Whistlers detected and analyzed by Automatic Whistler Detector(AWD) at low latitude Indian stations

Abhay K. Singh a,n, S.B. Singh a, Rajesh Singh b, Sneha A. Gokani b, Ashok K. Singh c,Devendraa Siingh d, János Lichtenberger e,f

a Atmospheric Research Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221 005, Indiab KSK Geomagnetic Research Laboratory, Indian Institute of Geomagnetism, Chamanganj, Allahabad, Indiac Physics Department, Lucknow University, Lucknow 226 007, Indiad Indian Institute of Tropical Meteorology, Pune 411 008, Indiae Space Research Group, Department of Geophysics and Space Sciences, Eötvös University, Budapest, Hungaryf Geodetic and Geophysical Institute, Research Center for Astronomy and Earth Sciences, Hungarian Academy of Sciences, Sopron, Hungary

a r t i c l e i n f o

Article history:Received 31 December 2013Received in revised form21 February 2014Accepted 6 March 2014

Keywords:Very Low Frequency wavesWhistlersLightning dischargesIonospheric/plasmaspheric parametersMagnetospheric physics

a b s t r a c t

Recently, at three Indian low latitude stations: Varanasi (geomag. lat. 141550N, geomag. long. 1531540E, L:1.078), Allahabad (geomag. lat. 16.051N; geomag. long. 155.341E, L: 1.081) and Lucknow (geomag. lat.17.61N, geomag. long. 154.51E, L: 1.104) an Automatic Whistler Detector (AWD) has been installed inDecember, 2010 for detection and analysis of whistlers. This instrument automatically detects andcollects statistical whistlers data for the investigation of whistlers generation and propagation. Largenumbers of whistlers have been recorded at Varanasi and Allahabad during the year 2011 which isanalyzed in the present study. Different types of whistlers have been recorded at Varanasi and Allahabad.The correlation between recorded whistlers and causative lightning strikes were analyzed using dataprovided by World-Wide Lightning Location Network (WWLLN). We observed that for both the stationsmore than 50% of causative sferics of whistlers were observed to match closely with the times ofWWLLN detected lightning strikes within the propagation times of causative tweeks. All of theselightning strikes originated from the region within 500–600 km radius circle from the conjugate point ofVaranasi and Allahabad supports the ducted propagation at low latitude stations. The dispersion of theobserved whistlers varies between 8 and 18 s1/2, which shows that the observed whistlers havepropagated in ducted mode and whole propagation path of whistlers lies in the ionosphere. Theionospheric columnar electron contents of these observed whistlers vary between 13.21 TECU and 56.57TECU. The ionospheric parameters derived from whistler data at Varanasi compare well with the othermeasurements made by other techniques.

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1. Introduction

Very Low Frequency (VLF, 3–30 kHz) electromagnetic wavesare a powerful tool for remote sensing of processes in the iono-sphere and magnetosphere (Helliwell, 1965). VLF waves, which aregenerated by lightning return stroke currents (sferics, whistlers)travel between hemispheres along ionization ducts before enter-ing the Earth-ionosphere wave-guide (Storey, 1953). Continuousmonitoring and analysis of these waves (such as radio atmo-spheric, tweeks, whistlers, VLF emissions) yield information aboutthe height of the waveguide, equatorial electron density profiles

(Tarcsai et al., 1988), electron/ion temperature (Sazhin et al., 1990),convective electric fields in the magnetosphere (Carpenter et al.,1972), duct width, duct lifetime (Singh et al., 1998), irregularities inthe electron density (in the magnetosphere as well as lowerionosphere), lightning induced electron precipitation (LEP) (Inanet al., 2010), etc. (Siingh et al., 2008 and references therein).Whistlers explain wave propagation and related properties of VLFwaves having right hand polarization and wave frequency smallerthan the electron gyrofrequency and electron plasma frequency(Helliwell, 1965, Sazhin et al., 1992). These waves have beenreceived both on the Earth's surface (Walker, 1976; Collier et al.,2006) as well as onboard satellites (Chum et al., 2006).

