to disturb the earth's

magnetic field, the protons draw the electrons onward, and the electrons exert a drag on the protons. They keep together, but with slight lateral deflections. If we look along the stream flowing onward from the sun, so as to see the earth with its north pole uppermost, the electrons move

slightly to the left and the protons to the right. The particles traveling along the line of centers of the sun and earth

are turned almost directly back in their tracks, at a distance that is determined by the density and speed of the gas. Those to left and right of this line of centers are deflected obliquely away from the earth. Beyond a certain distance from the line of centers the oblique deflection is very

small, and the particles travel practically straight onward. They pass

beyond the earth almost as if this were not present, or had no magnetic field. The magnetic field acts like a shield against the impact of the gas on

the earth; but the shield guards only a limited region. It creates a hollow

(Fig. 13) in the stream around the earth, a hollow which is smaller, the

greater the density and speed of the gas. The surface of the hollow is

nearest the earth on the sunward side, on the line of centers. Its length on the dark side of the earth depends on the random speeds of the particles relative to their mean speed of travel. Near the surface of this hollow the slight difference between the deflec

tions of the protons and the electrons constitutes an electric current.

Near the equatorial plane its direction is eastward. This current distrib

uted over the surface of the hollow has a magnetic field. For brevity it

is called the DCF field?D for disturbance and CF for corpuscular flux.

This DCF field annuls the earth's field within much of the gas-filled space behind the surface. Within the hollow space around the earth the

DCF field increases the earth's field, at least near the equatorial plane. The increase is regularly observed at the earth's surface at the beginning of a magnetic storm (Fig. 11). One can picture these events as arising from a slight compression of the earth's field within the hollow in the

gas, whose electrical conductivity prevents the penetration of the earth's

field into the gas. There is a slight penetration, but only in the thin layer near the surface of the hollow, where the electric currents flow in this

thin layer. As long as the flow of gas continues, there will be the turning aside of

the newly arriving electrons and protons with the slight opposite deflec tions to the left and right. This means the continuance of the DCF currents and their magnetic field. The particles that are turned aside,




away from the hollow, travel against the main stream of the oncoming

gas, before it is deflected, or across the part of the stream that is too far

to the side to be affected by the geomagnetic field. The density of the gas is so low that particles can thus travel against or across the main flow

with practically no collisions. The mathematical theory of image charges or dipoles provides a crude

model of the DCF field around the earth (Fig. 14). The field within the hollow is roughly like that of thei mage of the geomag netic dipole in a plane normal to the direction of flow of the gas, at the distance from the earth's center where the gas flow is re

versed. For a weak solar stream or cloud this distance may be ten earth radii or even more; for a

strong stream it may be only five or even three earth radii. The

image dipole is twice as far away. The image dipole field is clearly strongest on the sunward side of the earth.

The ejection of gas from a

stormy region of the sun is not

likely to be quite regular. There

may be gradual or sudden in creases or decreases of the rate of outflow. Such fluctuations in the onflow will cause changes in the

shape and size of the hollow around the earth, and in the

strength of the DCF field. Some times two or more solar flares oc cur within a few hours. The dura tion of onflow of the gas from each at the earth's distance

is likely to be greater than the duration of the flare. This is be cause the faster particles will arrive first, and the cloud will lengthen out as the slower particles lag behind. The faster particles from a new flare may arrive before the slower part of the gas from an earlier flare has all arrived at the earth's orbit. Then a new disturbance with its DCF increase will break in upon the original magnetic storm (see Fig. 11). Such double or multiple magnetic storms are not very uncommon.

Fig. 14. Schematic illustration of the

magnetic force lines in the plane of the noon meridian, at a time when the front "surface" of the solar gas can still be re

garded as plane and undistorted. The elec tric current near the stream surface shields the gas to the rear from penetration by the geomagnetic field. On the left the field (DCF) of the currents is the same as that of an "image" of the earth's mag netic dipole. The combined field shows a

"compression" of the earth's field by the stream surface.




