Indian Journal of Radio & Space PhysicsVol. 22, April 1993, pp. 69-81

Co-ordinated multi station VHF scintillation observations in Indiaduring March-April 1991

H Chandra', G D Vyas', DR K Rao1, B M Pathan1, A lype", B Ram Sekaran-, A Naidu", S M Sadique", C S Salgaonkar",T R Tyagi?, P N Vijay Kumar", Lakha Singh". K N Iyer", K N Pathak", A K Gwal", Sushil Kumar", R P Singh!", U PSingh'", Birbal Singh", V K Jain", G N Navneeth!", P V Koparkar!", P V S Rama Rao'3, P T Jaychandran' ', P Sriram!:',

N Y S Santa Rao 14, A DasGupta!", K Basu" & R G Rastogi"'Physical Research Laboratory, Ahmedabad 380009lIndian Institute of Geomagnetism, Bombay 400 0053Department of Physics, Aditnar College, Tiruchendur 628 2154Department of Physics, J NT U College of Engineering, Anantpur 515 022'Goa University, P.O. Santa Cruz, Bambolim 403 005, Goa6Department of Physics, Shivaji University, Kolhapur 416 004"Radio Science Division, National Physical Laboratory, New Delhi 1100128Department of Physics, Saurashtra University, Rajkot 360 0059Department of Physics, Barkatullah University, Bhopal 462026

'°Department of Physics, Banaras Hindu University, Varanasi 221 005"Department of Physics, R B S College, Agra 283 105'lDepartment of Physics, Nagpur University, Nagpur 440 010'3Department of Physics, Andhra University, Waltair 53000314Hindustan College of Engineering, Padur, Dist. Chingleput 603 \03, (via) Kelambakkam, Tamil NaduISInstitute of Radio Physics and Electronics, Calcutta University, 92 Acharya Prafulla Chandra Road,Calcutta 700 009'6CSIR Emeritus Professor, Physical Research Laboratory, Ahmedabad 380009

Received 15 December 1992

A number of YHF scintillation stations are currently operating in India as part of the All IndiaCo-ordinated Programme of Ionospheric and Thermospheric Studies (AICPITS). The chain coverslatitudes right from the magnetic equator to beyond the anomaly crest region. During March-April 1991co-ordinated observations were made at all the stations of the chain and the data collected analysedjointly.The quarter-hourly values of the occurrence of scintillations along with the start and end times of the patchesof scintillations during each night form the data base. Nocturnal variations of the percentage occurrence ofscintillations, histograms of percentage occurrence of the number of patches of scintillations during thecourse of a night and of the patch duration have been computed for each station. Scintillations generally startbetween 1930and 2000 hrsIST~he stations close to the magnetic equator show strong scintillations which lasttill early morning in a single patch or sometimes with a weakening or absence of scintillations for a short Limeduration. For the stations in the anomaly crest region or beyond, scintillations occur in small patches withperiods of no scintillations in between. The nocturnal variations show maximum scintillation activity ofabout 50 per cent in the equatorial region which drops to about 30 per cent in the anomaly crest region andfurther reduces to 10 per cent at Delhi, the Northern-most location in the chain. From the latitudinalvariations of the percentage occurrence of scintillations, the half width of the equatorial belt of scintillationshas been found to vary with local time. It extends right up to Bombay or even beyond it around 2000-2200 hrsbut is much narrower after midnight. The occurrence of spread-F during March-April 1991 from ionosondedata at Thumba, Waltair and Ahmedabad representing stations in the equatorial zone, intermediate zoneand the anomaly crest zone have been studied. There is a fairly good agreement between the nocturnalvariations of spread-F and scintillations.

