World wide study of horizontal drift and anisotropy of ionospheric irregularities in the E-region

Journal of Atmospheric and Terrestrial Physics, 1963, Vol. 25, pp. 553 to 569. Pergamon Press Ltd. l'rinted in Northern Ireland

World wide study of horizontal drift and anisotropy of ionospheric irregularities in the E-region G. L. NARAYANA RAO and B. RAMACI~ANDRA RAO Ionospheric Research Laboratories, Andhra University, "Waltair, India (Received 13 May 1963) Abstract An attempt has been made to study the latitude variation of anisotropy and drift parameters of ionospheric irregularities in E-region using IGY data of Waltair~ Yamagawa, Debilt and Halley Bay. A systematic variation in most of the drift and anisotropy parameters is observed from equatorial to very high latitude stations with the exception of polar regions for which the values are found to be entirely different. Harmonic analysis of N-S and E-W components has shown that the semi-diurnal component is predominant in all stations except Waltair. The sense of rotation is found to be clockwise in northern hemispher(~ for all stations except Yamagawa and anti-clockwise in southern hemisphere. The results were compar(~d with those of earlier investigators at different latitudes.

1. I~¢TRODUCTION SEVERAL investigators have studied the horizontal ionospheric movements and numerous methods have been developed for the observation of horizontal ionospheric movements. All these methods rely on the existence of ionospheric irregularities. Among the radio measurements the spaced receiver method is employed by m a n y investigators, MITRA (1949) applied this method to find the speed of the drift and the fundamental assumptions involved in this method are t h a t the amplitude p a t t e r n formed over the ground due to diffractive reflection of a radio wave from the ionospheric irregularity is statistically isotropic and t h a t it drifts bodily without a n y change in detail. I t is now widely known t h a t the amplitude p a t t e r n formed on the ground is anisotropic and is subjected to random changes in detail as it moves. The method due to [BRIGGSet el. (1950) considers the statistics of isotropic p a t t e r n drift over the ground which includes the possibility t h a t the pat t ern is changing due to ran d o m motions superimposed upon a steady drift. This method has been modified by PmLLIPS and SPENCER (1955) to take into account the fact t h a t the ground p a t t e r n is not always statistically isotropie. The following parameters were estimated using expressions derived by PHILLIPS and SPENCER (1955) and SALES ( 1 9 6 0 ) . (i) The size (a), axial ratio (r) and orientation of characteristic ellipse (~ measured anti-clockwise from east) the radius of which in any direction gives the separation in t h a t direction between two points having a correlation of 0.5. I f the ellipse is a circle, the correlation is independent of direction and the pat t ern is said to be isometric. (ii) The magnitude V and the direction ¢ of the mean velocity of the diffraction p a t t e r n over the ground. (iii) A p ar am e t e r V~ which has the dimensions of velocity and which is a measure of the rate at which the p a t t e r n changes as it moves. I f V, = 0 the pat t ern drifts without change of form. With a view to studying the world wide variation of drift parameters, original drift records from W~ltair, Yamagawa, Debilt and Halley Bay were obtained and the 553

554

G.L.

~NTARAYANA I:~AO a n d B. RAMACHANDRA RAO

records were analysed by the correlation method using manual methods for calculations. As the investigation of drift parameters by manual methods is very laborious, the investigation is confined to the above four stations each of which represents the characteristics of ionospheric irregularities in a particular range of latitudes. The investigation was confined to the period August-October, 1958. The results of the investigation of the horizontal movements and the anisotropy of ionospheric irregularities for the E-region are presented in this communication. The locations of observatories in the present investigation and the frequencies of waves used in the measurements are given in Table I. "Fable 1. L o c a t i o n s o f o b s e r v a t o r i e s in t h e p r e s e n t i n v e s t i g a t i o n a n d frequencies of waves used