Capture or Trapping of Some Solar Gas

Although much of the solar gas is turned away from the earth by the

shielding power of the geomagnetic field, some of the gas fares differently. Perhaps because of irregularities in the stream or cloud, some of the gas is caught or trapped in the field. It may break through the surface of the

hollow, because in speed or density or both it differs from the main body of the gas. Or the irregularity in the gas may consist of a magnetic field, a fragment of the general or a local magnetic field of the sun, transported away. The space probe Pioneer V has shown that such fields are sometimes

present. An ionized gas, being a good electrical conductor, is able to

carry away a magnetic field within itself. The solar gas may well be

turbulent, and in transit from the sun the magnetic field in it may become

tangled, somewhat as threads of colored liquid are seen to do if the liquid is turbulent.

Such a magnetic field in the gas may contain particles of energy much

greater than the generality of the gas particles. Whereas the energy of the forward motion of the gas is very small for the electrons, and only about five Kev for the protons, a tangled magnetic field can hold within the stream electrons and protons with energy of hundreds of Kev or even of several Mev. This requires a magnetic field of considerable ex

tent; but its volume may still be small compared with that of the stream. Thus particles may come to the earth after a day's travel from the sun, with speeds that would have brought them to us in one or a few hours if

they were not held in the transported field. Such particles may be more

able than most to burst through the surface of the hollow formed in the stream of slower particles. Until they reach the thin surface layer of these

particles, that carry the DCF current, they do not meet the earth's field. They quickly burst through the surface layer, and then become

exposed not only to the earth's field, but to this enhanced by the DCF

field, which is there stronger than anywhere else.

Prisoners of the Geomagnetic Field

A new life awaits the particles that thus enter the field within the hollow. While they are in the stream they and the transported magnetic field may interact on relatively equal terms, either swaying the other. But on entering the hollow the particles come into a field proceeding from the earth's core, and tightly anchored there. Its intensity increases

rapidly towards the earth's center. In this field the particles are tightly controlled.

Figure 15 illustrates their motion. They spiral round the earth's lines of magnetic force. Also, except for the few that move in the plane of the

magnetic equator, they travel back and forth along these lines. As they curve northwards or southwards, the increasing magnetic intensity turns



Fig. 15. Illustrating some aspects of the motions of charged particles trapped in

the geomagnetic field. The drift round the earth is not indicated. For penetration into our atmosphere, the particle motion must be nearly along the force line when

crossing the equatorial plane.

their motion more and more sideways. Finally the forward speed is en

tirely lost, and they turn back again toward the equator. They cross it and move toward a similar turning or "mirror" point in the opposite hemisphere. At the same time they drift sideways round the earth, the electrons eastward and the protons westward. The combined result of all these motions is that an electric current flows in the region where the

trapped particles are distributed. This current is partly eastward and

partly westward. But the latter is the stronger. The net effect is to weaken the earth's field within a certain distance around the earth, and to

strengthen it in the space beyond the region of trapped particles. Satellites have observed this weakening of the field. (Fig. 16a).

These events occur in the region of the radiation belts (Fig. 17), whose existence was revealed by the satellite instruments of Van Allen (U.S.A.) and Vernov (U.S.S.R.) and their colleagues. The inner belt is produced by the action of cosmic rays striking our atmosphere. The outer belt seems to be maintained by the intermittent capture of high energy par ticles that have come from the sun. The belts must suffer continual de

pletion. For the outer belt, at least, this is generally slow, but sometimes

rapid. Satellites have shown considerable variations in the extent and




I 23456789 10

I 23456789 10 earth radii

Fig. 16. (a) Satellite observations showing a considerable weakening of the mag netic field, near the equatorial plane, over the range of distance from 5 to 7 earth radii a. (6) A similar radial magnetic variation approximately calculated for a model belt of energetic particles, as shown in (c), below, mainly lying between 5a and 7a. The corresponding radial distribution of the electric ring current is shown in (c), and also its DR field.

intensity of the outer belt over periods of weeks and less (cf. Figs. 17a, 17b). One way in which protons are lost is by picking up an electron from one of the neutral atoms in the rare atmosphere that suffuses the belt. The protons may also be scattered away by hydromagnetic waves. Some




Fig. 17a. Schematic illustration of the density distribution of the energetic particles of the radiation belts (Van Allen). The orbit of the satellite Pioneer III is shown.

