The F2-region equatorial anomaly in the African, American and East Asian sectors during sunspot maximum

Journal of Atmospheric and Terrestrial Physics, 1963, Vol. 25, pp. 373 to 386. Pergamon Press Ltd. Printed in Northern Ireland

The F~-region equatorial anomaly in the A~rican, American and East Asian sectors during sunspot maximum A. J. LYON* and L. THOMAS D.S.I.R. Radio Research Station, Ditton Park, Slough, Bucks. t

(Received 28 February 1963) Abstract--A study has been made of the characteristics of the F2-region equatorial anomaly in the African, American and East Asian sectors during quiet days of the 1958 equinoxes, particular attention being given to its diurnal development. It is shown that the evening enhancement of tile crests north and south of the magnetic equator occurs first on the side near to the geographic equator. An examination of the latitude variation of hmF2, derived from M(3000)F2 data, for the evening hours shows a marked asymmetry about the magnetic equator, the greater heights also occurring on the side near to the geographic equator. It is suggested that this sequence in the development of the crests, together with the height asymmetry, supports the hypothesis that they result from diffusion of ionization along magnetic lines of force from above the magnetic equator. 1.

I N T R O D U C T I O l h~

THE equatorial a n o m a l y of the ionospheric F2-region was first recognised by APPLETON (1946) and has since been investigated b y m a n y workers. The main feature of the a nom al y is t h a t the peak electron density, NmF2, shows a smooth w~riation with magnetic dip with a m i ni m um near the magnetic equat or and crests near magnetic dips 30°N and S. The diurnal devel opm e nt of t he anom al y during years of sunspot m i ni m um has been studied by RASTOG: (1959), who showed t h a t the anom al y is most pronounced at a bout 1400 L.M.T. and t h e r e a f t e r declines steadily until it finally disappears by 2000 or 2100 hours. However, MARTYN (1959), using t he somewhat limited d ata available for the sunspot m a x i m u m of 1947, found t h a t the anom al y persists into the night until a bout 0300 hours b u t tends to decay during the evening hours. More recently, studies of I G Y dat a b y APPLETON (1960) and WRmHT (1960) have revealed a substantial e n h a n c e m e n t of the crests on either side of the magnetic e q u a t o r during the late evening hours. RAo (1962) has s t a t e d t h a t the diurnal development for this epoch of the sunspot cycle is generally similar to t h a t described b y RASTOGI (1959) for sunspot m i ni m um b u t delayed some 3 hours. Most of t he previous studies have made use of m o n t h l y median dat a which include the effects of a n y ionospheric storms t h a t occur. This complication is p r o b a b l y more serious near sunspot m a x i m u m when, in some months, almost all days are magnetically disturbed. T he present work deals chiefly with t he characteristics of the anom a l y at the two equinox periods of 1958 and is based on dat a for selected quiet days only. The extensive net w or k of stations operating during the I.G.Y. has made it * On leave from University College, Ibadan, Nigeria. + Official c o m m u n i c a t i o n . 373

374

A.J. LYoN and L. THOMAS

possible to s t u d y the equatorial anomaly in the African, American and East Asian sectors separately. The distributions of stations considered in these sectors are shown in Fig. 1 ; the data for Indian stations, situated between meridians 60°E and 90°E, were found to be consistent with those in the African sector and have been included to provide a more detailed variation. Also shown in Fig. 1 are lines of constant magnetic latitude (A); this parameter has been derived from magnetic dip (I) using the relation tan A---- ~ t a n I The main part of the investigation is concerned with the variation of NmF2, as indicated byfoF2, with magnetic latitude at different hours local time. In studying ~2o o I

/

60 N

~o I

oo

6o o

I

AMERICAN SECTOR

~2o o

I

I

"AFRICAN SECTOR

EAST A~A SECTOR

$o Z

•

"

_ 30°

. .