I IntroductionNighttime magnetic equatorial F-region is a seat

of intense plasma density irregularities encom-passing scales from about 1000 km to a fraction of ametre. These plasma density irregularities give rise tothe phenomenon of equatorial spread-F as seen in

ionospheric sounding, intense radiowave scintil-lations in transionospheric propagation and radarbackscatter signals. Several studies of thesephenomena have been made ever since the discoveryof equatorial spread-F by Booker and Wells! in 1938.Early studies, primarily based on ionosonde data,

70 INDIAN J RADIO & SPACE PHYS, APRIL 1993

had shown the equatorial spread-F occurring in thepost-sunset period, following a rapid rise of theF-Iayer and inhibition due to magnetic activity--'.The solar cycle variation of the equatorial spread- F islongitude dependent with a negative correlation inAmerican zone and a positive correlation in theIndian and African zones". Over the last two decadesmulti technique measurements such as rocket-borneand satellite-borne in situ measurements of theplasma density fluctuations, satellite radio beaconstudies of scintillations, coherent and incoherentbackscatter radar and other ground-based experi-ments like ionosonde and airglow measurementshave been made extensively. These measurementsalong with modelling/simulation studies havegreately added to our knowledge of the phenomenonof equatorial spread-F. Recent reviews" -9 coverthese aspects. It is believed that large scale plasmadepletions are generated first in the post-sunsetperiod through Rayleigh-Taylor instability and thenthese depleted regions rise fast to cover the entireF-region including topside ionosphere. Plasmaprocesses then give rise to instabilities acting on thesteep gradients available and generate smaller andsmaller scale sizes in a cascade process I 0. Since theequatorial spread-F irregularities extend along thefield lines, higher the altitude of plasma depletions,wider the latitudinal extent of the irregularities.However, there are still a number of points notunderstood yet. These include the day-to-dayvariability in the occurrence of spread-F, thecharacteristics of the plasma density fluctuations indifferent scale sizes, and the mechanisms ofgeneration.

Monitoring of the radio beacons onboard satellitesprovides a simple and convenient way of studying theirregularities associated with equatorial spread-F.The fluctuations in the signal strength, oftencharacterized by some form of scintillation index, aredirectly related to the fluctuations in electron density.Scintillations therefore provide a unique method ofstudying the temporal spectral behaviour of theelectron density fluctuations. Knowledge of thevelocity of irregularities enables conversion of thetemporal information into a spatial one. Indiaprovides a unique geographical feature of coveringlatitudes right from magnetic equator to well beyondthe anomaly crest region. Thus a chain of scintillationreceivers distributed closely can provide both thetemporal and spatial variations in the occurrencepatterns of scintillations. With this aim a number ofscintillation recording stations were set up under theAll India Co-ordinated Programme of Ionosphericand Thermospheric Studies (AICPITS). A specific

campaign was conducted during the months ofMarch-April 1991 for co-ordinated observationsusing the 244 MHz radio beacon from Fleetsat (73~).The data collected at sixteen stations covering dipequator to 40°dip were analysed jointly at a workshopheld at Kolhapur during 3-8 June 1991. The featuresof scintillation at Indian stations for this period aresummarized in this paper.

2 Observations and dataThe stations with scintillation recordings in this

period are shown in Fig. I. Table I lists the geo-graphical latitude and longitude, dip angle and alsothe dip angle of the subionospheric point computed at400 km for these stations. The receivers at the majorityof stations were developed at the Indian Institute ofGeomagnetism, Bombay, with recording on chartpapers. Apart from chart recording, digital datarecordings were available at Ahmedabad and Delhi.In the present study, however, the analog chart data atall the stations have been used. On examination of therecords following information has been compiled foreach day of the data collected at various staions:(i) start time and end time of every patch ofscintillations, and (ii) presence of scintillations everyquarter hourly (± 7.5 min),

o36

o32

,o.

·f1EQuatorial

Oi~-L~~~*::::r::±--~18elt -,:.•EQuator 840

Fig. I-Locations of stations in India recording VHF scintillationsunder AICPITS (TRD, Trivandrum; TCD. Tiruchendur; ANM.Annamalainagar; MDS, Madras; ANT. Anantpur; GOA. Goa;KLP, Kolhapur. BBY. Bombay. HYD, Hyderabad; WL T,Waltair; RJK, Rajkot; AHD. Ahmedabad; UJJ, Ujjain; BHP,Bhopal; NGP, Nagpur; CAL, Calcutta; VNS. Varanasi. AGR,

Agra; and DHL. Delhi).

