Day Time Whistlers Observed at Low Latitude Varanasi (L = 1.078)

J. Astrophys. Astr. (2019) 40:49 © Indian Academy of Sciences https://doi.org/10.1007/s12036-019-9617-y

Day time whistlers observed at low latitude Varanasi (L = 1.078)

S. B. SINGH, S. S. RAO and A. K. SINGH∗

Department of Physics, Atmospheric Research Lab., Banaras Hindu University, Varanasi 221 005, India. ∗Corresponding author. E-mail: [email protected]

MS received 27 August 2019; accepted 15 November 2019

Abstract. We present results of the ﬁrst time observations of whistlers during day time (sunrise) on 4th January 2017 at 01 UT (UT+5.30 = IST) at Indian low latitude ground station Varanasi(geomag. lat. 14◦55N, geomag. long. 153◦54 E, L.1.078). The main goal of analysis is to study the propagation characteristic of the observed whistlers during the day time (sunrise). These whistlers were observed during the quiet geomagnetic conditions (Dst-index = Ð8 nT). The dispersions of the observed whistlers are found between 11.16 and 14.78 s1/2, which shows that the observed whistlers have propagated in the ducted mode and the whole propagation path of whistlers lies in the ionosphere. Their columnar ionospheric electron contents lie between 23.57 TECU and 39.44 TECU. The ionospheric parameters derived from whistler data at Varanasi compare well with other measurements made by other techniques.

Keywords. Very low frequency waves—whistlers—ionospheric/plasmaspheric parameters—low latitude.

1. Introduction et al. 2012, 2014; Manninen et al. 2013, 2017; Chen et al. 2017). So, different propagation characteristics of Lightning’s discharge produces electromagnetic waves the whistlers are for different latitudes and longitudes from few Hz to many tens of MHz (Burke & Jones (Hayakawa & Tanaka 1978). The mode of propaga- 1992; Weidman & Krider 1986). Waves in the very low tion of the whistlers determines the occurrence activity frequency (VLF) range (3Ð30 kHz) penetrate the iono- and this occurrence activity has been governed by two sphere and propagate along the geomagnetic field lines main factors: (a) propagation conditions from the source without appreciable attenuation from one hemisphere (lightning) to magnetospheric channel and (b) existence to another hemisphere. This type of mode of propaga- of appropriate channels in the magnetosphere (Maeda tion is known as the whistler’s mode and waves are also & Kimura 1956; Smith 1960; Smith et al. 1960; Helli- termed as whistlers (Storey 1953; Singh et al. 1998). well 1965; Smith & Angeram 1968; Walker 1976, 1978; These waves have been received both on the Earth’s sur- Strangeways 1976, 1982). face as well as onboard satellites. Whistlers can have The electron density which is present in the iono- ‘duct’ and ‘non-duct’ propagation path during pene- sphere affects the observations of the whistlers (Crouch- tration through the ionosphere into the magnetosphere ley 1964; Singh et al. 2003, 2014). The field aligned (Helliwell 1965). During the propagation through the density irregularities guided the whistler-mode prop- ionized medium embedded in the geomagnetic field, agation. The whistler-mode waves can also reach the whistler’s waves dispersed and a particular form of the Earth’s surface by scattering on density inhomo- whistler’s spectrogram (time vs. frequency) is obtained. geneities in the ionospheric layer (Kuzichev 2012). These propagated waves carry the information about the During day time, the intensity of the whistler’s mode source region as well as the embedded medium through signal decreased due the D-regions of the ionosphere’s which they have propagated (Storey 1953; Helliwell absorption mechanism (Srivastava 1974). VLF propa- 1965; Singh et al. 1998). gation path were quite stable, but varying in a consistent The different viewpoints have been suggested for and somewhat predictable manner during day time the whistler-mode propagation for different latitudes (Andrew et al. 2011). Hence, it is important to under- (Hayakawa & Tanaka 1978; Singh et al. 1998; Singh stand how the whistlers propagate from source to