Station

Geographic latitude

Geomagnetic latitude

F r e q u e n c y in Mc/s

Waltair Yamagawa Debilt Halley Bay

17"7 ° N 31.2 ° N 52.1 ° N 75.5 ° S

7.5 ° N 20.3 ° N 58.8 ° N 65.7 S

2-2.5 2-3.0 2.3 2.2

2. EXPERIMENTAL TECHNIQUES AND OTHER DETAILS

The E-region drift speeds at Waltair were measured by a spaced receiver method which had been suitably modified by using a modified tripp]e beam oscilloscope for recording the fading pattern and the details of which were given by RAO et al. (1956). The aerials were placed at the corners of a right angled triangle with short sides equal to 108 m. At Yamagawa a pulse transmitter of 60 cycles repetition frequency with a peak power of 2 kW was used and the three aerials were placed to make pairs falling at right angles to each other in a span of l l 0 m. The amplitudes of the received echoes for each aerial were recorded in three lines on a photo film. At Debilt the aerials were placed at the corners of a right angled triangle with short sides equal to 93 m and the received echoes were photographed on a film. The three aerials at Halley Bay were arranged at the corners of a right angled triangle with short sides, 153 m, and the amplitude of the received echoes were recorded by a three pen recorder similar to t h a t of the method due to SALZBERGand GREENSTONE (1951). The records which are used in the present investigation are found to satisfy the following properties. The fading records are statistically stationary containing at least six fading cycles and the fading records do not possess high a degree of random fading, so t h a t an auto-correlogram drawn for one curve could be considered as representative of the three fading curves. About 60 equally spaced ordinates were measured on each of the three tracks and the correlation coefficients were calculated manually. In all about 100 records were analysed. Night time records for Debilt and Halley Bay are not available. 3. ANISOTROP¥ OF IONOSPHERIC IRREGULARITIES

(a) A x i a l ratio Histograms for the axial ratio " r " of the characteristic ellipse are shown in Fig. 1. For Waltair the frequency of occurrence is maximum in the range of 2.0-2.5 with

Study of horizontal drift and anisotropy of ionospheric irregulavit.ies

555

most probable value of 2-2. :For Yamagawa the peak has occurred in the range of 1.5-2.0 with a most probable value of 1.6 and for Debilt the frequency of occurrence is max imu m in the range of 1.0-1.5 with a most probable value of 1.4. :For Halley B a y in the polar region the axial ratio is higher as the peak occurred in the range of 2.0-2.5 with a most probable value of 2.2. This value is very near to the value observed at the low latitude station of WMtair. Considering the most probable values of axial ratio for the four stations, it m a y be noted t hat there is a gradual 12

WALTAIR

YAMAGAWA

IO

u) z 0

w u) I,n 0

--

DE B I L T

0

~E z

if O

I

.ALLEY L__

2

3

4

AXIAL

RATIO 'r"

1 1

5

Fig. 1. Histogram for the axial ratio "r" for the E-region. decrease of the axial ratio with increase of latitude up to polar regions where the value increases once again rather abruptly. The same conclusion m a y be drawn from the average values of " r " presented in Table 2, with the difference t h a t the average values of "r" for Waltair and Yamagawa are nearly the same. The diurnal variation of "r" is depicted in :Fig. 2. The variation of "r" for Waltair shows a sharp peak, with an a br upt drop in the morning hours, and there is not

556

G.L.

•ARAYANA

]~AO a n d B. I~AMACtIANDRA ]~AO

:Fable 2. A v e r a g e v a l u e s of d r i f t a n d a n i s o t r o p y p a r a m e t e r s of i o n o s p h e r i c irregularities for different stations Station

Axial ratio

Waltair Yamagawa Debilt Halley Bay

2-10 2-10 1.60 2.45

Length of s e m i - m a j o r axis 240 182 177 293

O r i e n t a t i o n of m a j o r axis

Drift speed

Vc/V

114" 127 '~ 133 ° 125 °

98 m / s e e 72 m / s e c 36 m / s e c .(}7 m / s e c

0.75 0.80 1.10 1.40

m m m m

much variation in "r" during the remaining period. For Yamagawa station the axial ratio attained the highest value at 2200 hours L.M.T. followed by a somewhat irregular variation up to morning hours and a broad minimum near about 1600 hours L.M.T. F o r Debilt the maximum occurred at 0800 hours L.M.T. followed by two