•

.Z._

J

3o°

WEST

LONG ITUDE

EAST

Fig. 1. Geographic location of stations. Also shown are curves of constant magnetic latitude. the diurnal development of the anomaly, particular attention has been paid to the details of the late evening enhancement of the anomaly. According to the "fountain t h e o r y " of MAI~TYN (1955) and DI;~cA~~ (1960) ionization is transported vertically upwards near the magnetic equator and diffuses down the lines of force to cause the concentrations at the latitudes of the crests north and south of the magnetic equator. I t is, therefore, of interest to investigate whether the latitude variation o f f oF2 is more symmetrical about the magnetic equator during the equinox period or during the period when the noon sun is overhead at the magnetic equator. For convenience the latter will be referred to as the "dip equinox" period. I t will be shown, however, t h a t the variations for these "dip equinox" periods show marked asymmetries similar to those which occur at the solstices and some of which have been described recently

The F2-region equatorial anomaly

375

b y Lyo_~" (1963). Hence the main part of the study relates to the geographic or true equinox periods. The details of the analysis including the locations of the stations considered, the selection of quiet days and their groupings into seasons, and the statistical t r e a t m e n t of the results are fully described in the Appendix. 2.

SOLSTICE

ASYMMETRIES

A:ND

THE

EQUINOX"

"DIP

Two types of a s y m m e t r y are observed in the latitude variations offoF2 near the solstices: in the daytime, as shown b y LYo~ (1963), foF2 is commonly found to be higher at magnetic latitudes on the winter side of the magnetic equator than at equal magnetic latitudes on the summer side. This a s y m m e t r y is observed even at quite low latitudes. It has been found in the course of the present investigation that at night an a s y m m e t r y occurs in the opposite sense, that is with foF2 higher on the summer side, especially in the hours prior to sunrise. The curves in Fig. 2 show the quiet-day variations of foF2 with magnetic I

~F2

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[

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I

I

I

I

[

I

I

I

I

I

I

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1

12

12

I0

[0

8

(Mc/s)

~

6

'

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/..

0400

.=)

4

4

SOUT HERN SOLSTICI PERIOD

DIP EQUIN 3X PERIOD, TRUE EC UINOX PERIOD, NOR[ SOLSTIC PERIOD MA tCH MAI

r.\

=/ir'~/,- ,f

14

•

12

/

8 I

i

I

60 S

40

20

I

0

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t

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20 40

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50 S

40

20

0

I I I I I t 2 0 4 0 60 5 0 4 0 N S MAGNETIC

l;o

A,~ # ;

13 IO

6

,Mcls~ 6

14

12

I I I I I I

60

0

20 40

60 60 40 N S

20

0

20

40 50 N

LATITUDE

Fig. 2. Variations off oF2 with magnetic latitude at 0400 and 1300 hours in tile American sector during 1958. latitude at 0400 and 1300 hours during different periods of 1958 for the American sector. The curves for the northern and southern solstice periods illustrate the .two types of a s y m m e t r y just mentioned for day and night conditions. I t is seen that the variations for the true equinox period are approximately symmetrical about the magnetic equator b u t those for the "dip equinox" period have marked asymmetries similar to those occurring during the southern solstice period. Since the magnetic equator in the American sector occurs south of the geographic equator, Fig. 1., the "dip equinox" period is nearer in time to the southern solstice than to the northern solstice. This general situation--fairly good s y m m e t r y about the magnetic equator at the true equinoxes and the appropriate solstice asymmetries appearing at the

376

A.J.