The whistler waves can propagate through the magnetosphericplasma in a ducted mode (Walker, 1976), a non-ducted mode(Smith and Angerami, 1968) and a prolongitudinal mode (Morgan,

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n Corresponding author. Tel.: þ91 9415303722; fax: þ91 542 2368390.E-mail address: abhay_s@rediffmail.com (A.K. Singh).

Please cite this article as: Singh, A.K., et al., Whistlers detected and analyzed by Automatic Whistler Detector (AWD) at low latitudeIndian stations. Journal of Atmospheric and Solar-Terrestrial Physics (2014), http://dx.doi.org/10.1016/j.jastp.2014.03.001i

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1980). When the wave frequency is lower than the lower hybridresonance frequency and the wave normal angle is relatively small(but not small enough to be trapped in ducts) the waves propagatein the prolongitudinal mode (PL) and such whistlers can bereceived on low altitude satellites known as PL whistlers(Morgan, 1980). Waves propagating in the non-ducted mode canbe received only on board satellites. It is expected that a lightningdischarge may illuminate a large area in the ionosphere throughwhich wave energy may enter the magnetosphere to appear as awhistler wave in the conjugate region (Storey, 1953). When alightning discharge is composed of multiple flashes and VLFenergy propagates in the same duct, the series of whistlers havingthe same dispersion and separated in time are called “multiflash”whistlers (Helliwell, 1965; Singh et al., 2011). When a lightningdischarge illuminates more than one duct simultaneously in themagnetosphere then “multi-path” whistlers are produced(Helliwell, 1965). The dynamic spectra of whistlers are controlledby the path-length, the distribution of electron density and themagnetic field along the path of propagation (Park, 1972).

The analysis of a whistler usually consists of the searching ofwhistlers in raw VLF data to obtain whistler spectrogram andapplying a model using the scaled f–t pairs to calculate plasma andpropagation parameters (Helliwell, 1965; Singh et al., 1998).However, it is not practical to manually search continuouslyrecorded VLF data for whistler events as a source of regularlymeasured plasmaspheric parameters. For this reason an AutomaticWhistler Detector (AWD) system (Lichtenberger et al., 2008) hasbeen developed as a step towards producing automatic and near-continuous ground based plasmaspheric measurements. The AWDsystem was first deployed in Tihany, Hungary (46.891N, 17.891E;L¼1.81), and identified 252,000 whistler traces over a 2-yearperiod (Collier et al., 2006). Using the AWD system operating inDunedin, New Zealand, Rodger et al. (2009) reported that Dunedinwhistler rates are hundreds of times higher than estimated fromthe conjugate lightning activity. Chum et al. (2006) have studiedthe penetration of lightning induced whistler waves through theionosphere by investigating the correspondence between thewhistlers observed on the DEMETER and MAGION-5 satellitesand the lightning discharges detected by the European lightningdetection network EUCLID. They demonstrated that the area in theionosphere through which the electromagnetic energy induced bya lightning discharge enters the magnetosphere is up to severalthousand kilometers wide. This suggested that stations situatedwithin 1000 km could record the same whistlers. The seasonaloccurrence of Tihany-observed whistlers was found to be stronglydependent upon lightning activity in the southern African con-jugate region (Collier et al., 2006). Whistler observations peaked inthe same month that lightning activity peaked, although thediurnal variation in whistler occurrence peaked in the nighttimeand was principally determined by ionospheric absorption ratherthan the late afternoon-early evening when conjugate lightningactivity peaked. Recently, Singh et al. (2012) reported the directcorrelation between whistlers observed at a low latitude station,Allahabad (L¼1.08) with their causative lightning dischargeslocated near the conjugate region using the Global LightningDataset 360 (GLD360) lightning location network. The World-Wide Lightning Location Network (WWLLN) currently detects thelightning discharges with peak currents larger than 50 kA withinthe spatial and temporal accuracy of 10 km and 10 μs and has theglobal detection efficiency of about 10% (Rodger et al., 2009).Recently, Srivastava et al. (2013) also reported one-to-one relation-ship between low latitude whistlers and conjugate source light-ning discharges using the WWLLN data.