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Fig. 2. D i u r n a l v a r i a t i o n o f t h e a x i a l r a t i o " r " t b r t h e E - r e g i o n .

minor peaks at 1200 hours and 1600 hours L.M.T. For Halley B a y the minimum occurred at 0800 hours L.M.T. and the m axi m um occurred at 1500 hours L.M.T. F r o m the s tu d y of the diurnal variation curves for the axial ratio it m a y be said t h a t the diurnal variation is not regular for all the four stations. However the ext ent of

Study of horizontal drift and anisotropy of ionospheric irregularities

557

the fluctuation of " r " is more for high latitudes and polar stations t han for equatorial stations. (b) Size of the characteristic ellipse The length of the semi-major axis has been taken as a measure for the size of the characteristic ellipse of the ground pattern. Histograms for the length of semi-major axis are shown in Fig. 3. The histogram for Waltair shows a peak value in the range 8!

o

WALTAIR

[. I

,

i2"

YAMAGAWA I" "

z O

8-

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bJ O O ~D

ol

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200 OF

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SEMI-MA,JOR

l l_ BAY

soo A X I S '~'

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IN M E T R E S

Fig. 3. Histogram for the length of semi-maj or axis of the spatial correlation ellipse "a" for the E-region. of 200-250 m. For ¥ a m a g a w a and Debilt the peak values are in the range of 150200 m and 100-200 m respectively. The most probable values for Waltair, Yamagawa and Debilt are 215 m, 165 m and 160 m respectively. For Halley B a y the frequency of occurrence is maximum in the range 250-300 m with a most probable value of 270 m. The latitude variation of the semi-major axis shows clearly a gradual decrease of the size of the ellipse with increase of latitude with the exception t h a t near the poles the value increases once again to a highervalue. Considering the average values of the semi-major axis reported in Table2 the same behaviour of latitude variation is noticed. The diurnal variation of the semi-maj or axis for the four stations is shown graphically

558

G . L . NARAYANA RAO and B. RAI~IACHANDRARAO

in Fig. 4. L e a v i n g out the minor variations, it is found t h a t at W a l t a i r the semi-major axis reaches a m a x i m u m value a r o u n d m i d n i g h t and a b r o a d m i n i m u m a r o u n d noon. I n the case of Y a m a g a w a two p r o m i n e n t m a x i m a are observed at 0600 hours and 1600 hours L.M.T. with a m i n i m u m slightly before noon. The v a r i a t i o n of s t r u c t u r e size in the case o f Debilt station is quite small during d a y t i m e . A broad m a x i m u m a r o u n d noon with a sharp pre-noon m i n i m u m is observed. I n the case of H a l l e y B a y

w I--

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MAGAWA

z

¢Y

0 DE B I L T

b-- I O O t

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1

l

HALLEY BAY

350

250

ISO O0

0 4'

OJ8 HOURS

Ii

16 '

2 '0

24'

IN L.M.T.

F i g . 4. D i u r n a l v a r i a t i o n o f t h e l e n g t h o f s e m i - m a j o r a x i s o f t h e spatial correlation ellipse "a" for the E-region.

station there is a sharp m i n i m u m just after noon with two p r o m i n e n t m a x i m a on either side at 0900 hours a n d 1600 hours L.M.T. The range of diurnal variation of the semi-major axis is v e r y m u c h larger at the low latitude station W a l t a i r and the polar station H a l l e y B a y . I t m a y be n o t e d for W a l t a i r which is a low l a t i t u d e s t a t i o n the night time values are higher t h a n the d a y t i m e values.