10,000 20,000 30,000 40,000 50,000 60,000 70,000 8 0,000 90,000 100,000 110,000

RADIAL DISTANCE FROM CENTER OF EARTH - KM

Fig. 17b. The density distribution along an equatorial radius as measured by two satellites Pioneer III and IV three months apart. On the later occasion the belt is denser and more irregular.

electrons and protons travel down along the lines of force into our atmos

phere, where scattering by other particles will be much more likely. In 1953 and later, Van Allen found evidence of the penetration of our atmos

phere down to heights of order 100 km, by electrons of energy up to 100




I i ui

bl >

GEOMAGNETIC LATITUDE Fig. 18. Energetic particles whose entry into the earth's atmosphere was regis

tered by rockoon-borne Geiger counters. The short vertical lines correspond to the normal cosmic ray incidence. The longer lines, for the latitudes from 65? to 76?, cor

respond to particles from the radiation belt (j. A. Van Allen and his colleagues).

Kev. This was observed in a range of about 11 degrees of latitude (65? to 76?), including the auroral zone (Fig. 18). In the auroral zone itself the downward electron flux was much more intense than to the south and north of the zone. These electrons were observed during the daytime (for convenience of rockoon launching), and when the magnetic condi tions were not specially disturbed. They consisted of the small minority of particles that are captured or scattered so that they cross the equator almost at right angles. In that case their turning or mirror points will be

unusually close to the earth. They form a fairly widespread feeble stream

entering the atmosphere, by day and by night. Over this band of latitude

they make a small contribution to the ionization of the atmosphere. They add also to the airglow, namely, the light, mainly chemilumines

cence, that comes to us from the high atmosphere.

The Auroral Particles; Diffuse Auroras

The aurora is not simply an intensification of this wide band of special airglow.

Perhaps the most striking of all the characteristics of the aurora is its thin ribbon-like structure. This is well shown in Figs. 1-3. Its extent in




height (Fig. 7b) is of order one or more hundreds of miles; its lateral extent (Fig. 9) is reckoned in thousands of miles.* Its thickness, even when diffuse, is far less, only two or three miles; and in the second phase, of rayed, folded and pleated aurora, the thickness may be only a few hundred yards.

* Thus (to use an expression familiar in connection with mass spectroscopy), the particles that produce the auroras must be "fo cused" by some special process.

As the particles travel substantially along the lines of magnetic force, one can tentatively trace back their motion before entering the atmos

phere. Two lines of force in the same magnetic meridian, on the N and S sides of a diffuse auroral arc, are (say) 3 miles apart near the earth; if

they are there in latitude 66? (near the auroral zone), their greatest separation elsewhere is on crossing the equatorial plane, at a distance of

63^ earth radii or about 25,000 miles. Ignoring any distortion of the

geomagnetic field, that separation is 29 times as great, namely, 87 miles. Thus an auroral band may come from a strip of this width at this great distance. This strip is an extremely small part of the radiation belt, whose total thickness is reckoned in thousands, even tens of thousands, of miles.

What can be the cause of this striking restriction of the source of the auroral particles? and why may there be many such parallel strips, from which particles flow into our atmosphere to produce the multiple auroral arcs? (Figure 3).

Only recently has an answer to these questions been offered. Its ac

curacy remains to be determined by further research. Some fellow

speculators in this difficult branch of geophysics have already rejected it out of hand. The answer proposed by my young Japanese collaborator Akasofu

and myself is based partly on our observational studies of magnetic storms, and on the partial theory of magnetic storms developed much earlier by Ferraro and myself. This theory explained the first phase of a

magnetic storm?the initial increase (Fig. 11)?by the DCF field. The first phase is followed by th? main phase, during which the hori

zontal magnetic intensity at the earth's surface is decreased?usually by more, sometimes much more, than the initial increase. Fig. 19 shows an outstandingly great decrease, the main phase of the storm of Septem ber 13, 1957.

This decrease is a manifestation of a new disturbing magnetic field that develops during the storm, and then dies away. It is due to addi tional electric currents encircling the earth, with the main flow westward. Their field is called the DR field (D for disturbance, R for ring current). It adds, often greatly, to the ring current field, detected by satellites

(e.g., Fig. 16a), that exists even during magnetically calm conditions. * As these numbers express only orders of magnitude, km may be read instead of

miles, and meters instead of yards.




