Magnetic field variations in the polar cap

Magnetic Field bJJ ANDRE

Variations in the Polar Cap

F. LEBEAU

Centre National d’Etudes Spatiales Bretigny-sur- Orge, France I. Introduction

Magnetic field variations in regions within the aurora1 belt follow a most unusual behavior pattern induced by upper-atmosphere phenomena as yet unidentified. As described by the K activity index, the magnetic activity exhibits both a diurnal and a seasonal variation, marked respectively by a daytime and a summer maximum. These are in sharp contrast to the variations usually observed in the temperate and aurora1 regions, which are marked by nighttime and equinoctial maxima. The specifically regional character of this magnetic activity was recognized as early as 1935 by Stagg (l), who based his work on the magnetic observations obtained during the Second Polar Year. Since then, Nikol’ski (24), Mayaud (5), Fukushima (6), Rourke (7) and Lebeau (8), among others, have enhanced our knowledge of the diurnal activity and tried to elucidate the cause of the unusual behavior. This paper is not intended to be a complete and critical review of the literature. Instead, it is an attempt to produce a coordinated description of magnetic phenomena observed within the polar cap. Since we are relying broadly on our own work and that of our close colleagues, it presents a personal rather than an impartial account of existing information. We will first consider the experimental laws that control the geographic distribution and time variations of the activity. Then we will try to show how these laws yield clues to the origin of the polar cap phenomena and the relation of these phenomena to those of the lower latitudes. ZZ. General

Approach

Two radically different explanations have been offered for the seasonal behavior of the diurnal activity within the polar cap. The summer solstice maximum, clearly visible in Fig. 1, is a characteristic of this variation. According to Stagg (l),this is a consequence of the strong seasonal variation that characterizes upper-atmosphere conductivity in the highlatitude regions. His view is that the diurnal activity is produced by the closing-within the polar cap-of a fraction of the auroral-belt electric currents and is therefore only a special aspect of the general aurora1 phenomena. Mayaud (5), on the contrary, considers the diurnal activity as a distinct phenomenon related to a specific feature of the Chapman-Ferraro magneticstorm model. Two neutral points exist on the surface of the cavity induced

297

Andre P. Lebeau in the storm solar plasma flow by the earth’s magnetic field. Along the lines of force starting from these neutral points, charged particles can travel down to the vicinity of the earth’s surface. Mayaud contends that this mechanism, which operates continually, produces the diurnal activity; the solardeclination seasonal variation, which changes the value of the angle between the dipole axis and the sun-earth line, exposing first one hemisphere and then the other, is responsible for the seasonal effect.

OUMONT

O’URVILLE

_

FIG. 1. Seasonal variation of the magnetic activity amplitude a at Godbavn (Arctic) and Dumont d’urville (Antarctic) from Lebeau (8).

More recently, Spreiter and Summers (9, 10) re-examined the theoretic analysis of this mechanism and computed the seasonal variation of the particle flux that could be injected into the upper atmosphere. They concur on the existence of a very substantial seasonal effect. In studying this problem, we came to the conclusion that a precise analysis of the time of maximum activity in the polar cap stations could resolve the contradiction between Stagg’s and Mayaud’s work-between a prediction of maximum activity at local noon and a prediction of maximum activity at magnetic noon. But first we had to accurately define the concept of magnetic noon (11). In the lower latitude regions the magnetic time can usually be determined from a rough model of the real magnetic field, but such approximations are not valid in the vicinity of the poles, where a slight change in position may make a large change in the time of the magnetic and local noons.