St,udy of horizontal drift and anisotropy of ionospheric irregularities

559

(e) Orientation of the major axis oj' the characteri~'tic ellipse Polar histograms for the orientation ~ of the major axis of the characteristic ellipse are presented in Fig. 5. For WMtair the major axis is mainly positioned along N - W - N with a peak value between 100°-ll0 ° and a most probable value of 1(~8 °. The major axes of the characteristic ellipses for Yamagawa and Debilt are mainly in WALTAIR /x~Oo

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to

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Fig. 5. Pol~tr histogram tbr orientation of nmjor axis of the spatial correlation ellipse "yJ" for the ~'-region. N - W quadrants with peak values in tile range of 130°-140 ° and 170°-180 ° and most probable values of 135 ° and 172 ° respectively. For Halley Bay the orientation of the major axis is in N - W quadrant with a peak value in the range of 150°-160 ° and a most probable value of 156 °. The diurnal variation curves for the orientation of the major axis are presented in Fig. 6. The value of ~ shows an irregular fluctuation for Waltair with two broad peaks around 0400 hours and 1600 hours L.M.T. The variation at Yamagawa is somewhat smooth with a t e nde nc y for a m axi m um value around midnight. Debilt shows a wider range of variation of ~ t ha n any other station. Very low values of V are observed in between 0800 hours and 1200 hours L.M.T. for Debilt indicating E - W orientation. At Halley B a y the variation is ahnost smooth but for a minimum at 0800 hours L.M.T. Fr om the diurnal variation plots it m a y be said t h a t the diurnal variations of yJ are not significant for all stations except Debilt. I t is interesting to note t h a t the most probable orientation of the major axis changes from N - W - N direction at low latitudes to nearly east directions at high latitudes. The orientation thus rotates in an anticlockwise direction with increase in latitude except near the poles where the values of y~are oaee again low. The same behaviour is observed when the average values of ~ presented in Table 3 are examined. I t appears from this stu d y t h a t the irregularities are elongated more nearly along N -S directions near the equator whereas at very high latitudes the irregularities are orientated more along E - W direction.

560

G . L . NA_RAYANARAO and B. RAMACHANDRA RAO 4. D I U R N A L V A R I A T I O N } { O R I Z O N T A L D R I F T S P E E D A N D D I R E C T I O N

(a)

Drift speed

Histograms for the distributions of true drift speeds V (half ground speeds) are presented in Fig. 7. From the histograms for Waltair it can be seen t h a t the true drift speed has the maximum frequency of occurrence in the range of 80 100 m/see with a most probable value of 90 m/see. The maximum occurrence of true drift speed 200

WA L T A I R

80

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160

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~'6°I

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200

BAY

150

80

OO 0- -

'40

0' ~8 HOURS

' 12

IN L . M . T .

Fig. 6. Diurnal variation of the orientation of the major axis of the spatial correlation ellipse "V" for the E-region.

lies in the range of 40-60 m/see for ¥ a m a g a w a with a most probable value of 50 m/see. For Debilt this maximum occurs in the range of 10-20 m/see. The most probable value for Debilt is found to be 19 m/see. It has not been possible to give any single most probable drift speed for Halley Bay as the number of records are few and the v a l u e s a r e w i d e l y d i s t r i b u t e d i n t h e r a n g e o f 10 m / s e e - 3 4 0 m / s e e .

The average vMues

of drift speeds for the four stations are presented in Table 2. I t will be evident from the results t h a t the average value of the true drift speed decreases from equator towards higher latitudes and increases again to higher values in the polar regions.