-100

-300

Akasofu and I conclude that the auroral arcs are connected with lines in the (geomagnetic) equatorial plane, where the magnetic field inten

sity is zero, and where lines of magnetic force cross one another. Lines of zero magnetic intensity are called neutral lines. Lines, and even points, where the magnetic intensity is zero, are decidedly exceptional. The earth's undisturbed dipole field has no such points or lines. To create such a point, the disturbing cause has to

produce there a neutralizing inten

sity of exactly the right magnitude and direction. If the main cause is an increased simple westward ring current round the earth, symmetrical with respect to the equatorial plane, such neutralization can occur only in this plane. If there is also symmetry about the earth's magnetic axis, then the existence of one neutral

point implies a complete circular neu tral line round the earth.

Actually in that case there must be not one but two neutral lines. Consider the graph of the magnetic intensity along a radius from the earth's center, in the equatorial plane. It will be somewhat like Fig. 16a. But if the earth's field is re

duced to zero at some distance, the

dip there shown in the graph must extend lower to touch or cross the horizontal axis: the former would be a very special case. At the points where the graph crosses the hori zontal axis, the intensity is zero. These points are on neutral lines.

Figure 20a shows the pattern of

-350

-500

-550

SEPTEMBER 13,1957 GMT

Fig. 19. The variation of north horizontal magnetic force (the mean for six stations in low latitudes) during the great magnetic storm of Sept. 13, 1957. The initial increase, due to the DCF field, is followed by the great main-phase decrease, due to the DR field of the enhanced ring current. In low latitudes, the changes of the east force (below) are much less.

the lines of force in the meridian

plane, when the earth's field is reversed within a short range of radial

distance, between the points 0 and X. There is a special line of force that crosses itself on one neutral line, and surrounds the other: hence it is

appropriate to call these neutral lines respectively X and 0 lines, corresponding to the shape of the lines of force through or near them. The X neutral line is connected with the earth by a line of force, the O line is not. (Note that the neutral lines are not themselves lines of force.)




Above and below the equatorial plane, near the neutral lines, the field direction differs completely from that found everywhere else near that

plane. On this account, and because the field near the neutral line is so

small, the motions of particles that come there can be changed in a way not possible elsewhere. The component motion along the lines of force can be increased at the expense of the motion around them; this enables the particles to descend lower into our atmosphere. But this happens only to particles that come very close to the X line, when this is brought into being by the additional DR magnetic field of the ring current.

Fig. 20a. Magnetic force lines in the midnight meridian plane, when the DR field (Fig. 16) is intense enough to reverse the earth's field between two points O, X where the field is zero. These two points lie on 0 and X neutral lines.

This, then, is the central idea in our explanation of why auroras are

thin ribbons of light. They are formed by particles that have passed close to an X line.

Probably the neutral lines are generally confined to the night side, instead of encircling the earth. This is because on the sunward side the

DCF field strengthens the earth's equatorial field and makes reversal

there, by the ring current, less likely. In that case the X and 0 lines form a loop (an "OX-loop)" or loops (Fig. 20b). According as the strength of the DCF field is greater or less relatively to the DR field, the loop will be shorter or longer. This will affect the length of the auroral arc, which is a ''transcription'' of the X line in the earth's atmosphere, linked with it by the lines of magnetic force.

When the ring current is strong but somewhat irregular it can produce a region where there is more than one field reversal (Fig. 20c). Each




corresponds to a pair of neutral lines, one X and one 0; and each X line is the source of an auroral arc (Fig. 20d). The latitude and spacing of multiple auroral arcs indicate the presence and spacing of such re

versals of the magnetic field many thousands of miles away, in or near

the equatorial plane.

Fig. 20b. Sketch of an OX-loop, along which the magnetic field intensity is zero.

The small arc round the earth's north (geomagnetic) pole N is a transcription of the

X-line; corresponding points on each are connected by magnetic force lines. Within

the OX-loop the field direction is reversed, so as to be southward; outside the OX

loop(s), the field in the equatorial position is northward.

The Second Auroral Phase

A physical theory of the aurora must explain not only the first phase but also the second one, which is notably different. The break up of the

first phase is usually sudden; then the auroral sheets become still thinner

than before; they often descend to a lower level and show a purplish red border: pleated and folded, marked by rays, they often move rapidly back and forth over the sky.