Initially, whistlers were thought to be a uniquely high- and mid-latitude phenomena. However, ground stations in India, Japan andChina showed the occurrence of whistlers at low latitudes (Lo2.3)

(Singh and Tantry, 1973; Hayakawa and Tanaka, 1978; Xu et al.,1989). The propagation characteristics of mid- and high-latitudewhistlers are well established and hence they have been usedextensively there in Earth's plasmaspheric investigations (Helliwell,1965 pp. 69; Al'pert, 1980; Tarcsai et al., 1988). For whistler wavepropagation, the wave normal angle must lie within the transmis-sion cone (Helliwell, 1965), which is much larger in a ductedpropagation than in non-ducted one. At low latitudes (geomagneticlatitude o301; Lo1.3) the Earth's magnetic field lines have sharpcurvature and are embedded in the ionosphere (o1000 km) whereducts are not supposed to regularly exist, so most of the waveenergy does not lie within the transmission cone. Hence, thepropagation characteristics of whistler waves at low latitudesremain poorly understood and they have not been used effectivelyas a diagnostic tool for the low latitude ionosphere. Thus for betterunderstanding the propagation characteristics of low latitudewhistlers and their associated lightning source location, the presentstudy have been initiated.

In the present study, we have analyzed the broadband VLF datamainly tweeks and multiple whistlers recorded by AutomaticWhistler Detector (AWD) installed at Indian low latitude stationsVaranasi and Allahabad during the year 2010. We have computedthe origin of these whistlers using the WWLLN data and presentarguments on the propagation mechanism of these whistlers. Thedetails about the experimental setup of newly installed AutomaticWhistler Detector (AWD) at Indian stations are presented inSection 2. The results and discussions of the study are presentedin Section 3 which includes the occurrence statistics of whistlersrecorded at Varanasi and Allahabad and the connection betweenwhistlers and causative lightning strikes. Finally, the conclusion ofthe study is presented in Section 4.

2. Experimental setup and data

VLF wave fields as a function of time and frequency are beingrecorded continuously using a crossed loop antenna, pre/mainamplifier, VLF receiver and Automatic Whistler Detector (AWD)which is installed simultaneously at three Indian low latitudestations: Varanasi (geomag. lat.¼141550N, geomag. long.¼1531540E, L¼1.078), Allahabad (geomag. lat.¼16.051N; geomag.long. 155.341E; L¼1.081) and Lucknow (geomag. lat. ¼17.61N,geomag. long.¼154.51E, L¼1.104). A detailed description of theAWD system operation and algorithm development can be foundin Lichtenberger et al. (2008). A brief description of the AWDsystem installed at Indian stations is presented here for simplicityto readers. The AWD system consists of the following parts:(i) antenna; (ii) preamplifier; and (iii) AWD software running ona personal computer (PC) with a Linux kernel. Antenna is crossedmagnetic loop antennae with areas �100 m2 orientated geomag-netic north–south and east–west so that the receiver picks upmagnetic fields parallel to the ground from any direction. Impe-dance matched pre-amplifier is placed at the bottom of theantenna for maximum power transfer as well as signal amplifica-tion. The amplified VLF signal is then digitized at a sampling rateof 44.1 kHz using a 16 bit sound-card in a PC. The PC time issynchronized with the Pulse per second (PPS) signal of a GPSreceiver. The PPS timing accuracy is better than 1 μs, which issmaller than the data sampling period (�22 μs) (Lichtenbergeret al., 2008).

Data collection is started through init system upon PC boot.There is a user level program called sampler.i386, it communicateswith Jackd (Jackd is a system for handling real-time, low latencyaudio on various computer operating systems, http://jackaudio.org) upon startup and during data logging, it writes data on thedisk. It opens a file in every hour. Each file is closed when round

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Please cite this article as: Singh, A.K., et al., Whistlers detected and analyzed by Automatic Whistler Detector (AWD) at low latitudeIndian stations. Journal of Atmospheric and Solar-Terrestrial Physics (2014), http://dx.doi.org/10.1016/j.jastp.2014.03.001i

hour is reached and a new one is immediately opened. The justclosed file is then processed by AWD which creates another filethat (may) contain whistler trace(s). A file can contain morethan one trace if consecutive events found. Whistler files are 4 slong (2 s before and 2 s after detection are saved) generally, butthey are extended in the case of multiple events (the 2–2 smargins are kept). False detection can occur if specific local noisesappear or a close storm generates ‘sferics curtain’ spoiling allsignal (Lichtenberger et al., 2008).