Study of horizontM drift and anisotropy of ionospheric irrcgulariti(~s

5(; l

With a view to studying the diurnal variation of drift speed at different latitudes the values of true drift speeds at different hours are averaged and the plots are presented in Fig. 8. Comparing the curves thus obtained for different stations it is found t h a t there is no similarity, h i general there are two prominent maxima and

:[

-ALT.,.

i l l

:)0 ~

,

,

DE B } L T

0 I-

16 i,,i u} ,.n

0 0 ~C

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60 |OO 140 180 220 -~60 300 TRUE DRIFT VELOCITY IN METRES P~'R SECOND-

31)O

F i g . 7. ~Iistogr&Tn f o r t h e t r u e d r i f t speed f o r t h e E - r e g i o n .

minima in the curves for Waltair and Yamagawa and one prominent m axi m um and minimum for the other two stations indicating the presence of a semi-diurnal component. As can be easily seen the range of diurnal variation of drift speed is large in the ease of the polar station Halley Bay. The ratio Vc/V is estimated for all the stations and the average values are presented in the Table 2. I t is at once evident t h a t the ratio VjV increases gradually from low latitude to high latitude. Near the polar stations the random motions are predominant over the horizontal drift. I t is interesting to note t h a t

562

G.L.

NARAYA~,'A ]=tAO and B. RAI~fACHANDRA ]:~AO

in this respect the behaviour of drift at polar stations is different from that of the low latitude stations. It may be noted that at all low latitude stations the random motions are smaller than the horizontal drifts whereas at very high latitude Stations and at polar stations the behaviour is opposite.

8 0

0

i

T6o

i

~

i

i

r

YAMAGAWA

3 w

-

T

20 t

- -

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~ eo

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HALLEY BAY

32C

24C

16C

80

0000

I 0400

I i 0800 1200 1600 HOU.S rN L.M.'r.

20 ~)0

I 2400

Fig. 8. Diurnal variation of true drift speed for the E-rogion. (b)

Drift

direction

Polar histograms of drift direction ¢ show a predominant direction of N - W for Waltair. In the ease of Yamagawa the drift direction is predominantly along N - E and E - S - E . The histograms for Debilt station show three peaks all in N - E quadrant with N - E direction as the predominant one. For Halley Bay station the drift direction is mainly towards E - N - E . No systematic trend of variation of general drift direction with latitude is noticed. 5. HARMONIC ANALYSIS OF E-REGION DRIFTS

With a view to studying the variation in N - S and E - W components of drift speeds, t h e s e v a l u e s are p l o t t e d against local t i m e and m e a n c u r v e s are drawn through the

Study of horizontal drift and anisotropy of ionospheric irregularities

563

points. In Table 3 arc presented the results of harmonic analysis. In the case of Halley Bay and Debilt stations the diurnal components could not be obtained as the data for daytime only is available. A study of the diurnal variation curves for N-S and E - W components, shown in Fig. 10(a) and 10(b), reveals t h a t the variation for Waltair has a predominent period of 24 hours whereas in the case of Yamagawa the variation is mainly semi-diurnal. The extent of variation of N-S and E - W componeuts is more at equatorial and polar latitude stations compared to the high wALTAIR ~-Oo

YAMAGAWA

N

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Fig. 9. Polar histogram for the true drift direction for the E-region. latitude stations like Debilt. At all the stations other than Debilt, the N-S component of drift is generally towards south during daytime. At Waltair the N-S component is towards north during night time whereas at Yamagawa it is towards south during night time. The E - W component is mainly directed towards east during daytime and west during night time at Waltair whereas at Yamagawa it is almost towards east all the time. At Debilt and Halley Bay the E - W component fluctuates between east and west during daytime. The following are the main conclusions drawn from a careful study of the results of harmonic analysis and polar plots of drift vectors shown in Fig. 11. The prevailing drift at Waltair which is directed towards W - S - W is smaller than the 24- and 12-hourly components. For Yamagawa station the prevailing drift, which is directed towards E - S - E , is larger t h a n 24-hourly and slightly smaller t h a n 12hourly drift vectors. In the case of Debilt and Halley Bay stations the prevailing

564

G . L . NARAYANA ]:~AO and B. I~AMACItAND:RARAO

drifts, which are towards N - E and S-E respectively, have smaller amplitudes compared to the 12-hourly drift vectors. Comparing the E - W components of the prevailing drift vectors for all the stations, it can be seen that the prevailing drift at Waltair is towards west while at other stations it is towards east. Ignoring the small WALTAIR

80

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-8o

hi

W

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hi

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16

IN L.M.T.