298

Journal of The Franklin

Institute

Magnetic Field Variations in the Polar Cap ZZZ. Magnetic

Noon

and Time

of Maximum

Diurnal

Activity

We define magnetic noon at a specified point of the earth’s surface as the time when the sun lies in the plane of the real-field line of force that starts from this point. We define a magnetic isochron as any locus on the earth’s

FIG. 2. Magnetic isochrons for the ,4ntarctic polar cap from Lebeau (8). surface where magnetic noon occurs at the same universal time. A set of magnetic isochrons covering the Antarctic polar cap is shown in Fig. 2. These curves converge on the invariant pole, on the part of the great circle that extends from the geographic pole to the invariant pole ; the difference between local and magnetic noon is 12 hr. The diurnal variation of the magnetic activity averaged over a period of 1 year is shown in Fig. 3 for eight Antarctic stations located within the central region of the polar cap (0 < 14’, where f3 is the invariant colatitude). These curves are derived from the K activity indices. The maximum activity time H

Vol. 290, No. 3, September

1970

299

Andre F. Lebeau (shown by an arrow) is obtained by parabolic interpolation, error less than 10 min.

300

-

vo

200

-

6

,oo-

with the absolute

x-2

S.B.

A 300

-

PO

i/:

Wk

-

200 -

100-

#A__

OLJ 0

’

0

E

3

6

9

” 12

” (5

18

’ 21

24

,

I

*

I

1

I

1

I

1

0

3

6

9

12

IS

18

21

24

FIG. 3. Mean diurnal variation of the activity amplitude for eight Antarctic stations situated within the central region of the polar cap. The amplitudes, given in gammas, are obtained from the K indices from Lebeau (8). Using the concept of magnetic noon, it is possible to uncover a fundamental feature of H (8). For a given station, let M be the time of magnetic noon, L the time of local noon and H the time of maximum diurnal activity and then

300

Journal of The Franklin Institute

Magnetic Field Variations in the Polar Cap plot the difference values of H - L vs. M-L. For the eight above-mentioned stations the points (Fig. 4) lie very close to a line drawn through the origin (the mean absolute deviation of H is 12 min ; the largest deviation is 22 min). It is possible to show that the coherence of the data depicted by Fig. 4 is reduced when any other parameter is substituted for the time of magnetic noon. For instance, using the geomagnetic dipole time increases the meansquare deviation to the regression line by a factor of 8 (12). We have come to

FIG. 4. Difference between time of maximum diurnal activity and local noon (H- L) plotted vs. the difference between magnetic noon and local noon (M- L) for eight Antarctic stations from Lebeau (8). the conclusion that the activity maximum is produced by the combined influence of two distinct factors, one associated with magnetic noon and the other with local noon. The first factor is indubitably linked with a chargedparticle mechanism, and the second is obviously photonic. The law defining the time of maximum activity intrinsically expresses the duality of the mechanism that controls this phenomenon. A schematic model might include an excitation mechanism and a modulation mechanism which for a given level of excitation determines the intensity of upper-atmosphere electric currents. Since excitation is maximum at magnetic noon, some physical parameter of the modulation mechanism must be directly related to the sun’s zenith angle. This requirement is satisfied by using E-region conductivity, which is strongly modulated by solar illumination and changes with it after a negligible time-lag. Thus, a precise analysis of the diurnal variation of the activity reveals that the mechanism postulated by Stagg does indeed play a part in the behavior of this variation; on the other hand, it also reveals that there is a particle mechanism superposed on it,. Analysis of the variation in amplitude of the diurnal maxima during the solar cycle and of the geographic distribution of the activity provides

Vol. 290,No. 3, September

1970

301

Andre F. Lebeau confirmation of the preceding results (8). Moreover, the modulation mechanism, that is, the effect of the E-region conductivity changes, at least partially explains the summer maximum that characterizes the seasonal variation ; however, we cannot a priori exclude the likelihood that the effect of some particle mechanism of the kind advanced by Mayaud (5) may superpose itself upon it. An attempt to confirm the presence or absence of such an effect was made by analyzing the observations obtained at a pair of stations (Wilkes and Dumont d’urville) chosen such that all the relevant parameters, except for the tilt angle of the dipole axis in the sun-earth direction, were equal. No corroborative evidence of the effect was found. Nevertheless, we do not consider this result to be absolutely conclusive. At present, the influence of the excitation factor on the seasonal variation of the activity remains an unsolved problem. IV.