Whistler traces are automatically identified in the incoming datastream. The whistler detection algorithm is based on two-dimensionalimage correlation: dynamic spectra are computed for 14 s overlappingdata windows and correlated with a pattern made from a modelwhistler. The model whistler is based on Bernard's (1973) approxima-tion. At low-latitude stations the appearance of low dispersionwhistlers is the main factor. Thus the mid-latitude algorithm withlowered dispersion (D) and higher nose frequency (fn) can be applied(Lichtenberger et al., 2008). The practical values are fn¼50–100 kHzand D¼5–15 s1/2 for low latitude. However, the task here is morecomplicated because of the frequent appearance of mid-latitudewhistlers in conjunction with our low-latitude ones. The algorithmapplies various techniques to remove the effects of unwanted signalsincluding sferics, power line harmonics, and military and navigationVLF transmitters.

The AWD system has been installed and working at Varanasi,Allahabad and Lucknow since December, 2010 and has collected alots of broad band as well as narrow band VLF data. In the presentstudy we have analyzed the whistlers recorded at Varanasi andAllahabad only and due to poor data quality for Lucknow stationwe have not included it in the present analysis. The broad banddata consists of variety of whistlers and tweeks. Large number ofwhistlers has been observed during routine recording of VLF databy AWD at Varanasi in the month of March 2011. Various types ofwhistlers have been recorded at Varanasi which is shown in Fig. 1.The whistlers are marked by W1, W2, W3, etc. and the causativelightning sferics are marked by S1, S2, S3, etc. Fig. 1a shows atypical example of single whistler trace observed at Varanasi on19th March 2011 at 21:58:04.4024 UT. There are doublet whistlersW1 and W2 in the Fig. 1b recorded at 21:07:55.6264 UT on 21March, 2011. Fig. 1c contains three whistlers observed at20:58:34.0688 UT on 30th March, 2011. Similarly Fig. 1d–i showsfour, five, six, seven, eight and ten whistlers observed at 21:47:12.6616 UT, 22:15:05.5976 UT, 22:18:08.0816 UT, 21:42:27.8992UT, 21:48:25.4824 UT on 21st March, 2011 and at 21:17:29.64 UTon 30th March, 2011 respectively. It is clearly observed that all therecorded whistlers are accompanying with causative lightningsferics. To find the causative lightning strikes corresponding towhistlers, lightning data from WWLLN has been used in thepresent study.

3. Results and discussions

3.1. Whistlers occurrence statistics

It is observed that, in general, whistler activity at low latitudeIndian stations is low and sporadic (Singh, 1993; Singh et al., 2004,2007, 2012). The maximum monthly occurrence rate occurredduring January to April, i.e. during the winter months (Singh et al.,2007; Srivastava et al., 2013). This is obviously due to the lightningactivity in the conjugate hemisphere. After analysis we found thatAWD captured 1081 whistlers at Varanasi and 1219 whistlers atAllahabad during the year 2011. In the present study, we reportobservations of large number of whistlers during the months ofMarch, 2011 at both the Indian stations Varanasi and Allahabad.During the month of March, 2011 we recorded plenty of whistlers

only on the following dates: 19th March, 30th March and 31stMarch, 2011 simultaneously at both stations Varanasi and Allaha-bad and on 21st March, 2011 only at Varanasi. The hourlyoccurrence rate of whistlers recorded at Varanasi during March,2011 is shown in Fig. 2a. The hourly occurrence rate of whistlersrecorded at Allahabad during March, 2011 is shown in Fig. 2b.In the month of March, 2011 AWD detected 1081 whistlers atVaranasi but only 323 whistlers were detected at Allahabad. It isclearly seen from Fig. 2 that Allahabad has less recording of VLFdata (marked ND) due to fault in recording system. About 48% ofwhistlers observed at Varanasi are made of a single trace andremaining 52% are of multi-trace whistlers. All these whistlerswere recorded during nighttimes and no whistler activity wasfound in the daytime. This may be attributed to the heavyabsorption of VLF waves by the daytime D-region of ionosphere.