24

W O0

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i

16

i

24

IN L.M.T.

Fig. 10(a). Variation of N - S and E-'VV components of the true drift speed for Waltair, Yamagawa and Debilt for the E-region.

northward component of prevailing drift at Debilt, it is found that the N-S component of prevailing drift is predominantly towards south for all the stations. This is surprising in view of the fact that one of the stations, namely Halley Bay, is in the southern hemisphere. No systematic trend of amplitude of the prevailing drift or its direction with latitude is noticed. There are only two stations, Waltair and Yamagawa, for which the results of 24hourly component are available. At Waltair the amplitude of the 24-hourly component is higher than 12-hourly, whereas at Yamagawa the 12-hourly component is

Study of horizontM drift and anisotropy of ionospheric irregularities HALLEY N

320

565

B~Y E

240

160 ct

z 0 u

80

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08 16 24 HOURS IN L-M.T.

$

w

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OJ8 16 24 HOURS IN L.M-T.

Fig. 10(b). Variation of N-S and E-V~r components of the true drift speed for Halley Bay for the E-region. Table 3. Results of harmonic analysis Station ~Valtair Yamagawa Debilt Halley Bay

N-S component --3.5 ÷ 25.4 sin (t -t- 121) ÷ 20.5 sin (2t + 242) --13.2 + 5.3 sin (t _r 37) + 40.5 sin (2t + 33) 3.6 + 4sin(2t + 63) --37-3 + 46.3 sin (2t -/- 24)

E - W component --21.7 + 49.6 sin (t ÷ 111) ÷ 26.1 sin (2t ~ 165) 51"9 + 18"4 sin (t -÷ 250) + 29.3 sin (2t -b 112) 1.7 + 2sin(2t + 334) 36-7 5- 48.1 sin (2t -- 126)

p r e d o m i n a n t c o m p a r e d to the 24-hourly c o m p o n e n t . The sense of r o t a t i o n of the 24-hourly drift v e c t o r is clockwise b u t since the differences in the phase angles bet w e e n the N - S a n d E - W c o m p o n e n t s are close to 0 ° a n d 180 ° for W M t a i r a n d Y a m a g a w a respectively, the degree of ellipticity is quite large. The phases of 24h o u r l y drift vectors a t W a l t a i r a n d Y a m a g a w a are more or less opposite as can be seen from the results in Table 3, as well as the polar d i a g r a m s in Fig. 11. The 24h o u r l y drift is directed t o w a r d s east at W M t a i r a n d west at ¥ a m a g a w a during n i g h t time. Thus the E - W c o m p o n e n t s of t h e 24-hourly drift v e c t o r at W a l t a i r a n d

566

O.L.

NA-RAYANA

"P~Aoand ]3. I'{AiV~AOItANDBARAO

Yamagawa are exactly opposite in phase. Although it is difficult to draw any definite conclusions regarding the latitude variation of the 24-hourly component, it may be said t h a t there is a trend of decrease of amplitude of the 24-hourly component with increase of latitude. STEADY COMPONENT

DIURNAL COMPONENT WALTAIR

SEMI-DIURNAL COMPONENT

SOI N

SOFN

SOl N

|

-2S

(21)

(,IS

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(12)

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w

SCALE IN m/SEC.

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sO (O~6) _ S O ~ S

(O, 9)

Fig. 11. Polar plots of the drift vectors in E-region. The presence of the 12-hourly component is of considerable importance in conneetion with the tidal oscillations of atmosphere (WrLKIES, 1949). According to the tidal theory the rotation is clockwise in the northern hemisphere and anticlockwise in southern hemisphere. The amplitude of the semi-diurnal component of drift is predominant for all the stations. In the ease of Debilt the drift velocities are generally unusually low. Leaving out of consideration the unusually low velocities of Debilt, it can be easily seen t h a t the 12-hourly drift vector increases steadily with increase of latitude, reaching a high value at Halley Bay. For Waltair the N-S component of the 12-hourly drift vector is lower t h a n the E - W component whereas in the case of Yamagawa and Debilt the converse is true. For Halley Bay these components are of the same order.