CHANDRA et al.: VHF SCINTIUATIOH OBSERVATIONS IN INDIA 71

Table I-Locations of the scintillation recording stations in theIndian region

Geog. lat. Geog. long. Dip angle Dip angledeg deg deg of subionos-

pheric pointat 400 km

deg

-0.6-0.56.0

14.918.119.322.926.629.628.730.830.730.734.037.740.2

Name ofstation

TrivandrumTiruchendurAnnamalainagarGoaKolhapurWaltairBombayNagpurRajkotCalcuttaUjjainAhmedabadBhopalVaranasiAgraDelhi

8.38.3

11,4

\5.216.717.719.021.1

22.322.623.223.023.225.327.228.6

76.978.179,4

74.074.283.373.0

79.170.788.475.872.477.683.0

78.077.2

-0.6-0.77..6

17.220.621.9

25.729.532.631.9

33.933.833.837.341.0

43.7

3 Results3.1 Characteristics of scintiUatioJlS

The characteristics of scintillations occurring atthe stations along the entire chain can be broadlyclassified into two types. The one occurring atstations close to the magnetic equator with onset ofscintillations in the post-sunset period and strongscintillations lasting throughout the night. Figure 2shows one such example with recordings atTiruchendur during the night of 18-19 Mar. 1991.Scintillations started around 2030 hrs and lastedbeyond 0500 hrs in the morning as shown in thefigure. Though there were brief periods whenscintillations weakened for a while, namely, between2040 and 2047, around 2115,0225 to 0237 and 0315 to0325 hrs. Later after 0400 hrs scihtillations wereobserved to be comparatively weaker. In contrastUjjain, a station situated at the anomaly crest region,showed on the same night scintillations intermit-tently lasting for an hour or even for a smaller periodwith patches in between (Fig. 3). The scintillationsstarted around 2040 hrs for a minute or so and thenreappeared again around 2050 hrs. This patch lastedtill around 2200 hrs. There were three more patchestill 0200 hrs, In the pre-sunrise period (after 0300 hrs)scintillations were weaker. One point of interest,however, is that both at Tiruchendur and Ujjain theonset or disappearance of scintillations was abrupt.

The occurrence characteristics of scintillations atTiruchendur in the equatorial region, at Bombay

situated in between the equatorial zone and theanomaly crest zone, and at Ahmedabad situated inthe anomaly zone are also shown in Figs 4(a), (b) and(c) respectively for the entire period of the campaign.The length of the lines denotes the time period forwhich scintillation activity was present. It is apparentthat the number of days with scintillation activity wasmaximum at Tiruchendur, less at Bombay and least atAhmedabad. Another point worth rroticing is that ondays when scintillations lasted continuously for alonger duration at Tiruchnendur, one sees longduration of scintillations at Bombay also. While ondays when scintillations at Tiruchendur occurred insmall patches of duration, there was very likelihoodof the absence of scintillations at Bombay, forexample, on to March, 12 March, 27-28 March.Comparing the scintillation characteristics atAhmedabad one notices that the number of patchesof smaller duration was more than those of longerduration seen at Tiruchendur and Bombay.

3.2 Characteristics of patches at different stationsFrom the examination of the scintillation records

at different stations of the chain and tabulation of thescintillation patches, following two parameters of thescintillation patches have been compiled for eachstation: (i) number of patches during a night, and(ii) time duration of each patch.

Histograms of the percentage occurrence of thenumber of patches over a night and the time durationof each patch were made for each station. However,we show here the results after grouping the stations inthe following four different latitude 'zones:

(I) Magnetic equatorial zone consisting of thestations Trivandrum, Tiruchendur, and Anna-malainagar.

(II) Zone intermediate to the eq uatorial zone andthe anomaly crest zone consisting of the stations Goa,Kolhapur, Waltair, and Bombay.

(III) Anomaly crest zone consisting of the stationsNagpur, Rajkot, Ahmedabad, Bhopal, Ujjain, andCalcutta.

(IV) Zone beyond the anomaly crest consisting ofthe stations Varanasi, Agra, and Delhi.

The histograms of percentage occurrence of thenumber of patches during a night for the fourlatitudinal zones are shown in Fig. 5. For theequatorial zone I, the number of patches is five or lesswith a median value between 2 and 3 on about 85per cent of the nights. For zone II, the number ofpatches is six or lesson about 88 per cent of nights witha median value of3.ln the anomaly crest zone III, thenumber of patches is six or less on about 90 per cent ofnights with a median value close to 3. In zone IV, thenumber of patches is three or less on 90 per cent of the

72 INDIAN J RADIO & SPACE PHYS, APRIL 1993

TIRUCHENDUR18-19MAR. 1991

.- t~". .~04'O- _ •.•. _. ~.