During this second phase, which may continue for half an hour or up to a few hours, the magnetic storm-changes become more irregular. One may say that a substorm is superposed on the main storm : there may be one or more such substorms during the course of the storm. Their




Fig. 20c. (Above) Graph of the field intensity along a radius in the equatorial plane, indicating several field reversals. (Middle) Corresponding pattern of magnetic force lines in the midnight meridian plane. (Below) Corresponding OX-loops in the equatorial plane, and their transcriptions in the polar ionosphere as multiple auroral arcs.

number and intensity vary from one storm to another. The more, and more intense, the substorms, the greater is the main phase of the storm?

indicating greater intensity of the westward ring current. The substorms,




their current systems and magnetic field may be denoted by DP (D for disturbance and P for polar).

Such substorms are produced by concentrated electric currents? electro jets?along the part of the auroral zones where the second-phase aurora appears. These electrojets usually are westward. They may carry as much as a million amperes. Their circuits are completed mainly over the polar caps, but they spread to a less extent over the whole earth between the two zones.

Akasofu and I suppose that these substorms and active auroras result

Fig. 20d. Auroral arcs and bands photographed by an all-sky camera at College, Alaska. The circle shows the region well photographed by the camera. The arcs are

regarded as transcriptions of X lines in the equatorial plane, about 25,000 miles away.

from the entry of new supplies of solar gas into the earth's field. The gas is probably trapped on the sunward side of the earth. Thence it spreads round the earth in somewhat the same way as did the electrons pro duced by high level nuclear bombs during the Argus experiment. This

strengthens the ring current and the DR field and the main phase of the storm.

We think that while the gas is spreading around the earth to form a

complete ring?which may occupy one or more hours?an eastward electric field is set up across the gap. This electric field is perpendicular to the lines of magnetic force. The magnetic force hinders or prevents the




electric field from driving electric current, except very close to the X

neutral line or lines, where the magnetic intensity is very weak. Electric current flowing eastward along the X line changes the angle of inter section between the two parts of the line of magnetic force that cross on

the X fine (Fig. 20e). This narrows the strip from which the auroral

particles can travel down into our atmosphere. This reduction of width

correspondingly narrows the sheet-stream of charged particles that flow

along the magnetic force lines into our atmosphere, and produce the aurora.

The electric field along the X line accelerates the

adjacent electrons of the radiation belt to greater en

ergy. It also accelerates the background electrons that lie near the X line. They belong to our outermost

atmosphere, that suffuses the radiation belt region. Hence the electrons that travel from near the X line into our atmosphere have greater energy and pene trate deeper than during the quiet auroral phase.

The electric current flowing along the X line is

unstable, and will become wavy and change its

shape irregularly, whipping from side to side. These motions affect the stream of particles flowing along the magnetic force lines into our atmosphere, thus

producing the waving auroral draperies we see. A smaller-scale instability of the sheet of gas connecting the X line and our atmosphere produces rays and

pleating in the aurora. Both these kinds of instability are known in laboratory experiments on ionized

gases?called plasmas?in magnetic fields.

During this phase the electric field along the X line reduces the flow and the mean energy of the

protons. (Small consequent readjustments are made, in the ionized background atmosphere in the radia tion belt region, which maintain almost complete

electric neutrality and permit only very weak electric fields to exist.) The auroral light is produced more by electrons than by protons, espe cially during this phase. Moreover the mean latitude of entry of the

protons becomes slightly different from that of the electrons. This sets up an electric field in the ionosphere, across the width of the auroral zone?

usually southward. On account of the peculiar non-isotropic electrical

conductivity of gases in the presence of a magnetic field, this will drive a westward current along the zone. This is achieved mainly by east ward flow of electrons along the auroral sheets. This flow has been ob served by radar.

In our explanation of the aurora in its second phase we venture fur

Fig. 20e. Illus

trating in meridian section the narrow

ing of the region of low field intensity near an X-line, when an electric current flows east ward along the X line.




ther from the known facts than in the case of the first phase. We have considerable confidence in the main lines of the explanation, but further effort both of theory and observation will be needed before its soundness can properly be assessed.

As the DP substorm and the rayed auroral draperies die away, the

sky often shows most irregular patterns of auroral light. The pattern of neutral lines in the equatorial plane may be correspondingly compli cated before conditions there return to calm and stability.

Thus the beautiful changing aurora (we believe) mirrors events that take place thousands of miles away, near the equatorial plane. Their

prior cause is storms on the sun, which eject ionized hydrogen gas into

space.