Study of horizontal drift and anisotropy of ionospheric irregularities

567

The rotation of the 12-hourly drift vector is expected to be clockwise for all the northern stations b u t this is not found to be so for Yamagawa, which shows anticlockwise rotation. The reason for this unusual behaviour is not known. TSUKAMATO and OGATA (1959) also reported anticlockwise rotation for Yamagawa station. Halley B a y showed an anticlockwise rotation as is expected theoretically for a southern hemisphere station. The phase difference between the N - S and E - W component of the 12-hourly drift vector decreases with increase of latitude until at very high latitude it is nearly equal to 90 °. Comparing the polar diagrams of 12-hourly drift vectors one can see the interesting feature that the N - S component of the 12-hourly drift vector at Waltair is exactly opposite in phase with the N - S components of the 12-hourly drift vectors of the other three stations. 6. COMPARISON WITI-I THE RESULTS OF PREVIOUS INVESTIGATORS

Analysing the data for autumn 1960 at Waltair, RAo (1962) reported average values of 2.02,272 m, 87 m/see and 123 ° for the axial ratio, the size of the irregularities, the true drift speed and the orientation of the major axis respectively. The results obtained in the present investigation are in general agreement with those of R~o (1962). The slight deviations m a y be due to the fact that the values in the present investigation correspond to a period of maximum solar activity whereas the value reported b y RAO (1962) relate to a period of lesser solar activity. TSUKAMATO and OGATA (1959) reported a mean axial ratio of 1-6 and an average value of 0.4 for the ration Vc/V for Yamagawa using the IGY data on 2-5 Mc/s and following the method described by BRINGS and SrENCEn (1955). These values are in general agreement with the corresponding values for Yamagawa reported in the present investigation following the PmLLIPS and SPENCER (1955) method. B~mGs and PmLLIPS (1950) working at Cambridge (52°N 0°E) and using a simple approximate method reported sizes of irregularities of the order 200 metres which are of the same order as the size of the irregularity of 160 metres reported for Debilt in the present study. PHrLLIPS and SPENCER (1955), working at Cambridge on 2.4 Me/see, analysed fifteen records using the correlation function method, and found no evidence of very elongated patterns of a consistent direction of elongation. YERG (1956) working at Puerto Rico (18°N 67°W) found an axial ratio of 1.8 and in all cases the ratio of V~/V is greater than unity indicating that the fading is mainly due to random motions. The direction of elongation was most frequently in the N - E S - E quadrants. Although the axial is in reasonable agreement with the value reported for Waltair, the direction of elongation is different from that of Waltair. YERG (1956) found V~/V values of the order of 2.5 which are much higher than the values obtained for Waltair. S K I ~ E R et al. (1958) working at Ibadan (2½°S magnetic latitude) found very elongated amplitude patterns for waves reflected from both E- and F-regions. The mean axial ratio is found to be 5. The direction of elongation was found to be parallel to the earth's magnetic field. Ibadan is a station which is very much closer to the geomagnetic equator compared to Waltair and this m a y be the reason for larger values of axial ratio and closer alignment of the correlation ellipse parallel to the earth's magnetic field at Ibadan compared to Waltair.