Nature

of the Excitation

Mechanism

The preceding considerations shed no light on the intrinsic nature of the excitation mechanism that introduces the energy necessary to cause variations in diurnal activity. Further, they also leave entirely open the question I I I I I I I I I I LITTLE AMERICA 1958 t’ I: 200

0

’ 7.5

1,

I 13.5

FIG. 5. Mean diurnal variation

,

Mt,

t.

1”

19.5

of the activity

,

I

1.5

at Little

I

HEURES I

75

T.U. b

135

America

from Lebeau

(8).

formulated by Mayaud: Is the diurnal activity intrinsically different from the aurora1 latitude activity or is it only a speci$c aspect of the aurora1 activity ? A first clue is obtained from a study of data obtained at the stations located between the central region of the polar cap (6 < 14’) and the aurora1 zone (0 = 23”). Three Antarctic stations-Little America, South Pole and Gaussland-belong to this category. The diurnal variation at these stations is marked by two daytime maxima symmetric with respect to the time predicted for the single maximum observed in the central region (Figs. 5, 6). Within the model proposed earlier, these results are interpreted by assuming that in the region lying lower than 6 = 14”, the excitation mechanism tends to create two separate maxima that are symmetric with respect to the magnetic noon. Obviously, the modulation mechanism is inadequate to account for this effect.

302

Journal

of The Franklin

Institute

Magnetic Field Variations in the Polar Cap The injection of particles along the lines of force through the neutral points, which could explain the diurnal variation observed in the central region, can hardly account for the splitting of the maximum. On the other hand, this feature is easily understood in terms of the diurnal movement of the aurora1 oval if we assume that the excitation intensity increases as the -8 t

I

-6 I

I

-4 1

-2 I

I

0 I

II

6

4

2

I

I

I

I

I

8 I

I

I,

M-++L)

5 -* vo

ct

._

. DU

Wk se=

l

PO

+ l

00

FIG. 6. Deviation of maximum activity time from +(A4 + L) for the Antarctic station of the central and intermediate regions from Lebeau (8).

distance between the station and the aurora1 oval decreases. In the central region of the polar cap (t?< la’), a station is nearest the aurora1 oval at magnetic noon. If the effect of ionospheric conductivity could be removed, a single maximum would be observed at magnetic noon. But, in the 14” < 0 < 23” region, the aurora1 oval passes over the zenith of the station twice a day. This gives rise to two maxima, which ionospheric conductivity shifts toward local noon. We are therefore led to the conclusion that the same particle phenomena that produces the diurnal polar cap activity also produces the activity observed in the aurora1 region. The magnetic activity indices are inadequate to prove or disprove this theory and so we must rely on analyses of the mean hourly values of the magnetic field. V. Diurnal

Variation

of the Field

The hourly mean values of the field at the polar cap stations reveals a regular diurnal oscillation whose amplitude varies with the season and with

Vol.

290, h‘o.

3, September

1970

303

Andre F. Lebeau the level of planetary activity as represented by the Kp indices. From Fig. 7, this variation appears to be stronger during summer and during disturbed days. At first glance the daily variation seems to be controlled by the same factors as the activity described by the K indices. This raises the question

HIVER

-600

1

-600

-

-400

-

H

T.U.

-300,“““““““““““” 0

3

6

9

12

15

16

21

i4

FIG. 7. Diurnal variation of the Y component during summer and winter 1958 [solid line represents disturbed days; broken line represents quiet days; from Lebeau and Schlich (S)] .

of whether the diurnal variation and the K activity in the polar cap are two distinct phenomena or two aspects of the same physical entity, that is, of the fluctuating electric current system deriving its geometry from the aurora1 oval..

VI.