It seems likely that at least some of the earth's fellow planets have

magnetic fields. If these fields are at all similar to our own, there will be

magnetic storms and auroras there also. But their auroral zones may be nearer to or further from their magnetic poles than on the earth. And in color their auroras, depending on the nature of their planetary atmos

pheres, may also differ from ours. A final reflection bears on the interconnection between natural phe

nomena seemingly far dissociated. It is now generally accepted that our earth's magnetic field proceeds from its liquid core, about 2000 miles below the surface. In this core, perhaps owing to unequal radioactive

heating, there is a slow circulation, with eddies. By dynamo action this convection created and maintains the earth's magnetism. The processes that go on there are copartners with sunstorms in producing the aurora. If the earth's core were to cool and become rigid, the geomagnetic field would die away or become vestigial. Sunstorms might still continue, but the earth would no longer be magnetically protected from the gases they eject. The gas would impinge on the sunlit hemisphere, facing the

sun, and light like that of the aurora would be produced in our atmosphere. But, lost in the glare of scattered sunlight, it would not be seen. We

enjoy the beauty and the interest of the aurora through the joint action of sunstorms and the core of the earth.

REFERENCES

Akasofu, S.-L, 1960, Large-scale auroral motions and polar magnetic disturbances?

I., J. Atmosph. Terr. Phys., 19, 10-25.

Akasofu, S.-L, and S. Chapman, 1960, Some features of the magnetic storms of July 1959, and tentative interpretations, UGGI Monograph No. 7, 93-108.

Akasofu, S.-L, and S. Chapman, 1961a, A neutral line discharge theory of the aurora

polaris, Phil. Trans. Roy. Soc, A, 258, 359-406.

Akasofu, S.-L, and S. Chapman, 1961b, The ring current, geomagnetic disturbance, and the Van Allen radiation belts, Geophys. Res., 66, 1321-1350 (May).

Akasofu, S.-L, and S. Chapman, 1961c (in press), A new theory of the aurora

polaris, J. Amer. Rocket Soc. Alfv?n, H., 1950, Cosmical Electrodynamics, Oxford Univ. Press. Armstrong, E. B. (editor), 1956, The Airglow and the Aurorae, Pergamon Press,

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Chamberlain, J. W., 1961, Physics of the Aurora and Airglow, Academic Press, New York.

Chapman, S., and V. C. A. Ferraro, 1931a, 1931b, A new theory of magnetic storms, Terr. Mag., 36, 77-97, 171-185.

Chapman, S., and V. C. A. Ferraro, 1933, A new theory of magnetic storms, Part

II, The Main Phase, Terr. Mag., 88, 79-86.

Chapman, S., and V. C. A. Ferraro, 1940, The theory of the first phase of a geo magnetic storm, Terr. Mag., 45, 245-268.

Chapman, S., and J. Bartels, 1940, Geomagnetism, Oxford Univ. Press.

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Fritz, H., 1873, Verzeichniss beobachteter Polarlichter, Gerolds Sohn, Wien.

Fritz, H., Petermanns Geog. Mittheil, 20.

Fritz, H., 1881, Das Polarlicht, Leipzig. Harang, L., 1951, The aurorae, Wiley, New York.

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Meredith, L. H., M. B. Gottlieb, and J. A. Van Allen, 1955, Direct detection of soft radiation above 50 kilometers in the auroral zone, Phys. Rev., 97, 201-205.

Singer, S. F., 1957, A new model of magnetic storms and aurorae, Trans. Amer.

Geophys. Union, 88, 175-190.

Smith, E. J., P. J. Coleman, D. L. Judge, and C. P. Sonett, 1960, Characteristics of the extraterrestrial current svstem: Explorer VI and Pioneer V, J. Geophys. Res., 65, 1858-1861.

St?rmer, C, 1955, The Polar Aurora, Oxford Univ. Press. Van Allen, J. A., 1957, Direct detection of auroral radiation with rocket equipment,

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Vernov, S. N., A. E. Chudakov, P. V. Vakulov, and Yu. I. Logachev, 1959, Study of terrestrial corpuscular radiation and cosmic rays during flight of the cosmic

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SUN STORMS AND THE EARTH: THE AURORA POLARIS AND THE SPACE AROUND THE EARTH