LYON

and L. THONAS

"dip equinoxes"--is found throughout the day and night both for the March and September equinoxes and in both the African and American sectors. It is not possible to estimate from the present analysis at what solar declination the asymmetries characteristic of the solstice periods first develop. It is, however, clear that they are present very soon after the sun has crossed the geographic equator towards its solstice position, and may be present even earlier. For the remainder of the paper attention will be confined to the true or geographic equinox period. 3. THE DIURNAL DEVELOPMENT OF THE ANOMALY The quiet-day variation offoF2 with magnetic latitude at each hour has been used to examine the diurnal development of the equatorial anomaly during the March and September equinoxes of 1958 for each of the three sectors. An examination of all the results has shown that the development of the anomaly in each sector is very similar in the two equinox periods. The sample curves of Fig. 3 2O

20

AFRICAN

,,,,,,,

,2.oo,

15

SECTOR tS

.foF:

0800

I

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tS

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;

20.00

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25

5O N S

N S MAGNETIC

i

I,o

~ 0000

S

J. F2 i0 {M~$)

N

S

04 oo 5025 N S

O

25

SO N

5

LATITUDE

Fig. 3. Variations offoF2 with magnetic latitude at two-hourly intervMs during the equinox period, 5Iareh, 1958. show the latitude variations at 2-hourly intervals for the African and American sectors during March, 1958; the corresponding curves for East Asia show similar changes to those in the African sector. Attention is drawn to the following features of the development and decay of the anomaly: (a) The anomaly persists throughout the night at least until 0200 hours and sometimes until sunrise. This is in contrast to the diurnal changes at sunspot minimum when, according to RASTOGI (1959) and MARTYN (1959), the anomaly has completely disappeared by 2000 or 2100 hours. There is also some evidence that the anomaly persists longer in the African and East Asian sectors than in the American sector. (b) After sunrise there is usually no distinct anomaly and for several hours the ionization tends to show a m a x i m u m near the equator. In most eases this m a x i m u m is displaced away from the magnetic equator towards the geographic equator.

The F2-region equatorial anomaly

377

(c) A pronounced anomaly is present b y 1000 hours in the African and E a s t Asian sectors b u t does not appear until a b o u t 1200 hours in the American sector. (d) From about 2 hr after the time of the first appearance of the anomaly until about 1800 hours the whole latitude variation between 50°N and 50°S remains fairly constant. During this period NmF2 at the equatorial trough is only about 60 per cent of its value at the crests on either side. (e) After 1800 hours the anomaly becomes more pronounced as shown b y APPLETON (1960) and W~mHT (1960). (f) From 0100 hours the anomaly decays and this is chiefly due to a rapid decrease in foF2 at the crests. 4. THE EVENING ENHANCEMENT OF THE EQUATORIAL ANOMALY

The median values offoF2 for the September, 1958, equinox period at Bogota (A =- 18°N) and Buenos Aires (A ----- 17°S) shown in Fig. 4 indicate the variations

20

14 I

18

16 I

1~, I

BOGOTA

Jo F2

2O

22

I

I

O0 I

}

(Mc/s) 16

14

15

14

12

HUANCAYO

12

t

I0

5o F2 (Me/s)

Bo .os A,RES ttt ~,

I 14

4

,t

I 16

,t

,t

I IB

I 20

I 22

.I O0

L.MZ

Fig. 4. Median values o f f o F 2 for the equinox period, September, 1958. Bogota and Buenos Aires are located near the northern and southern crests of the anomaly respectively in the American sector, while t t u a n c a y o is situated near the equatorial trough.

occurring near the northern and southern crests respectively in the American sector; the values for H u a n c a y o (A = 0) show the corresponding variation near the equatorial trough. In each case the vertical lines represent the standard error