Properties Variation

of the Mean

Diurnal

Amplitude

of the Magnetic

Field

It is possible to define a parameter that is a measure, for a given day, of the mean amplitude of the variation of the horizontal component of the field. Let us assume that the effects of all external and induced currents are removed from the field observed at a given station then the horizontal

304

Journal of The Franklin

Institute

Magnetic Field Variations in the Polar Cap components take the values X0 and Y, and let X and Y be the real values of the same components at a given time. Then the expression i = [(X - X,)2 + ( Y - Yo)2]*

2OC

I

0

100

0

FIG. 8. Activity

I

I

200

300

amplitude 08. current I et Dumont Lebeau (8).

d’urville,

b

for 1957 and 1958 from

is an approximate measure of the field deviation as well as of the equivalent zenithal electric current density. Approximate values of the “undisturbed field” components X0 and Y, are obtained by averaging a selection of quiet night values. Let Xi and Yi be the mean hourly values of X and Y for the ith hourly interval of a given day; then the parameter I, a measure of the mean value of the field variation-or, equivalently, of the mean value of the zenithal electric currentis given by 1 i=24

1= -

24

_

c [(xi-Xo)2+(Yi-Yg)2]'. i=l

An analysis of the correlations between I and the local and planetary magnetic activities, as described by the K and A, indices respectively, elicits some important results. Figure 8 shows the relationship between I and

Vol. 290. No. 3, September

1970

305

Andre F. Lebeau a parameter a, which measures the amplitude of the activity maximum from K indices converted into amplitudes: for the Dumont d’Urville station, a is directly proportional to I. This result implies that the magnetic activity as measured by the 3-hr K indices only resects the$uctuations of the current system that produces the daily variations. It therefore appears that the activity indices and the mean hourly values give two different approaches to the study of the same phenomenon. The variations of I with the planetary level of magnetic activity were studied by Mendel (13). The 1 index undergoes an important seasonal variation similar to the seasonal variation in magnetic activity (Fig. 9). The effect of this variation must be eliminated in order to study the correlation between I and A,. To achieve this, the days are arranged into categories corresponding to a definite scale of A, values. For every station, the mean value f of I and .&, of A, are determined in each category. Since the A, seasonal variation is not very pronounced and the categories comprise days evenly distributed over the different seasons, the seasonal variation has no effect on 1. Figure 10 shows 1 vs. _&, for the Oasis station. All of the stations give typically similar results. In a logarithmic system of coordinates the representative points are very nearly aligned, and so the relation between j and xP can be written as I’ = “2i;

(cu< 1).

The values of 01and k have been determined by Mendel-Berthelier (14) and Guerin (15)for thirteen polar stations (see Fig. 11). From the invariant pole to the lower latitudes, k steadily decreases and 01increases. Thus, for a definite level of planetary activity, I is maximum at the invariant pole and decreases evenly with the invariant latitude. This continuous decrease is not altered by the crossing of the aurora1 belt. This property of the mean intensity is a marked deviation from the behavior of the magnetic activity as measured by the K index-the K activity in fact exhibits a flat minimum in the center of the polar cap and a sharp maximum on the aurora1 belt. The increase of a: toward the low latitudes also shows that the lower the latitude, the faster the change of I with A,. Thus, when the magnetic activity is increased, the current system in the polar cap tends not only to reinforce but to expand toward the low latitudes, a behavior analogous to the spreading of the aurora1 belt activity during perturbations. The most remarkable property of the variation in I vs. the invariant latitude 6 is undoubtedly the absence of any definite influence by the aurora1 belt. Considered together with the relations obtained between I and K activity at the polar cap stations, this property leads to the following conclusions : (1) The same general magnetic phenomenon that is observed in the aurora1 belt is observed in the polar cap. (2) The aurora1 belt is associated with a strong increase in variability of the ionospheric electric currents but has no effect on the mean intensity of these currents.

306

Journal

of The Franklin

Institute

Magnetic Field Variations in the Polar Cap

01

’

,

v

1

Jan

I

Feb. Mar.

FIQ. 9. Seasonal variation

I

Apr.

,

I

1

Mai. Jun

Jul.