The occurrence of whistlers at our low latitude Indian stationscompared to mid-latitudes stations is very low. The first AWDsystem has been working in Tihany, Hungary (L¼1.8), andLichtenberger et al. (2008) reported that it has collected 100,000whistler traces per year. Using AWD system operating in Dunedin,New Zealand, Rodger et al. (2009) reported the detected 92,528individual whistler events containing 236,019 whistler tracesduring 530 days. Rodger et al. (2009) showed that about 58% ofthe Dunedin, New Zealand observed whistlers are made of a singletrace; that is, roughly 42% are multi-trace. Whereas Lichtenbergeret al. (2008) showed that vast majority of the whistlers observedby the Tihany AWD are single-trace events. The conjugate point ofVaranasi and Allahabad lies over the ocean and hence thunder-storm activity is relatively low as compared to that over the land(Christian et al., 2003). This may be one of the reasons for overalllow whistler activity at Varanasi and Allahabad. The whole periodof observations of whistlers at Varanasi and Allahabad presentedabove is geomagnetically quiet having maximum Kp¼1þ and Dst-index¼�13 nT.

3.2. Whistler-lightning connections

For one-hop whistlers, one would expect the appropriatelightning source population to be that in the geomagnetic con-jugate hemisphere. Initially for low latitude station Yamako(geomag. lat. 251N) Ohta and Hayakawa (1990) compared whistleractivity with thunderstorm activity in the conjugate region anddid not find any correlation between the two. Kumar et al. (2007)also reported similar results for whistlers observed at low latitudestation Suva, Fiji (geomag. lat. 22.11S) and suggested that whistleroccurrence at any station is controlled more by the propagationeffects in the ionosphere and magnetosphere than by the lightningactivity in the conjugate region. However, Singh et al. (2012)reported that low latitude ground whistlers are most likely linkedwith their causative lightning strikes located near the conjugateregion using the GLD360 lightning location network. Recently,Srivastava et al. (2013) also reported one-to-one relationshipbetween low latitude whistlers and conjugate source lightningstrikes using the WWLLN data. Hence, to confirm the recentfindings that low latitude whistlers are correlated with theconjugate source lightning strikes, we have analyzed large numberof whistlers recorded simultaneously at Varanasi and Allahabad on30th March, 2011 between 20 UT and 21 UT hours and corre-sponding lightning data from WWLLN.

The exact time of the causative sferics for the observedwhistlers is computed using the method described by Helliwell(1965) by plotting the delay time ‘t’ versus ‘f-1/2’ of the whistlerfrequencies ‘f’ and extrapolating the line to meet the origin. If theline passes through the origin, the selected sferics is the causativesferics and the corresponding time is the time of causative light-ning stroke for the particular whistler. The dispersion ‘D’ of the

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whistler is obtained from the product of travel time of the whistlerfrequency components and square root of frequency components(Helliwell, 1965). A typical plot of delay time ‘t’ versus ‘f-1/2’ for awhistler (shown in Fig. 1a) observed on 19th March, 2011 at21:58:04.4024 UT is shown in Fig. 3. It is clearly observed that theline passes through the origin and the selected sferics, S1 is theexact causative sferics for the particular whistlers W1. Hence thetime of this causative sferics which is 21:58:04:3444 UT is thetime of causative lightning stroke for this whistler. The reciprocal

of the slope of the line gives the dispersion of the whistler,D¼13.2 s1/2.

The lightning locations detected by WWLLN around the con-jugate point of Varanasi and Allahabad between 20 UT and 21 UThours on 30th March, 2011 are plotted in Fig. 4a. Fig. 4b shows thezoomed view of the conjugate area to show the conjugate pointsof the receiving stations with WWLLN detected lightning strikes(pink stars) and whistlers producing lightning strikes (blue cir-cles). All of the lightning strikes originated from the region within

Fig. 1. Various types of whistlers observed at Varanasi during the month of March, 2011. The whistlers are marked by W1, W2, W3, etc. and the corresponding causativelightning sferics are marked by S1, S2, S3, etc.