378

A.J. LYoN and L. T~OMAS

associated with each median (of. Appendix); only lower limits for these standard errors could be estimated for the evening hours at Buenos Aires. I t is evident t h a t the enhancement of the anomaly during the evening hours is caused both by a reduction in N m F 2 at the equatorial trough and increases near the crests. A comparison of the variations at Bogota and Buenos Aires suggests t h a t the northern crest is enhanced before t h a t in the south; the absolute value reached b y f o F 2 at Buenos Aires is limited by the movement of the southern crest towards the magnetic equator after 2000 hours which will be shown in section 5. This feature of the evening enhaneement of the anomaly in the American sector' is shown more clearly by a s t u d y of the variations offoF2 with magnetic latitude at each hour. I t is found t h a t after the northern crest has attained its m a x i m u m value of about 19 Mc/s at 2000-2100 hours, it declines steadily but the southern peak continues to increase, and by 0100 is predominant.. A similar sequence occurs in the African and East Asian sectors but in both the southern peak is enhanced first. This reversal in the development of the northern and southern crests between the American and African sectors can be seen in Fig. 3. To illustrate the phenomenon more clearly the detailed results for selected hour's in the March, 1958, equinox period are given in Fig. 5. From the relative positions of the magnetic and geographic equators in each section, Fig. 1, it can be concluded t h a t in all three sectors it is the crest situated near the geographic equator which shows the earlier enhancement, some 2 or 3 hr after sunset, while the other crest attains its m a x i m u m some 3 hr later. I t has been suggested by WRm~T (1960) and others t h a t this evening enhancement is due to diffusion of ionization along lines of force from great heights above the magnetic equator and it is, therefore, of interest to examine the latitude variation of hmF2 during the evening hours. In the absence of' adequate hmF2 data, estimates of height have been made using M(3000)F2 data. A comparison of the available hmF2 data and values of M(3000)F2 at a number of stations between latitudes 40°N and S shows t h a t from 1800 to 0600 hours an approximately linear 1

relation exists between hmF2 and ~ ,

hmF2--

namely 1650

M

225

(!)

Figure 6 shows the latitude variations of M(3000)F2, with values increasing downwards, for the African and American sectors for 1800 and 2000 hours during March, 1958. The corresponding scale for hmF2, as given by (1), is also included. There is clearly a marked a s y m m e t r y in hmF2 about the magnetic equator in the period during which the evening enhancement of the anomaly occurs. The more limited data available for the East Asian sector also shows such an asymmetry. Furthermore, for the American sector the northern latitudes show the greater heights while for the African and East Asian sectors the southern heights are greater. The difference in hmF2 between equal latitudes in the range 10-20 ° north and south of the magnetic equator is of the order of 100 kin.

T h e F 2 - r e g i o n equatorial anomaly

379

, ,AFRICAN SECTOR JoF2 f5

I

I

/"

I

I

AMERICANSECTOR

{

/

to

J

i / ~,

2tOO

O

4O

4NI o £5 MAGNETIC

6

4o N

LATLTUDE

Fig. 5. Variations of median values o f f oF2 with magnetic latitude for selected hours during the equinox period, March, i958. 5.

CHANGES IN L A T I T U D E OF THE CREST AND OF THE E Q U A T O R I A L T R O U G H

RASTOGI (1959) has reported that during the development of the equatorial anomaly at sunspot minimum the northern crest shifts to higher latitudes during the early afternoon and later approaches the equator again. The present data have, therefore, been examined to enquire whether any such latitude shift is observable at sunspot maximum. The large amount of data available for the African and American sectors has made it possible to study changes in the positions of crests and of the equatorial trough in each separately, as shown in Fig. 7. For the East Asian sector the data are insufficient to study these changes. There is some uncertainty in the positions of the crests and troughs at certain hours and 2

A. J. LYON and L. THOMAS

380 t'6q hmF2 M(3000)F2 I (knt,t, 700

~

2'0---600

AFRICAN

SECTOR

/

/

~

800

2"4-- 500/

/

/~ !2000

i

-400

2.8-

I

I I J

I

30 20 I0 S

0

W

IO 20 30 N

I

30 20 I0

J

O

IO

20

S

30 N

AMERICAN SECTOR 20-

-600 -50(

2'4-

/

/

~2OOO

-40(/ 28-

Z f

I

O 40 20 3(D N

I

[

3O 20 I0 O IO 2O 3O S N MAGNETIC LATITUDE Fig. 6. Variation of median values of M(3000).F2 with magnetic latitude for 1800 and 2000 hours during the equinox period, March, 1958. The scale of 2V/(3000)F2 is plotted downwards and that of hmF2, derived using (1), is plotted upwards. :30 2 0 $