568

G.L. NARAYANAI:~AOand B. RA~AC~A~DRARAO

FOOKS and JONES (1961) reported the drift and anisotropy characteristics for the Eregion at Cambridge. They found the median value of axial ratio to be 1.5 and the orientation of the major axis to be in N - W and S-E quadrants. Vc/V values are found to be less than 1-0 on 56% of occasions for the E- region. It can be seen that the values obtained at Debilt in the present study, although slightly lower, are in general agreement with the results reported from Cambridge. 7. CONCLUSIONS

The following are the main conclusions drawn from the study of the present investigation. (i) The axial ratio is found to decrease systematically with increase in latitude except near poles. (ii) The orientation of the semi-major axis of the characteristic ellipse is found to rotate systematically from north to west with increase of latitude. From this it appears that the orientation of spatial correlation ellipse tends to be aligned along the earth's magnetic field at latitudes lower than Waltair, a result which is in agreement with that of SKINNER et al. (1958) at Ibadan. (iii) The length of the semi-major axis also shows a systematic decrease in average value with increase in latitude except near the poles. (iv) There is also a systematic increase in Vc/V from equator to polar latitudes. (v) The true drift speed also shows a systematic decrease with increase in latitude but near the poles the value increases once again. (vi) There is no systematic change in diurnal variation of anisotropy parameter for different latitudes. (vii) From the study of harmonic analysis it is found that the prevailing drift is east at all stations except Waltair for which it is directed towards west. (viii) The amplitude of diurnal component is predominant in low latitudes and is found to decrease with increase in latitude whereas the amplitude of the semi-diurnal component generally increases in magnitude with increase of latitude. (ix) The rotation of the diurnal drift vector is clockwise for Waltair and Yamagawa and the rotation of the semi-diurnal drift vector is clockwise in all the northern hemisphere stations except Yamagawa. The rotation of the semi-diurnal drift vector is anticlockwise for the south polar station Halley Bay. (x) The ellipticity in the polar plots tends to be circular with increase of latitude.

Acknowledgements--The authors express their sincere appreciation to Mr. W. R. PmCOTT, D.S.I.R. Radio Research Station, Slough, to Dr. J. VELDKAMP, Royal Netherlands Meteorological Institute, I)ebilt and to Dr. Ishida, Radio Research Station, Kokubunji, J a p a n , for kindly supplying the original fading records for the IGY period which enabled us to carry out these investigations. Our thanks are due to the Council of Scientific and Industrial Research of India for financial support to carry out this work.

S t u d y of h o r i z o n t a l d r i f t a n d a n i s o t r o p y of i o n o s p h e r i c i r r e g u l a r i t i e s

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R.EFERENCES BRIGGS B. H., PHILLIPS G. 0-. a n d SHINN D. H . BRIGGS B. H . a n d PHILLIPS G. J . BlalGGS B. H . a n d SPENCER M.

1950

Prec. Phys. Soc. Lend. B63, 106.

1950 1955

Prec. Phys. Soc. Lend. B63, 907. Physics of the Ionosphere. P h y s i c a l

FOOKS G. F. a n d JONES I. L. MITRA S. N. PHILLIPS G. J . a n d SPENCER M. ]~AO P. B.

1961 1949 1955 1962

J. Atmosp]~. Terr. Phys. 20, 229. Prec. I~st. Elect. Engrs. 96, 441. Prec. P]~ys. Soc. Lend. B68, 481.

]~AO B. I~., t~AO M. S. a n d MURTHY D. S. SALES (~. S. SALZBERG C. D. a n d GREENSTONE :Ft. SKINNER N. J., HOPE J . a n d V~rRIGHT R . ~V. TSUKA~ATO K. a n d OGATA Y.

]956 1960 1951 1958

WILKES M. V.

1949

Rep. Io~tosp. m~d Space Res., Japan. 13, 48. Oscillatio*~s qf Eartl£s Atmosphere.

YERG 1). C.

1956

J. Atmospl~. Terr. Ptlys. 8, 247.

Society, L o n d o n .

1959

P h . D . T h e s i s of A n d h r a U n i v e r s i t y , ~Valtair, I N D I A . J. Sci. Industr. Res. A15, 75. P e n n . S t a t e U n i v . R e p o r t , No. 131. J. Geopl~ys. Res. 56, 521. Not~zre, LorM. 102, 1363.

C a m b r i d g e U n i v e r s i t y Press.