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500–600 km radius circle from the conjugate point which sup-ports the ducted propagation at low latitude stations. Out of 68whistlers observed at Varanasi during one hour (between 20 UTand 21 UT) 39 whistlers were associated with coincident lightningstrikes within 500 km from the conjugate point of Varanasiwhereas at Allahabad out of 117 whistlers 63 were associatedwith coincident lightning strikes within 600 km from the con-jugate point of Allahabad. Thus we observed that for both thestations more than 50% of causative sferics of whistlers wereobserved to match closely with the times of WWLLN detectedlightning strikes within the propagation times of causative tweeks.Singh et al. (2012) observed a large number of 864 whistlers atAllahabad, in the night of 26 January 2011 and using lightning datafrom GLD360 geo-location network, they showed that 311 (36%) ofwhistlers were associated with coincident lightnings within 200and 450 km from the conjugate point of Allahabad.

Correlation study between whistler and lightning activitiesshowed that causative lightning of whistlers were widely distrib-uted around the conjugate point and the distance of the causa-tive sferics from conjugate point varied from 500 to more than2000 km (Weidman and Krider, 1986; Carpenter and Orville, 1989).Ondoh et al. (1979) suggested nearly overhead ionospheric exitpoints for whistlers observed at a Japanese station and proposedducted propagation. Yoshino (1976) observed most of whistlers atSugadaira, Japan, when there was thick cloud cover �500 km

southwest of conjugate point. Collier et al. (2009) have found thatwhistlers observed at Tihany, Hungary, having source region of1000 km surrounding conjugate point, and the source region wasnot circular rather displaced towards magnetic pole from con-jugate point. Other studies by Collier et al. (2010, 2011) forwhistlers observed at Dunedin and Rothera have shown oppositeresults that the source region is rather towards the equator fromthe conjugate point.

One of the advantage of low latitude whistlers while establish-ing correlation between whistlers and lightning activity is that thepropagation delay for the sferics is much less as compared to mid-and high-latitudes (Srivastava et al., 2013). Hence, uncertainty inidentifying the occurrence time of causative sferics is very small.In addition, for low latitude whistlers, the sferics associated withwhistlers easily stand out in the spectrogram and there is reallyvery little ambiguity in linking the sferics and whistlers which isnot so common for mid- and high-latitudes where the propagationtime is high.

The analysis of whistlers from a ground-based station has longbeen regarded as inexpensive and effective tools for ionosphere/plasmasphere diagnostics (Park, 1972; Sazhin et al., 1992; Singhet al., 1998; Carpenter, 2007). For longitudinal propagation thewhistler mode dispersion relation is given as (Helliwell, 1965)μ2 ¼ Π2

ω Ω�ωð Þ where μ is the refractive index, П and Ω are the plasmaand electron gyrofrequency, and ω is the wave frequency. Since therefractive index depends on frequency, the whistler mode isdispersive, which is commonly characterized by the dispersionD¼ t

ffiffiffif

pwhere ω¼ 2πf and t is the time delay of the signal at

frequency f. The dispersion and group delay are primarily depen-dent upon the cold plasma electron density and the McIlwainL-shell (McIlwain, 1961) of the whistler duct. The computedpropagation parameters of the various types of whistlers shownin Fig. 1 are tabulated in Table 1. Dispersion of the observedwhistlers varies between 8 and 18 s1/2. Previous studies haveshown that the dispersion of the whistlers observed at variousIndian stations range from 10 s1/2 to 70 s1/2 (Siingh et al., 2008 andreferences therein). The dispersion analysis also shows that theobserved whistlers have propagated in ducted mode and afterexiting from the duct they penetrate the ionosphere and arereceived at Indian low latitude stations, Varanasi and Allahabad.For our low latitude Indian stations whole propagation path ofwhistlers lies in the ionosphere.