=~_03t3'o-~-',c;~'"''T.~ -. - -~. ---r-----------'

- .- - ~ -- -- -- --. :". --.-... -, . .. /" .~~~~:--0".~'..,\\,,~

.. t/

10 min.

Fig. 2- Typical nighttime long duration scintillation record at an equatorial station, Tiruchendur

\

CHANDRA et al: VHF SCIN1TI.1.ATION OBSERVATIONS IN INDIA 73

UJJAIN18-19 MAR. 1991

Fig. 3- Typical nigh time scintillation record at Ujjain, a station in the anomaly crest region.

74

)0

2~

c;:;~ 20...J

~U. I~<I:

rc

>-<I: NO0

2~

c;:;~ 20

IUa: I~«z

10

,

18

)0

2'

en 20en

>-)0«

0

2~

20c;:;~I I~ua:«

10~

INDIAN J RADIO. SPACEPHYS,APRIL 1993

TIR\JCHENOUR

NO -

-

--

-~--

00

TlME,hrs 1ST

Fig. 4(a)-Daily occurrence of scintillations at an equatorialstation, Tiruchendur, during March-April 1991 (ND, No data).

---------------------ND---~

21 0)

Bombay

--

,00

,. TIME,hrslST

18 21

Fig. 4(b)-Oaily OCcurrence of scintillations at Bombay duringMarch-April 1991.

~r---~r---~r----'-----'----~-----'AHMEDABAD

--=---=====- - --

- ---- ----20

\11 IS>-C0

10

eft~ 5-'~a..c

~

25

110

110110

110

110

20

III

1. 20 22 00 02

TIME, hrs 1ST06

Fig. 4(c)-Oaily occurrence of scintillations at Ahmedabad in theanomaly crest region during March-April 1991 (NO, No data).

nights. But one must note here that at stations in zoneIII and zone IV, the occurrence of scintillations is less,and it is absent in these two zones particularly whenscintillations do occur in small patches in theequatorial F-region.

The histograms of the percentage occurrence of thepatch duration are shown in Fig. 6. In the equatorialzone I, patch duration extends up to 10h. In zone II,patch duration extends up to 5 h which is again thecase in the zones in the anomaly peak region andbeyond.

3.3 Onset of scintillations at different latip.desAn examination was made to study the latitudinal

variation, if any, on the onset time of the occurrence ofscintillations. Figure 7(a) shows an example of thescintillations recorded at a few stations during thenight of II Mar. 1991. The scintillations started at2015 hrs at Tiruchendur, at 2028 hrs at Kolhapur, at2037 hrs at Rajkot, and at 2046 hrs at Ujjain. Thusthere is a delay of about half an hour in the onset timeof scintillations from magnetic equator to the onsettime in the anomaly peak regions. Another example isshown in Fig. 7(b) from the scintillations recordedduring the night of 8 Mar. 1991. During this nightscintillations started at 2019 hrs at Tiruchendur, at2139 hrs at Kolhapur, and at 2147 hrs at Bombay,

CHANDRA et al.: VHF SCIN1UJ..ATION OBSERVATIONS IN INDIA 75

40

30

20

10

wuZWOCOC::::>uu0wC><{

0~Zw 20uocwn,

10

MARCH -APRIL 1991

ZONE IV

Varonasi t Agra

ZONE mRajkot ,Ahmedabad,UllainBhopal & Calcutta

ZONE DGoo. Kolhapur, Watfair

'--_-, & Bombay

O~------------------------~-L~20

10ZONE ITiruchendur. Trivaldrum& AnnamalainagarL~-----,

OL-------------------~-L~--~2 3 4 5 6 7 8 9 10 11 12

NO.OF PATCHES

Fig. 5--Distribution of the occurrence of scintillation patchesduring a night at four different latitudinal zones, namely, (I)Equatorial, (II) Intermediate between equatorial and anomalycrest, (III) Anomaly crest, and (IV) Beyond the nnomaly crest.

showing a delay of about an hour and a half betweenTiruchendur and Bombay. In both the examples theonset of scintillations was sudden. On some occasionsthe onset of scintillations was slow. Such an exampleis shown in Fig. 7(c). On 13 Apr. 199J scintillationsstarted building up slowly from around 1950 hrs atTiruchendur and strong scintillations set in around2000 hrs. Similar types of onsets were seen at Goa andBombay with strong scintillations from 2030 hrs atGoa and at 2045 hrs at Bombay which were precededby slow scintillations. At Ujjain strong scintillationsstarted around 2200 hrs rather abruptly.