I

Aug

I

I

Sep. Oct.

of current I at, Wilkes, Mirny, (1958) from Mendel (13).

,

Nav. Dec.

Vostok,

and Scott Bases

vi-

150 y#---

&P

ICO-

_& _%

f

’

-

50-

,)/i” -tT-

I

I

20 1

I

I

30

I

a

40 AP

,

,

50

,

,

60

,

,

70

A, , ech logorithmique

FIG. 10. Mean annual relation between I and A, for the Oasis station, showing the regression line on the logarithmic diagram from Mendel (13).

Vol.290.No.S,September1970

307

Andre F. Lebeau

FIG.

11. a: and amplitude a vs. the invariant latitude for thirteen Antarctic stations from Mendel-Berthelier (14).

VII.

Daily Variation the Polar Cap

in Density

and Direction

of Ionospheric

and Arctic

Current

in

Provided that a precise value of the undisturbed field H,, is obtained, it is possible to compute the perturbation vector for a given time and thereby obtain information on changes in the direction and intensity of the variable ionospheric currents at the zenith of the station [see Mendel(13) and Lebeau and Mendel (IS)]. To obtain the undisturbed field, we use the variation of the mean ionospheric current density with the level of planetary activity. For a given level of activity, we determine twenty-four Hi vectors that define the horizontal field value for each hourly interval i (1-c i < 24). Figure 12 shows the behavior of the twenty-four H, vectors vs. the activity level. When the activity decreases, the broken line joining the extremities of the vectors shrinks, and it is assumed that at a zero level of activity, the line would reduce to the extremity of the undisturbed field H,. By extrapolation, we obtain H, with an accuracy of about 10~ and thence the equivalent zenithal current density vector i with corresponding precision.

308

Journal of The Franklin

Institute

Magnetic Pield Variations in the Polar Cap A study of the diurnal variations of i discloses the following properties. For the central polar cap stations, i experiences a quasiuniform rotation. Figure 13 shows the azimuth variation in i at different levels of activity at the stations Vostok (Antarctic) and Thule (Arctic). The properties of the azimuth of i, seen in these diagrams and confirmed by analysis of data from the other stations, are: (1) The azimuth of i is ahead of the sun’s azimuth in both hemispheres. (2) The difference between the azimuth of i and the sun’s azimuth does not depend markedly on the level of magnetic activity. (3) The difference between the azimuth of i and the sun’s azimuth shows a diurnal variation, with a minimum at the time of the maximum current intensity.

z x = zi

VI

-1000

-

FIG. 12. Determination of the H, field for Scott Base from Mendel (13).

These properties of the i vector azimuth appear to be similar to a property of the perturbation vector associated with nighttime magnetic bays that was recognized in the Dumont d’urville st,ation data [Lebeau (17)]. Such nighttime bay disturbances, which are frequently observed in the central polar cap, account for the small nighttime maximum that appears in several curves in Fig. 3. These bays obviously coincide with polar substorms in the amoral oval. When they occur in the middle of a magnetically quiet period, a quietfield variation can be empirically interpolated through the bay; the perturbation caused by the bay is then defined as the deviation of the observed

Vol. 290, No. 3, September

1970

309

locale

and Thule [---,

p

all days; from Mendel (13)].

FIG. 13. Diurnal variation of the azimuth of i at Vostok

(at)

Heure

I

I

I

/’

14

I

I6

I

I

/’

18

I

i

I

I

20

I

I

I

I

24

(b)

Heure

22

I

I

I

locale

2

4

I

,

disturbed days; - - - - -, quiet days; -

I2

J, ,

I’

#’

,I i

i

:I

P

I’./’

215

. - * - . -,

Magnetic Field Variations in the Polar Cap field value from this interpolated value. An analysis of a hundred Dumont d’Urville’s bays reveals a regular rotation of the perturbation vector azimuth. Starting from this observation, Gillon (18) recently showed that the bays observed in the central polar cap increase the intensity of i but leave its direction strictly undisturbed. A study of the diurnal variation of the intensity of i shows the existence of a maximum that is close to local noon in the central region stations (Fig. 14).