Various models have been used to calculate the plasmasphericparameters (Helliwell, 1965). But no suitable model is availablefor low latitude whistlers whose whole path of propagation liesin ionosphere. If the path lies in the upper ionosphere (Lo1.5) thenplasma density irregularities and plasma blobs are frequentlypresent there; these may affect wave propagation through theprocesses of scattering, diffraction, reflection and non-linear inter-action (Sonwalker and Harikumar (2000)). Park (1972) gave anempirical formula to estimate the time delay due to the ionosphericpath, assuming that the maximum contribution comes from the F2layer of the ionosphere. Using the formula 3.5 in Park (1972), theionospheric dispersion Di is related with columnar electron contentNi as Di¼1.15� (Ni)1/2 where Ni is in 1012 el cm�2. Since for our lowlatitude Indian stations all the propagation took place in theionosphere and due to lack of any suitable model, this ‘correction’can now be taken as responsible region for all time delay (disper-sion). In this way, from the dispersions of the observed whistlers wecan estimate the columnar electron contents along the whistlerpaths using the computed dispersions for all the whistlers recordedat Varanasi. We know that the whistler propagation path passestwice in ionospheric propagation, hence the columnar electroncontent along the complete whistler path is defined as Ni¼Di

2/5.29.The large numbers of whistlers observed at our low latitude stationVaranasi in the month of March, 2011 have been analyzed using

Fig. 2. The hourly occurrence rate of whistlers recorded at (a) Varanasi and(b) Allahabad during March, 2011 (ND¼No data).

Fig. 3. A typical plot of delay time ‘t’ versus ‘f�1/2’ for a whistler (shown in Fig. 1a)recorded at Varanasi on 19th March, 2011 at 21:58:04.4024 UT. The inverse of theslope of the line gives the dispersion.

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above method. The computed propagation and ionospheric para-meters of the various types of whistlers shown in Fig. 1 aretabulated in Table 1. The columnar ionospheric electron contentsof these observed whistlers vary between 13.21 TECU and 56.57TECU, where 1 TECU¼1016 el m�2. This is in good agreement withionospheric total electron contents (TEC) measured using GlobalPositioning Systems (GPS) based measurements over Indian lowlatitude stations (Chauhan and Singh, 2010; Kumar et al., 2012).Kumar et al. (2012) reported that TEC over low latitude Indianstations varies between 10 TECU and 50 TECU. The InternationalReference Ionosphere (IRI) modeled TEC over Indian stations alsocompares well with our computed columnar ionospheric electroncontents using observed whistlers (Chauhan and Singh, 2010;Kumar et al., 2012). For better comparison of the GPS TEC, IRI TECas well as whistlers computed TEC for particular day (times) ofMarch 30, 2011 over Varanasi are plotted in Fig. 5. The figure clearlyshows that the ionospheric total electron contents derived fromwhistler data at Varanasi compare well with the other measure-ments made by other techniques. Note that the TEC values obtainedfrom whistlers are twice as large as the ones obtained from IRImodel or GPS measurements as whistlers propagates twice (‘up’and ‘down’) through the ionosphere.

The analysis of whistlers recorded at a ground station yieldsinformation about the duct properties through which it haspropagated. Dispersion analysis of observed whistlers (Table 1)clearly shows that there exist different ducts in the ionosphere andit is also evident that different whistlers have propagated alongthe geomagnetic field lines in closely spaced different ducts.Kumar et al. (2007) reported two component whistlers at Suva,Fiji and concluded that those whistlers were associated with twoclosely separated lightning strikes and also propagated in twoclose ducts. The analysis of occurrence rate and diffuseness of the

whistler trace give an estimate of the lifetime and width of theduct. The lifetime of ducts may vary from a few minutes to manyhours (Singh, 1993). However, statistical analysis suggests that theduct formation and decay are cyclic phenomena with the timescale of an hour (Hansen et al., 1983; Hayakawa et al., 1983; Singhand Singh, 1999). At low latitudes duct formation is difficult due tothe required density enhancement (�100%), the presence ofcurvature in geomagnetic field, and plasma turbulence. This maybe the reason at low latitude Indian stations AWD recordedcomparatively less number of whistlers in comparison to mid-latitude (Lichtenberger et al., 2008; Rodger et al., 2009).

