Indian Journal of Radio & Space PhysicsVol. 30, October 2001, pp. 249-253

Study of travelling ionospheric disturbances using IPS array at Rajkot

A K Jadeja', R M Jadav2, Hari am Vats3 & K N Iyer'

IChrist College, Rajkot 360 005

2Department of Physics, Saurashtra University, Rajkot 360 005

3Physical Research Laboratory, Ahmedabad 380009

Received 4 September 2000; revised 19 February 2001; accepted 2 March 2001

Using a novel radio astronomical technique, the presence of travelling ionospheric disturbancJ (TIDs) are detectedfrom observations of radio star signals by the interplanetary scintillation (IPS) system at Rajkot for t~e first time in India.The periods of the TIDs (20-24 min) and north-south component of their velocity (66-273 rnIs) indicate that they belong tothe medium scale category. The results are discussed in the light of a model of atmospheric gravity waves (AGWs)originating in auroral electrojet region and propagating meridionally equatorward.

1 Introduction "Travelling ionospheric' disturbances (TIDs) are

ionospheric manifestations of propagatingatmospheric gravity waves (AGWs). They arefrequently detected by a variety of ionospherictechniques. Their properties such as occurrence,periods, velocity of propagation, horizontalwavelength, etc. .have been quite extensively studiedby a variety of radio techniques, including fixed-

. frequency sounders I, Ionosondes=:" total-electron-content systems5,6,7, Doppler soundersi", backscatterradars with azimuthal scanning ability" anddifferential Doppler measurements using the satellitesof the Navy Navigation Satellite System!' (NNSS). Inaddition, they have been detected by in situinstruments "aboard satellites'? and by tomographicreconstruction+', Based on these observations they areclassified into (i) large scale with periods greater than30 min, horizontal wavelengths up to a few thousandkilometers and speeds of 300-1000 m s-\ (ii) mediumscale with typical periods of 10-30 min, horizontalwavelengths between 100 and 300 km and speeds of100-300 m S-I and (iii) small scale with speeds up to1000 m S-I, but periods less than a few minutes.

Recently, radio astronomical techniques like radiointerferometry in which signals from two separateaerials are fed into one receiver!", have been used forTID studies. Bougeret" showed that the NancayRadioheliograph, while observing solar radio sources,provided useful data on TIDs. Mercierl6,17 andMercier et ai.18

, using the same instrument, extendedthe technique to study the propagation characteristics,temporal period, polarization and seasonal

climatology of TIDs. Jacobson et al.", viewing fourexceptionally strong, quasi-point celestial sources at50 MHz from the Clark Lake Radio Observatory,observed quasi-periodic perturbations· in thefrequency rangeJ< 2 mHz (medium scale TIDs).

Measurements with Westerbrook SyntheticAperature Radio Telescope combined· withdifferential Doppler shifts observed by' NNSSsatellites'", show that the average' direction ofpropagation of a sample of medium scale TIDsobserved during the first three months of 1982 and1983 is towards about 10° west of south, the speedand horizontal wavelengths being typical for mediumscale TIDs. The results are in satisfactory agreementwith the observations from Nancay (France). Most ofthe. observations are in mid atid high latitudes, thusindicating a paucity of observktions at low latitudes.In this paper, the radio astronomical technique, usingthe interplanetary scintillation (IPS) system at Rajkot(220 18' N, 70° 48' E), has been used to study TIDsfor the first time in India.

2 Experimental system and methodologyBlock diagram of IPS system at Rajkot is shown in

Fig. 1. It can be divided into four main parts: (a)Antenna array, (b) Pre-amplifier and multibeamforming Butler matrices (BM), (c) Correlationreceiver and (d) Data recording system.

Antenna array is an array of 64 rows of open wiretransmission lines in E-W plane, connected with each .are 16 full wave dipoles in N-S plane. Parallelreflectors are kept below these to enhance thecollection of radio waves frfm sky. The 64 pre-

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250 INDIAN J RADIO & SPACE PHYS, OCTOBER 2001

amplifiers are connected, one each to all transmissionline in order to enhance the signal. The output of thepre-amplifiers is brought to the BM inside thelaboratory via underground cable in order to maintainphase and gain stability ..

The signal is then fed into BMs, which aremultibeam forming matrices with small aperature,from different directions. The signal from requiredbeam from BMs output is fed to correlation receiverwhich reduces background noise and all otheruncorrelated components.

The quadrature outputs of the receiver consisting ofcosine and sine channel are recorded both on analogchart recorder and on digital data acquisition system(computer) which form the primary raw data.