L FIG.

14. Maximum

intensity time Hin+, vs. time of the local noon from Mendel (13).

This result deviates markedly from the result obtained for the K activity. We offer the following interpretation. The mean hourly values 1i 1are a measure of the mean intensity of the ionospheric current whereas the K indices represent the product of the mean intensity and the variability. We know that the mean intensity is maximum at local noon. If we assume that the variability is maximum at magnetic noon, we are led to predict a maximum of the K activity (product of the mean intensity and the variability) at an intermediate time. This interpretation of the time of maximum activity is consistent with the fact (see Section VI) that the crossing of the aurora1 belt has no bearing on the mean intensity level. VZZZ. Conclusions

and Unsolved

Problems

The partial results obtained for the K activity and the mean hourly values of the fields warrant the following general conclusions: The magnetic phenomena observed in the polar cap are revealed as a special aspect of the more general aurora1 phenomenon. Their main features are

Vol. 290, No. 3, September 1970

311

Andre F. Lebeau consistent with properties of the aurora1 phenomenon as presently known, provided that due allowance is made for the important effect of the ionospheric conductivity variations. It should be obvious that the study of certain aspects of the very-highlatitude phenomena could shed some light on the aurora1 phenomenon. More specifically, we have in mind a study of the relationships between the permanent phenomena and the polar perturbations since their relative levels in the central polar cap and in the aurora1 belt differ so greatly. Numerous problems remain unsolved and others are created by this analysis of very-high-latitude phenomena. The questions are : (1) The strong seasonal variation produced by the modulation of the ionospheric conductivity completely screens a possible variation associated with a particle mechanism. Is there or is there not a particle mechanism component in the seasonal variation ‘1 (2) Is the injection mechanism operative along the lines of force starting from the neutral point? And if so, do the injected particles create a specific magnetic phenomenon that could well be blurred by the main phenomena ? (3) What are the relations between the aurora1 arcs aligned in the sunearth direction, a specific feature of the central polar cap and the nighttime magnetic phenomena in the same region P Specific questions that arise in interpreting the aurora1 phenomena are: (1) Why does the mean density of the ionospheric current decrease steadily from the invariant pole to the lower latitudes and why is this decrease unaffected by the crossing of the aurora1 belts 1 (2) Why is this decrease lessened by a higher level of perturbations ? Undoubtedly, a general theory of the aurora1 phenomenon will one day provide the answers to these questions. It is not improbable that formulation of this theory will be contributed to by the phenomenologic knowledge drawn specifically from studies of the polar cap. References

(1) J. M. Stagg, “The diurnal variation of magnetic disturbance in the high latitudes”, Proc. R. Sot. Lond., A, Vol. 149 (867), pp. 298-311, 1935. (2) A. P. Nikol’ski, “Diurnal variation of magnetic disturbance in the high latitudes”, Probl. Arktiki, Vol. 4, pp. 5-43, 1938. (3) A. P. Nikol’ski, “Double aspect of magnetic perturbations at high Latitudes”, Probl. Arktiki, Vol. 2, 1948. (4) A. P. Nikol’ski, “Geographic distribution of magnetic activity in the Antarctic”, Dokl. Akad. nauk SSSR, Vol. 112, pp. 846848, 1957. (5) P. N. Mayaud, ‘ ‘ActivitB magnbtique dans les regions polaires”, in “Expdditions Polaires Franpaises, Terre-Ad&lie, 1951-1952, Magn&isme Terrestre”, Fast. 11, Paris. Published by Expeditions polaires franpaises, 1954. (6) N. Fukushima, “Gross character of geomagnetic disturbance during the international geophysical year and the second polar year”, in “Report of Ionosphere and Space Research in Japan”, Vol. 16 (l), pp. 37-56, 1962. (7) G. F. Rourke, “K index magnetic activity in the Antarctic”, in “Geomagnetism and Aeronomy, Antarctic Research Series” (edited by A. H. Waynick), Vol. 4, pp. 123-157, American Geophysical Union, 1965.