Thus it is clear from the above examples thatscintillations start earlier at the equatorial stationsand later at latitudes away from the equator.

MARCH - APRIL 1991

wuzWIt:It::::>ugWI!I«t-zWUIt:WQ.

80ZONE I f- ZONE Ii

60Goa.Kolhcipur. Waltair Varansi and AgrO

40and Bombay

20

~L-..

-- ZONE I ZONE.60

Trivandrum, TiruchMdur RaiMot. Ahm.dabad

40and Annamalaonagar Ujjain. Bhopal and

Calcutta

20 L,o 4 6 8 10 0 2 4 6 8 10

PATCH DURATlON,h

Fig. 6-Distribution ofthe occurrence of duration of the patchesof scintillations at four different latitudinal zones, namely, (I)Equatorial, (II) Intermediate between equatorial and anomalycrest, (III) Anomaly crest, and (IV) Beyond the anomaly crest.

3.4 Daily variatiom in scintiUation occurrenceFrom the quarter-hourly occurrence of scintilla-

tions during the campaign period of March andApril 1991, average quarter-hourly percentageoccurrences have been computed for each month.The monthly averaged daily variations of thescintillation occurrence at all the sixteen stations forthe month of March 1991 are shown in Fig. 8. Similardaily variations are noted for the three equatorialstations of Tiruchendur, Trivandrum and Anna-malainagar with peak occurrence between 50 and 60per cent which drops to less than SO per cent at Goaand Waltair and to less than 40 per cent at Bombayand Kolhapur. For stations Nagpur, Rajkot,Ahmedabad, Calcutta and Ujjain in the anomalypeak region., peak occurrence rate is close to 30per cent, while for Varanasi and Agra it furtherreduces to less than 20 per cent. At Delhi peakoccurrence is only 10 per cent. Another systematicdifference noted is the scintillation occurrence in thepost-midnight period which drops considerably withincreasing latitude. It must be noted here that thedaily variations are shown in hrs 1ST for all thestations. Therefore the onset times show differencesdue to longitudinal differences. This is clearly seen inthe case of stations close to the anomaly crest wherefive stations lie from Rajkot to Calcutta covering alongitudinal extent of 1°. The onset of scintillationsover Calcutta occurred about one hour earlier than atRajkot.

3.5 Latitudinal. variatiom in scintillation occurrenceTo study the latitudinal variations in scientillation

occurrence during the course of night, three-hourly

76 INDlAN J RADIO & SPACE PHYS, APRIL 1993

11Mar. 1991

/ 0,: C"iI ! :

!i ~ f,... !.l

/

\ ;;\\ Q ....•,

\

\t

2028\\~ \ -

c.., '. --ICtl,"~1r : .,.

,-f";I 7~ -.-

s r •• \

i .\ \ i.._\\.., - \ \ '___-_-_.__'.."•..l ,,- . .1 C:IIA T';•• 1.1".

mi n

•

TIRUCHENDUR. . - e :-.

! ' ••

\ \, - ~ \

··g'~10'.

Fig. 7(a)--Scintillation records observed at Tiruchendur, Kolhapur. Rajkot and Ujjain on II Mar. 1991 showing the timedelays in the onset of scintillations

CHANDRA et aL: VI-W SCINTIllATION OBSERVATIONS IN INDIA 77

8 Mar.1~91

\ \-O-.Ji\-,_. '-'"--'-- ~- -J,--\---..:...