2.1 Identification of TID and period determinationTo study the ionospheric irregularities, signals from

different radio, stars are recorded daily using the radiotelescope at Rajkot at 103 MHz. In the absence ofTills, ionosphere remains unperturbed and radiowaves suffer no deviation and intensity remainsconstant [Fig. 2(a)], while in the presence of TIDs,when the radio wave passes through undulatedionosphere, it suffers ray deviation which gives rise toapparent position shift of the source as illustrated inFig. 2(b), resulting into quasi-periodic variation ofintensity. In the present case beam is fixed. Therefore,the apparent position shift of the source will give riseto a corresponding slow variation of the signalstrength, as the radio rays go in and out of the beam asillustrated in the bottom traces of Fig. 2(a). When TIDoccurs, such fluctuations in 'SIN' and 'COS' channels

32 33 4_._._._._._._._._._._._.. ~ DIPOLE ARRAY

DAS = DATA ACQUISITION SYSTEMBM = BUTLERMATRL'X .

L.N.A. = LOW NOISE AMPLIFIER

Fig. I-Block diagram of IPS array at Rajkot

of chart recorder are observed. A typical example ofsuch case is shown in Fig. 3. The typical record ofradio source signal in the absence of TIDcorresponding to the situation in Fig. 2(a) is shown inthe Fig. 3(a) for comparison. The tracings of cosinechannel of the source 3C123 in 4R beam and 3C119source in 9R beam recorded sifultaneously using twoseparate receivers and recorders on 22 June 1998 areshown in Fig. 4. The mead time shift betweensuccessive peaks or troughs in the record is a measureof TID speed.

. 2.2 Determination of TID velocityTo find out the velocity component of TID,

simultaneous observation of two sources are made bytwo different receivers. If a particular peak or troughof TID is observed at time t on beam-x and after atime delay, Ot, if it is observed on beam-y then onecan find the velocity component of this TID, providedthe angular separation 8 between the two beams and

RAYS FROM RADIO STAR

IONOSPHERE

x

TIME TIME

. (a) 1 (b)

Fig. 2-Schematic drawing of the I' clio waves path through theionosphere, (a) unperturbed ionosphere : the radio waves suffer nodeviation with variation in signdl intensity; (b) c1iSlUrbcdionosphere : produces quasi-periodic shift to the radio sourceposition with variation in signal intensity [I - signal intensityresulting from the' convolution between the source positionfunction and receiving antenna beam]

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JADEJA et al.: STUDY OF TIDs USING IPS ARRAY 251

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<IFig. 3- Typical recording of a radio source (a) in absence of TIDs and (b) in presence of TIDs

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Fig. 4-Tracings of cosine channel of the source 3C123 on 4Rbeam and 3C119 source on 9R beam recorded simultaneouslyusing two separate recorders on 22 June 1998.

vertical height of ionosphere where the TIDpropagates (which is assumed to be 350 km) isknown. The velocity component from the beam in adirection in which it appears earlier, to the beam inwhich its appears later is given by

v = 350 tanSOt

.:. (1)

1146

3 Observations and resultsVarious sources were detected by Rajkot radio

telescope and the periods of wavelike perturbationswere determined. The histrograms of occurrencetimes and monthwise distribution of TID occurrencesare shown in Fig. 5 [(a) and (b)]. They showmaximum occurrence during noon-afternoon hoursand in the month of March. Histrograms of perioddistribution for the period January 1997-March 2000are shown in Fig. 6. From the histrograms of perioddistribution the maximum occurrence 'of TIDs isfound to. be in the period 20-24 min which suggeststhat these belong to the category of medium scaleTIDs.

The different radio source pairs were observedduring January 1997-March 2000. It may be pointedout that the probability of two radio sources beinglocated on any two suitable beams simultaneously,and the probability of a TID ~ropagating parallel tothe present baseline which is north-south, reducegreatly the number of suitable events for velocity

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252 INDIAN J RADIO & SPACE PHYS, OCTOBER 2001

determination. Hence, in this span of time we foundonly five simultaneous events as given in Table 1.The velocity components of TIDs derived from Eq.(1) is also given in Table 1.

The TIDs were also found to vary in number withsolar activity ,(Table ,2). The number of TIDoccurrences is considerably more in solar maximumyear (1999) indicating strong solar activity effect onTID occurrences.