In order the whistler waves could escape the ionosphere to theground, their wave vector should be close to vertical because therefractive index in the neutral atmosphere is 1, and the horizontalcomponent of the refractive index (wave vector) should be con-served (assuming negligible horizontal electron density gradients)during the propagation. At low latitudes, this is a problem withlongitudinal propagation since the magnetic field has a smallinclination (it is far from being vertical). Hence observation ofwhistlers at low latitude is possible if there are strong horizontalgradients (e.g. edge of the equatorial ionization anomaly) in theionosphere or presence of some irregularities like spread F orplasma bubbles at the time of whistler observations. Kumar andSingh (2009) using GPS based measurements showed that Var-anasi is situated at the edge to equatorial ionization anomalyregion. We have also checked our GPS scintillation data for anypossibility of observation of some ionospheric irregularities orplasma bubbles and found that some weak ionospheric irregula-rities were present at the time of whistlers observation havingscintillation index S4 varied between 0.2 and 0.3. The rates ofchange of TEC during the time of observations of whistlers onMarch 30, 2011 varied between 5 and 9 TECU indicating the

Fig. 4. (a) Location of receiving stations Varanasi and Allahabad along with the lightning locations detected by WWLLN around the conjugate point of Varanasi andAllahabad between 20 UT and 21 UT hours on 30th March, 2011. (b) Zoomed view of the conjugate area to show the conjugate points of the receiving stations with WWLLNdetected lightning strikes (pink stars) and whistlers producing lightning strikes (blue circles). (For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

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Please cite this article as: Singh, A.K., et al., Whistlers detected and analyzed by Automatic Whistler Detector (AWD) at low latitudeIndian stations. Journal of Atmospheric and Solar-Terrestrial Physics (2014), http://dx.doi.org/10.1016/j.jastp.2014.03.001i

presence of plasma bubbles at the time of observations ofwhistlers at Varanasi. This supports the possibility of ductedpropagation of whistlers at our low latitude stations Varanasiand Allahabad.

The ionospheric and other parameters derived from whistlermeasurements compare well with direct rocket and satellitemeasurements (Park et al., 1978; Singh et al., 1998; Chauhan andSingh, 2010; Kumar et al., 2012). The study of the latitudinal andlongitudinal distribution of ionospheric parameters and its longterm variations using rockets and satellites is financially andtechnically challenging, whereas these can be studied very readilyby the whistler technique at a number of stations spread inlatitude and longitude.

4. Conclusion

Propagation features of the whistlers recorded at the low-latitude Indian stations, Varanasi and Allahabad, have been pre-sented to identify the location of causative lightning strikes of thewhistlers using WWLLN data, and to compute various propagationand plasmaspheric parameters. There were significant lightningstrikes detected by WWLLN in the conjugate region around thetime of occurrence of whistlers, suggesting whistler occurrence isstrongly controlled by lightning activity. We observed that for boththe stations more than 50% of causative sferics of whistlers wereobserved to match closely with the times of WWLLN detectedlightning strikes within the propagation times of causative tweeks.All of these lightning strikes originated from the region within500–600 km radius circle from the conjugate point of Varanasi andAllahabad which supports the ducted propagation at low latitudestations. The large numbers of whistlers observed at our lowlatitude station Varanasi in the month of March, 2011 have beenanalyzed using Bernard's method. Dispersion of the observedwhistlers varies between 8 and 18 s1/2. The dispersion analysisshows that the observed whistlers have propagated in ductedmode and after exiting from the duct they penetrate the iono-sphere and are received at Indian low latitude stations, Varanasiand Allahabad. The columnar ionospheric electron contents ofthese observed whistlers vary between 13.21 TECU and 56.57TECU. The ionospheric parameters derived from whistler data atVaranasi compare well with the other measurements made byother techniques.

Acknowledgments

The work is financially supported by ISRO, Bangalore, underISRO-CAWSES India Phase-II Program. We are thankful to boththe reviewers for providing valuable comments/suggestions toimprove the quality of the manuscript.

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Fig. 1 Whistlers Dispersion D (s1/2) Columnar ionosphericelectron contents (TECU)

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