tO

care is n e c e s s a r y in d r a w i n g conclusions. H o w e v e r , it is believed t h a t the following features are significant: (a) F o r several h o u r s a b o u t sunrise t h e s i t u a t i o n is c o m p l e x b u t t h e r e is evidence t h a t t h e a n o m a l y is shifted 5-10 ° n o r t h w a r d s in t h e A f r i c a n sector. (b) I n t h e African s e c t o r t h e crests t e n d to m o v e a p a r t b y a few degrees d u r i n g the p e r i o d 1000-1500 h o u r s a f t e r w h i c h t h e y r e m a i n b e t w e e n 16 ° a n d 19 ° n o r t h or south. T h e r e is little evidence of such a m o v e m e n t in t h e A m e r i c a n sector d u r i n g these h o u r s a n d t h e crests o c c u r at slightly smaller l a t i t u d e s d u r i n g the a f t e r n o o n , n e a r 13-17 ° .

The F2-region equatorial anomaly AMERICAN SECTOR

AFRICAN SECTOR

S 20

IO

O

IO

I

~

I

I

,,

381

N L.M.'I: S 20 20 I I

IO

O

I

IO

N 2O

I

I

0000

x,

I

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%

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x

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I i"

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,~

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/ s

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~l

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ii1 I

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i

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I/

I II

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iii

1600

,

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x

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x I

I ' , I

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= I

l

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i

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,i

2000

1

,' f

! !

%

t

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11

i

20

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)

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11

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x i

oooo I

20 N

MAGNETIC

! ¢

Ih

l

I

20

10

S

,

0

1

I0

I

2o N

LATITUDE

Fig. 7. Changes in the magnetic latitudes of the equatorial trough and of the sub-equatorial crests with time, during the equinox period, September, 1958.

(c) T h e r e is s o m e e v i d e n c e t h a t in t h e A f r i c a n sector t h e e q u a t o r i a l t r o u g h t e n d s to o c c u r 1 ° or 2 ° to t h e s o u t h of t h e m a g n e t i c e q u a t o r d u r i n g t h e a f t e r n o o n a n d n i g h t w h e r e a s in t h e A m e r i c a n sector t h e r e t e n d s to b e a slight d i s p l a c e m e n t to t h e n o r t h d u r i n g t h i s period. (d) F r o m a b o u t 2100 h o u r s t h e crests a p p r o a c h t h e e q u a t o r a n d o c c u r a t l a t i t u d e s 10-13 ° n o r t h a n d s o u t h b y 0000 h o u r s in t h e A m e r i c a n sector a n d b y 0300 h o u r s in t h e A f r i c a n sector.