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4 Discussion and conclusionsTwenty five measurements of horizontal and

vertical trace. velocities of medium size TIDs weremade by Davies and Jones21 at Bouider, Colorado(mid latitude), using HF propagation technique. Theyfound that average speed of the TIDs was slightlylarger in summer than in winte~ with a mean value of143 rnJs. In the present results, the mean values ofTID speed is obtained to bJ 153 rnJs. The TIDobservations using NNSS and radio interferometerwere made bySpoelstra'", The velocity and period ofTIDs observed by Spoelstra'? range from 118-30.3 rnJsand 20.-30.min, respectively. The present observationsare in satisfactory agreement with the above results.The present results differ from those derived byMercier22 = ~poelstra ~nd K~lder23 that TID occursgenerally m wm~er mormngs. Ir th~ present stl.id~ wehave found maximum oc.curre,ce in March (equinoxmonth) and during noon and I afternoon hours. Thedifference may be due to the variation in the latitudezones in the two cases. Balthazor and Moffetr", usinga global coupled' thermosphere-ionosphere-plasmasphere model, simulated. large scale AGWsoriginating at northern and sduthem:,auroral regions

C/)W 20oZW0:: 15a:::::>0o0 10I..L.

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Fig. 5-Histograms of (a) occurrence times and (b) monthwisedistributionof TIDs for the period January 1997-March 2000

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Fig. 6-Histrograms of period distribution of TIDs for the periodJanuary 1997-March 2000

Table I-Velocity components of TIDs using various source pairs during Junuary 1998-March 2000

Date of Source pair Time shift Velocityobservations (M) (m/s)

Direction orvelocity

componen

North to SOL th

South to North

North to South

min

22 June 1998 3Cll9 & 3Cl23 10 ll4

22 Mar. 2000 3Cl19 & 3Cl23 5 273

24 Mar. 2000 3Cll9 & 3Cl23 14 78

25 Mar. 2000 3Cl44 & 3CI47 13 237

25 Mar. 2000 3C47 & 3C48 19 66

North to South

North to South

I-J

JADEJA et al.: STUDY OF TIDs USING IPS ARRAY

Table 2-Solar cycle dependence of TID observations which isvarying with average sunspot number

Year Average Totalsunspot number TID observation

1997 32 06

1998 88 27

1999 140 36

and propagating meridionally towards the equator.The enhanced auroral electric field (during magneticdisturbances) produces a family of gravity waves inthe thermosphere. The resulting TID is parametrizedin terms of hmF2 and NmF2. Its characteristics arelatitude and altitude dependent and bear complexrelationship with the forcing AGW. Thecharacteristics of TIDs observed in the present study(for example, 4 out of 5 cases showing southwardpropagation) are in agreement with this model, whilethe above reported results for large scale TIDs may beof auroral zone origin. The medium scale TIDs canhave both auroral and meteorological origin, forexample, the tropospheric jet stream. Bertin et al.25

studied medium _scale TIDs using three-stationobservations of Faraday rotation of geostationarysatellite signals! at mid latitudes and found bothauroral and meteorological sources for these.Employing incoherent scatter measurements ofthermospheric temperature and neutral wind and raytracing technique, thel5 could trace the source regionfrom where the TIDs could have originated. Theyfound 12 out of 14 cases to have originated fromwarm and cold fronts, depressions or jet streams,while the other two had auroral origin. Out of the fiveevents given in Table 1, 22 and 24 Mar. 2000 weremagnetic storm days with a maximum K index of 4 asper magnetic storm data of NGRI, Hyderabad. On allother days, apparently there was no storm. However,the TIDs observed on 22 March moved from south tonorth and hence cannot be interpreted as originated inthe northern auroral zone as a result of the storm.Thus, the observed Till direction of this event favoursmore of meteorological origin.

In India, the measurements of TIDs (especiallytheir propagation and source identification) are rare.Sen Gupta et al?6 studied large scale TIDs frommeasurements of Faraday rotation of ATS-6 signalsfrom a wide network of - stations in India. Theyidentified one case of large scale TID, probablyoriginating from auroral zone, on a magnetically

253

disturbed day with large surges of auroral electrojetcurrent and propagating equatorward. Another event,probably originating from th~ surges in equatorialelectrojet current and propagating away from theequator, was also observed by them"'.

Observations of different radio star signals usingthe radio telescope at Rajkot give the informationabout TIDs. In this study, a mean velocity of 153 mJsand a mean period of 22 min are observed whichbelong to medium scale TIOs. The present resultsindicate that occurrences of T Ds are correlated withsolar acti vity.

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