-\ -- Q

. <>, n'

Fig. 7(b}-Scintillation records observed at Tiruchendur, Kolhapur and Goa on 8 Mar. 1991 showing the time delays in theonset of scintillations.

groupings of the data were made. These were thepost-sunset (1900-2100 hrs), pre-midnight (2200-0000 hrs), post-midnight (0100-0300 hrs), and pre-sunrise (0400-0600 hrs) periods. The mean occur-rence of scintillations during these four groups hasbeen plotted as a function oflatitude (Fig. 9). Duringthe evening hours scintillation occurrence remainsbetween 20 and 27 per cent at latitudes up to 20° andthen decreases with increasing latitude. The halfwidth of the scintillation belt (considered as thelatitude at which occurrence reduces to half of thevalues at magnetic equator) appears to be 1SO for thisperiod of the night. For the pre-midnight hours themaximum occurrence is 50 per cent at equator and thehalf width comes out to be 14°.During post-midnightperiod the peak occurrence is once again 50 per cent atequator which drops to halfin a latitude width of 6°

only. Similarly the half width for early morning hoursis again 6° though peak occurrence of scintillation hasdropped to 30 per cent at equator. Thus the belt ofequatorial region of scintillations is widest duringonset phase which decreases with the progress ofnight in the post-midnight period. For the epoch ofMarch 1991, the width estimated is about 30° in thepre-midnight period and 12° in the post-midnightperiod.

3.6 Spread-F occurrence during March 1991The occurrence of scintillations in the nighttime

equatorial region is well correlated with theoccurrence of equatorial spread-F. During thecampaign period ionosonde data were available atThumba, Waltair, and Ahmedabad, which representthree distinct zones - equatorial, edge of the

78 INDIAN J RADIO & SPACE PHYS, APRIL 1993

13Apr.1991.~ --,~ =; ------ -.---.-!"-----------------

f r t=r=r»: --------; ~!

--'-- ..- r .-

t2200~ ~~------~

-, UJJA IN ------,o:--~-----'--

: \ !ti, ".·-,-,-; .T/-,'-7' ,~--r'-- 7' t=:

{ : Ii" : .' If I

!

BOMBAY\ ' \ '\ I! \I. " \ \

" \ '

oa

..-.----jl---- --i- __ ~ __

-;-...--1~-----,--'----------. _. ~'

GOA

t :.2030 ~ __.•...•----,.,G,........,t;O;--·~-----I, I,>.

I~

I

TlRUCHENDUR'

y..10 min

Fig. 7(c)--Scintillation records observed at Tiruchendur, Goa, Bombay and Ujjain on 13 Apr. 1991 showing the time delays in.the onset of scintillations. / .:

CHANDRA et ilL: VHF SCIN11I..lATION OBSERVATIONS IN INDIA 79

MARCH 1991O,lhi

Agra

Ujjoin

20

20 22 o 24 2

TIME,hrs 1STFig. 8-Nocturnal variations in scintillations at all the stations of the chain covering a latitude belt of 8°N-29°N during

March 1991.

equatorial zone, and anomaly peak region. The dailyvariations based on quarter-hourly values in thepercentage occurrence of spread-F at these threestations is shown in Fig. 10. The onset of spread-F isseen to occur around 1900 hrs at Waltair and about15-20 min later at Thumba which is likely due to thelongitudinal difference. The maximum occurrence

rate exceeds 80 per'cent at Thumba, 70 per cent atWaltair and 35 per cent at Ahmedabad. The peakoccurs around 2100 hrs at Ahmedabad. There is adecrease in the occurrence rate after 2200 hrs to about40 per cent at Thumba and to about 20-25 per cent atWaltair. For Ahmedabad the occurrence ratedecreases continuously and there is no spread-F seen

80 INDIAN J RADIO & SPACE PHYS, APRIL 1993

•••ARCH - APRIL 1991

)0(04 - 06 IIrs'

20

I10

(01-0)IIr5 ,

)0

UJ 20uZUJ0: 10ex:::Ju~UJI!l«•....zUJ (0u0:UJ

)0a..

20

10

122-00llrsl

II

(19-21I1rsl

8 10 12 14 16 18 20 22 24 26

GEOGRAPHIC LATITUDE,deg

Fig. 9-Latitudinal variatiens in the percentage occurrence ofscintillations at four different time periods of the night.

after 0300 hrs. For comparison the nocturnalvariations in the percentage occurrence of scintil-lations at three stations, namely, Tiruchendur,Bombay and Ahmedabad, representing the threelatitudinal zones are shown in Fig. II.The maximumoccurrence at the three stations is 70, 50 and 30per cent respectively around 2200 hrs. Unlike rapiddecrease in spread-F occurrence after midnight thescintillation occurrence decreases rather slowly. Theoccurrence characteristics of spread- F are consistentwith the occurrence of scintillations observed at thethree latitudinal zones.