382

A.J. LYON and L. THOMAS

6. DIscussIol~ AND CONCLUSIOI~S From the foregoing results some details of the equatorial anomaly at sunspot maximum have been clarified and the following features have emerged: (a) During the solstice periods the latitude variation of foF2 tends to show marked asymmetries about the magnetic equator and these are in opposite senses during the day and night hours. (b) These asymmetries appear at small solar declinations and are well marked during periods when the noon sun is overhead at the magnetic equator. (c) The latitude variation for a period centred on the true equinox shows very good symmetry about the magnetic equator. (d) The late evening enhancement of the peaks of the anomaly occurs earlier on the side of the magnetic equator near the geographic equator and the layer is higher on that side. Theories of the asymmetries observed during the solstice will not be considered here. It is clear, however, that such a theory must account both for their occurring over a remarkably wide range of latitude and for the marked reversal in sense occurring between day and night. The result that the evening enhancement occurs earlier where the layer is higher may, perhaps, be regarded as evidence in favour of theories of the anomaly which involve a diffusion mechanism. For example, if the layer is say a scale height higher at 15 ° N than at 15 ° S magnetic latitude during the evening hours, as our evidence suggests, then the line of force passing through the F-layer peak on the northern side will be about a scale height higher at the equator than the corresponding line on the southern side. Since the coefficient of diffusion increases as e z, where z is the height measured in scale heights, the above result implies that this coefficient should be about 3 times greater on the northern side. It is well known (APPLETON, 1960) that hmF2 begins to rise rapidly at about 1800 hours in a rather narrow belt centred on the magnetic equator. If this represents an upward vertical drift of ionization, the evening enhancement may be due to a renewed evening "fountain-effect" similar to that suggested by MART¥¢~ (1955) and DUNCAN (1960) to explain the anomaly in daytime. Alternatively, this diffusion along lines of force may be associated with thermal contraction which might begin at sunset, also near 1800 hours. In either case then the enhancement develops in some 2-3 hr on one side of the magnetic equator and in about 6 hr on the other. This separation in time is therefore consistent with the suggested factor of about 3 between the diffusion coefficients deduced from the differences in height. Other points of detail which have appeared in the present study are as follows: (a) The anomaly appears earlier and persists longer in the African and East Asian sectors t h a n in the American sector. (b) During the period 1000-1500 hours the separation of the peaks in the African sector increases but no such effect has been detected in the American sector. (c) From about 2100 hours the peaks approach the equator and occur at magnetic latitudes 10-13 ° north and south by 0000 hours in the American sector and by 0300 in the African sector.

The F2-region equatorial anomaly

383

Acknowledgements--The work described above was carried out as part of the programme of the Radio Research Board, and this communication is published by permission of the Director of Radio Research of the D e p a r t m e n t of Scientific and Industrial Research. REFEREIqCES APPLETON E . V . APPLETON E . V .

1946 1960

DUNCAN 1:~. A. I~ozo T. LYoN A . J .

1960 1931 1963

MARTYN ]). F. MAICTYN D . F . PIGGOTT W . R .

1955 1959 1960

RAO C. S . R . ]~ASTOGI R . G . W~IGHT J . W . YULE G. U. and KENDALL M. G.

1962 1959 1960 1949

Nature, Lond. 157, 691. Some Ionospheric Results Obtained during the I . G . Y .

(Ed. by W. J. G. BEYNON), p. 3. Elsevier, Amsterdam. J. Atmosph. Terr. Phys. 18, 89. Biometrika 23, 315. The Ionosphere, p. 88. Institute of Physics and Physical Society, London. The Physics of the Ionosphere, p. 254 Physical Society, London. Proc. Inst. Radio Engrs., N . Y . 47, 147. Some Ionospheric Results Obtained during the I . G . Y . (Ed. by ~V. J . G. BEY~oN), p. 116. Elsevier, Amsterdam. J. Atmosph. Terr. Phys. 24, 729. J. Geophys. Res. 64, 727. J. Geophys. Res. 65, 185. A n Introduction to the Theory of ~b'tatistics. Griffin, London. APPENDIX

DETAILS OF METHODS OF ANALYSIS

(i) The selection of quiet days In selecting quiet days the following criteria, based on the magnetic Ap figure, have been adopted: (a) Ap ~ 15, on the d a y considered. (b) Ap < 25, on the day before. (c) Ap < 60, 2 days before. As ionospheric disturbances sometimes occur without any obvious connection with magnetic disturbance, use has also been made of a catalogue of ionospheric disturbance compiled by I:)IGGOTT(1960) for days during the IGY. In this catalogue au F2 disturbance index W, based on a study of conditions at a world-wide network of stations, was assigned to every day. This index ranges in integral steps from 0 to 5 in which 0,1 represent ionospherically quiet conditions. The further requirement has, accordingly, been applied t h a t days selected as ionospherically quiet should have a W value of 0 or 1. In fact, only a few days had to be rejected on this basis after the initial selection based on the Ap figures. Finally, sample f-plots from a number of stations were examined for a number of the days selected. I t was found, t h a t in any one season, the diurnal variations of fl>F2 were sufficiently similar on the selected days t h a t an averaging procedure would give a good representation of typical variations. (ii) The true and "dip" equinox periods For the purposes of the present s t u d y the true equinox periods are defined as the periods within 30 days of the equinox days. This means t h a t the sun's declination will v a r y from about 12 ° north to 12 ° south. A similar number of days have been included in the solstice periods for which results have been included in Fig. 2.