312

Journal

of The Franklin

Institute

Magnetic Field Variations in the Polar Cap (8) A. Lebeau,

“Sur l’activite

magnetique

diurne dans les calottes polaires”,

Ann.

Gkoph., Vol. 21 (2), pp. 167-218, 1965. (9) J. R. Spreiter and A. L. Summers, “On conditions near the neutral points on the magnetosphere boundary”, Planet. Space Sci., Vol. 15, pp. 787-798, 1967. (10) J. R. Spreiter, A. Y. Alksne and A. L. Summers. “External aerodynamics of the magnetosphere”, in “Physics of the Magnetosphere” (edited by R. L. Carovillano, J. F. MacClay and H. R. Rodoski), pp. 301-375, New York, Reidel, 1968. (11) A. Lebeau, “Sur la notion de midi magnetique”, C. r. Acad. Sci., Vol. 260, pp. 627-630, 1965. (12) J. Berthelier and A. Lebeau, “Calcul d’un temps magnetique approach6 et fondement experimental de la notion de temps magnetique”, Ann. Geophysique, Vol. 23, pp. 381-386, 1967. (13) A. Mendel, “Courants Blectriques dans l’ionosphere des regions de haute latitude”, D.E.S. Faculte des Sciences de 1’Universite de Paris (Groupe de Recherche Ionospherique, Note Technique 63), 1966. (14) A. Mendel-Berthelier, private communication, 1969. (15) C. Guerin, Etude synoptique des variations du champ magnetique dans les regions polaires. These de Doctorat 3* cycle, Universitd de Paris, 1970. (16) A. Lebeau and A. Mendel, “Courants Blectriques dans l’ionosphere des calottes polaires”, C. r. Acad. Sci., Vol. 266, pp. 643-646, 1968. (17) A. Lebeau, “Sur une propriete de l’activite magnetique nocturne a la station Dumont d’urville (Terre-Ad&lie), C. r. Acad. Sci., Vol. 253, pp. 1094-1096, 1961. (18) M. Gillon, “Arcs auroraux circumzenithaux et perturbations magnetiques a la station Dumont d’urville. These de Doctorat 3e cycle, Universite de Paris, 1970. (19) T. Hatherton, “Geometry of the southern aurora1 zone and the evidence for the existence of an inner zone”, Nature, Lord, Vol. 196, pp. 288-290, 1960. (20) E. R. Hope, “Low latitude and high latitude geomagnetic agitation”, J. Geophys. Res., Vol. 66, pp. 747-776, 1961. (21) A. Lebeau and J. Bitoun, “Sur l’activite magnetique diurne dans les regions de haute latitude et sur une propriete du vent solaire”, C. r. Acad. Sci., Vol. 255, pp. 3205-3207, 1962. (22) A. Lebeau and A. Mendel, “Intensite moyenne des courants Blectriques dans magnetique l’ionosphere des calottes polaires en fonction de l’activite planetaire”, C. r. Acad. Sci., Vol. 266, pp. 745-747, 1968. (23) A. Lebeau and R. Schlich “Etude des observations realisees it la station Dumont d’urville (Terre-Ad&lie)“, An&e Geophysique Internationale, Serie III, Fast. 3, Centre National de la Recherche Scientifique, 1962. (24) A. Lebeau and R. Schlich, “Sur une propriete de l’activite magnetique diurne dans les regions de haute latitude”, C. r. Acad. Sci., Vol. 254, pp. 1014-1016, 1962. Space Sci. Rev., (25) K. D. Cole, “Magnetic storms and associated phenomena”, Vol. 5 (6), pp. 699-770, 1966. (26) D. H. Fairfield, “Ionosphere current patterns in high latitude”, J. Geophys. Res., Vol. 68, pp. 3589-3602, 1963.

Vol.290,No. 3, September1970

313