4 Discussion and conclusionsThe salient features emerging out of this study

based on scintillation observations at a chain of stations

21 00TIME, hr. 1ST

Fig. IO-Nocturnal variations in the percentage occurrence ofspread-F at Thumba, Waltair, and Ahmedabad, representingrespectively equatorial, low latitude, and anomaly crest regions in

India.

I.&JuZI.&Jcrcr;:,uuoI.&J<!><l>-ZI.&JucrI.&Je,

I.&J

!f soI.&Jcrcr S;:,(J

g '0I.&J<!>:! )0

zwu 10crwo, 10

SP~EAD -F OCCURRENCE,MARCH 1991100·r---=--=;:...:-=-----=-r-:---=-.=.:.:;:=.!--=:..:..:.....--:~-..,.....--..,

80

60

10

11 04 06

MARCH-APRIL 1991

70

o ~I __ ~ __ ~I~~I __ L-~ __ ~~ __ ~18 11 1J 00 01 01 0) 0' 05 06

TIME,hrsIST

Fig. II-Nocturnal variations in the percentage occurrence ofscintillations at Tiruchendur. Bombay, and Ahmedabad,representing respectively equatorial. low latitude, and anomaly

crest stations in India.

extending from equator to about 400N dip in India forthe first time are:

(a) Scintillation occurs continuously throughoutthe night in the equatorial zone while it breaks up intomore discrete patches at anomaly peak region andbeyond.

(b) The duration of scintillation patches is longer atequator compared to those at other latitudes.

(c) There is a systematic time delay in the onset ofscintillations as one goes away from the equator.

(d) The percentage occurrence consistentlydecreases away from the equator with peakoccurrence in the pre-midnight period.

(e) The half width of the equatorial scintillationbelt is 15°, 14°and 6° at the pre-midnight, midnightand post-midnight periods respectively.

CHANDRA et al: VHF SCINTILlATION OBSERVATIONS IN INDIA 81

(f) The occurrence patterns of scintillation andspread-Fare well correlated in the equatorial as wellas anomaly crest latitudes. A noteworthy feature isthat if the irregularities are already broken up intopatches at the equator itself, then scintillation is lesslikely to be observed at the crest latitudes andbeyond.

The above result of the consistent time delayconforms to the belief that the scintillation-producmgirregularities are generated at the bottomsideF-region above the equator under favourableconditions and are lifted up by E x B forces and thenextended to farther latitudes along field lines. Radarobservations!' ,!2 indicate that at altitudes below about50U km there are strong irregularity layers whichtrigger weaker irregularities in between, while abovethis altitude the weaker irregularity regionsdisappear. At the equator the integrated effect of allthese vertically displaced irregularity regions givesalmost continuous scintillation, while the verticallydrifting striated irregularity regions at higheraltitudes when mapped to higher latitudes givediscontinuous patches of scintillation. The decrease inthe width of the scintillation belt during midnight andpost-midnight may also be due to the reduction ofvertical drift velocities during these periods. Thegeneral chracteristics of scintillation occurrencereported here are consistent with the previousobservations at the magnetic equator and at theanomaly crest region for similar levels of solar'activity. Further studies using a larger data base'covering low and high sunspot conditions will benecessary to hring out the seasonal and solar activitydependence and their comparison with otherlongitude sectors so that a better understanding of the

irregularity generation mechanism can be obtained.This will be possible only with the complete data setfor the whole AICPITS project duration.

AcknowledgementsThe authors are grateful to the Department of

Science and Technology (DST) for providing theopportunity to a number of research groups in Indiato work jointly under the All India Co-ordinatedProgramme of Ionospheric and ThermosphericStudies (AICPITS). Thanks are also due to the Headsof various university centres and research institutesfor providing the facilities for these researchactivities. The joint analysis of the data andpreparation of this research paper were done at thedata analysis workshops at Kolhapur (June 1991)and Varanasi (June 1992) funded by DST.

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43 (1938) 249.2 Wright R W, Koster J R &1ikinner N J, J Almos & Terr Phys

(GB), 8 (1956) ?.40., :f Bhargawa B N, Indian J Meterol & Geophys, 9 (1958) 35.

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