384

A.J.

LYOl~ a n d

L . THOMAS

Geographic

Name

Magnetic Latitude

African

Longitude

Latitude

sector

Kiruna

67050 ' N

20026 , E

65°N

Sodankyla

67022 , N

26°39 ' E

65

Uppsala Slough De Bilt

59o48 , N 51031 ' N 52o06 ' N

17°35 ' E 00034 , W 05°11 ' E

59 50 50

Dourbes Pruhonice Freiburg

50°06 ' N 49°59 ' N 48003 , N

04035 ' E 14°33 ' E 07o35 , E

49 47 46

Graz Schwarzenberg

47o06 , N 46050 , N

15°27 ' E 07°21 , E

45 45

Poitiers Genova Rome Rabat *Delhi *Ahmedabad

46o34 44033 41o54 33o56 28035 23001

00o21 08o57 12°30 06o50 77013 72°36

44 42 38 30 24 18

*Calcutta Tamanrasset

22033 ' N 22048 , N

88021 , E 05031 , E

17 14

*Bombay Dakar *Madras

19o00 , N 14o40 ' N 13000 , N

73°00 , E 17 ° 2 6 ' ~V 80°15 , E

13 10 5

Djibouti

11°33 ' N

43o09 , E

4

10°50 ' N 10o14 , N 08o25 , N

78o50 , E 77029 , E 77o00 , E

2 2 0

Ibadan Bangui Bunia

07022 ' N 04o36 , N 01o32 ' N

03°58 , E 18035 , E 30°11 ' E

3°S 8 11

Leopoldville Elizabethville Tananarive

04022 , S 11°39 , S 18°55 , S

15°15 , E 27028 , E 47033 ' E

19 28 34

Salisbury Tsumeb Johannesburg

17o50 ' S 19°14 ' S 26°12 ' S

31°00 ' E 17o43 ' E 28002 ' E

36 36 44

Cape Town

34°08 ' S

18o19 ' E

46

Winnipeg St. Johns W'ashington

49054 ' N 47032 , N 38o44 , N

97°24 ' W 52°47 ' W 77o08 , W

66°N 57 57

San Francisco White Sands Grand Bahama Puerto Rico Panama Bogota Talata

37°26 32°18 , 26040 ' 18030 ' 09o24 , 04o32 , 04034 ,

*Tiruchirapalli *Kodaikanal *Trivandrum

* Indian

stations

American

sector

, , , ' , ,

N N N N N N

, , ' , , '

E E E W E E

included

N N N N N S

122°10 ' 106°30 ' 78°22 ' 67010 , 79o54 ' 74o15 ' 81°15 '

V~" W W" W V~" V~T W

43 4i 40 32 21 18 6

The F2-region equatorial anomaly

385

Geographic Name

Latitude

Chiclayo Chimbote Huancayo Natal La Paz L a Quiaea Tucuman Sao P a u l a B u e n o s Aires Concepcion Trelew Port Stanley l)ecepcion Port Lockroy Ellsworth

Magnetic Latitude

Longitude

06°48 ' 09004 ' 12°03 ' 05°20 ' 16°29 , 22o06 , 26o53 ' 23033 ' 34o36 , 36035 ' 43°14 ' 51o42 , 62059 ' 64o50 ' 77o43 '

S S S S S S S S S S S S S S S

79°19 , 78°35 ' 75020 , 35o07 , 68°03 ' 65o36 , 65023 ' 46o38 , 58°29 ' 72059 , 65°18 ' 57o51 ' 60o43 ' 63°31 ' 41°08 '

W W" W ~T ~V ~ W W" W W" ~V Vv~ W W W

5 4 0 0 2° S 7 11 12 17 20 23 27 36 38 50

45°24 ' 39o44 ' 35°42 ' 31°13 ' 26o19 ' 25°02 ' 22°12 ' 16025 ' 01o19 ' 02°30 ' 19019 , 27o32 ' 35°19 ' 42o55 '

N N N N N N N N N N S S S S

141°41 ' 140o08 ' 139°29 ' 130°38 ' 127o47 ' 121°31 ' 113°33 ' 120°35 ' 103o49 ' 140°31 , 146°44 ' 152o55 ' 149 ° 0' 147°10 '

E E E E E E E E E E E E ]g E

40 ° N 33 30 25 20 18 16 10 9° S 10 29 38 47 59

E a s t Asian sector "Wakkania Akita Kokobunji Yamagawa Okinawa Taipei Macao Baguio Singapore Hollandia Townsville Brisbane Canberra Hobart The

"dip

declination equator. between

equinox"

were confined to intervals

during

which the solar

This was considered desirable in order to distinguish as clearly as possible the "dip equinox"

the true and "dip" (i)--are

periods

did not vary by more than 5 ° below and above its value at the magnetic and solstice conditions.

equinox periods--selected

listed in Table

The actual

days included

in

according to the criteria described in

1.

T a b l e 1. Q u i e t d a y s d u r i n g M a r c h 1958 a n d S e p t e m b e r , 1958, e q u i n o x p e r i o d s M a r c h , 1958 T r u e e q u i n o x : F e b . 15, 2 4 - M a r . 2, 28, A p r . 8-13, 21-24. D i p e q u i n o x : A f r i c a : A p r . 8-13, 21-27, M a y 3. D i p e q u i n o x : A m e r i c a : F e b . 1-3, 15, 2 4 - M a r . 2. S e p t e m b e r , 1958 T r u e e q u i n o x : A u g . 30, 31, Sep, 1, 2, 11-15, 18-24, 28, 29, Oct. 5, 6, 9-21. D i p e q u i n o x : Africa: Aug. 12-16, 20, 21, 29-31, Sep. 1, 2, 11, 12. Dip e q u i n o x : A m e r i c a : Oct. 9-21, 31, N o v . 1-9.

386

A . J . LYON and L. THOMAS

(iii) The distribution of stations The stations included in the three longitude sectors are listed in order of decreasing magnetic latitude. (iv) Statistical treatment of data The foF2 and M(3000)F2 data for the 20-30 quiet days selected in each season were extracted from the hourly data published for each station, and median values for each hour have been calculated. The diurnal variations obtained from these median values at each station have been used to allow for any difference between the longitude of the station and the meridian adopted. Naturally, there are some missing data at all stations b u t the inclusion of 20-30 days has ensured that a reasonable number of values are available at most stations at most hours of the day. Although different sets of days have sometimes been used at different stations, the medians should be comparable provided the variations from day to d a y at each station are not too large. In order to estimate the magnitude of these variations the interquartile range has also been determined for each hour and the standard error of the median computed. Medians and quartile ranges have been used in preference to means and standard deviations to make it possible to include values qualified by D (greater than) or E (less than). For a large sample taken from a normal distribution the standard error, Sin, of the median is derived from the quartile range, Q, using the relation (YVLE and KENDALL, 1949) Sm=

(0.941 \~-~ ] Q

(2)

where ~ is the number in the sample. For the smaller samples encountered in the present study, different numerical factors for each value of ~ replace the bracketed term in (2). These factors have been determined using some results of H o J o (1931).