0123456789().: V,-vol 49 Page 2 of 10 J. Astrophys. Astr. (2019) 40:49 observation point, when they leave the magnetosphere (Lichtenberger et al. 2008). Data collection is started through the ionosphere during day time. through init system upon PC boot. There is a user level Day time whistlers are a rare observation at low lat- program called as sampler.i386, which communicates itudes (Hayakawa & Tanaka 1984). The low latitude with jacked upon startup and during data logging, it cutoff occurrence of the day time whistlers at geomag- also writes data on the disk and opens a file every netic latitude is ∼20◦ (Tanaka & Hayakawa 1985). The hour. The AWD system has a window length of 10 s geomagnetic latitude of Varanasi (14◦55N, L = 1.078) with a 2 s overlap with the preceding and the subse- is well below this cutoff and hence the present report quent data window. The length of the FFT window is of whistlers during daytime is a unique and rare phe- 256 points and no window overlap is used when pro- nomena. Till now, there are only two observations ducing the spectrogram (Lichtenberger et al. 2008). of the whistlers for the Indian low latitude ground A file can contain more than one trace if consecutive stations during day time: ∼90 whistlers observed at events are found. Whistler files are 4 s long (2 s before Allahabad (geomag. lat. 16.05◦N, geomag. long. 155◦E, and 2 s after detections are saved) generally, but they L = 1.08) (Gokani et al. 2018)and∼100 whistlers at are extended in the case of multiple events (the 2Ð2 Jammu (geomag. lat. = 22◦26N, geomag. long. 74◦E; s margins are kept). False detection can occur if spe- L = 1.17) (Altaf & Ahmad 2013) respectively. A large cific local noises appear or a close storm generates numbers of whistlers have been recorded at our low lat- ’sferics curtain’ spoiling all signal (Lichtenberger et al. itude ground station Varanasi (geomag. lat. 14◦55N, 2008). geomag. long. 153◦54E, L = 1.078) using Automatic The whistler detection algorithm is based on two- Whistler Detector during night time (Singh et al. 2014). dimensional image correlation (Lichtenberger et al. For the first time, we have reported the observations of 2008). The dynamic spectra are computed for 14 s day time whistlers at Varanasi during the sunrise period overlapping data windows and correlated with a pat- on 4th January 2017. tern made from a model whistler. The model whistler In the present paper, we have studied the whistler’s is based on Bernard’s (1973) approximation. At low- observations at 01 UT (01 : 00 + 5.30 = IST) at low latitude stations, the appearance of low dispersion latitude Indian station Varanasi. The general features whistlers is the main factor. Thus the mid-latitude of these day time whistlers have been analyzed. The algorithm with lower dispersion (D) and higher noise experimental setup of AWD is presented in Section 2 frequency ( fn) can be applied (Lichtenberger et al. in detail. The results and discussion of the study is pre- 2008). However, frequent appearance of mid-latitude sented in Section 3 and finally the conclusion is given whistlers in conjunction with our low-latitude ones in Section 4. makes the task more complicated. Both broad band as well as narrow band has been collected by this instrument AWD at low latitude 2. Experimental set-up data Varanasi since December 2010. Typical examples of the observed whistlers during day time at 01:00 UT A unique Automatic Whistler Detector (AWD) sys- on 4th January 2017 has been shown in Figure 1. tem (Lichtenberger 2009) is installed in the premises The spectrum of the whistler is characterized by its of Banaras Hindu University and the VLF waves are discrete tone that decreases in frequency with time, continuously recorded at Varanasi (geomag. lat. = due to the velocity difference between higher and 14◦15N, geomag. Long. = 153◦54E, L = 1.078) lower frequency components as the whistler propagates since December 2010. The main objectives of this through the plasmasphere (Menietti & Gurnett 1980; unique AWD system are the derivation of the plasma Daniell 1985). The whistlers and their causative light- parameter and counting of whistler’s traces for various ing sferics are marked by W1 and S1 respectively at purposes related to whistler’s generation. AWD con- different times. Figure 1 shows 6 traced typical example sists of the following parts: (i) antenna, (ii) preamplifier of whistlers among the 17 whistlers. Their observa- and (iii) AWD software running on the personal com- tion times are 01:21:06.948 UT, 01:45:13.852 UT, puter (PC) with the Linux kernel. A detailed description 01:48:09.5376 UT, 01:49:06.9768 UT, 01:51:55.2656 about the AWD has been found in Lichtenberger et al. UT and 01:51:33:438 UT respectively. For the low (2008). This system is faster than real time when imple- latitude (below 1.3 kHz) and for the high latitude mented on an average PC, hence processing of 1 h of (greater or equal to 1.7 kHz), the cutoff frequency of raw VLF data does not take more than 1 h. AWD runs the whistlers determines their ducted mode propaga- 6Ð12 times faster than real time on a 3 GHz P4 PC tions (Carpenter et al. 1968). The cut-off frequency J. Astrophys. Astr. (2019) 40:49 Page 3 of 10 49

Figure 1. Typical examples of observed whistlers during day time at Varanasi on 4th January 2017. of the observed whistlers is found to be below ∼1.6 3. Results and discussions kHz. The geomagnetic condition is investigated using dis- 3.1 Whistlers occurrence statistics turbance storm time (Dst) index which is taken from http://wdc.kugi.kyoto-u.ac.jp/index.html and the inter- Sun’s ionization generally overpowers any effects of planetary medium parameters like solar wind veloc- lightning during day time. The ionized ionosphere is ity (Vx), interplanetary magnetic field (IMF) Bz and responsible for the radio wave’s bounce. The radio solar wind pressure (Psw) have been taken from waves between the ground site observation and the Goddard Space Flight Center, NASA, USA (http:// source/emitter (transmitter) are also reciprocated by the cdaweb.gsfc.nasa.gov). The possibility for ionospheric lightning. Lightning’s global occurrence rate is found to F-region irregularities over Varanasi is investigated be 40 to 80 per second all the time and the ionosphere using GPS data available at BHU, Varanasi. The is ionized by it. The relation between the lightning and ionospheric conditions for the entire month (Jan- the occurrence activity has been linked by many authors uary, 2017) are analyzed using Vertical Total Electron for the Indian low latitude (Singh et al. 2012, 2014; Content (VTEC) obtained from international GPS Srivastava et al. 2013). Thus it has been concluded that service (IGS) (ftp://cddis.gsfc.nasa.gov) for the path the occurrence activity of the whistlers is determined between Varanasi and its conjugate point. by the lightning. Singh et al. (2014) has also matched 49 Page 4 of 10 J. Astrophys. Astr. (2019) 40:49 closely about 80% of causative sferic of whistlers with rate enhances during the storm (Somayajulu & Tantry the time of WWLLN detected lightning strikes for the 1968; Rao et al. 1974; Kumar et al. 2007). To study the Indian low latitude Varanasi for night time observa- relation between the occurrence activity of the observed tions. It has been reported that the source/conjugate whistler and the geophysical situation in the interplan- region of the observed whistler of Varanasi lies over etary medium as well as Earths’ surface, we have taken the ocean where lightning activity is less compare to the data from Goddard Space Flight Center, NASA, that over land (Singh et al. 2014). We have observed USA of Dst index, solar wind velocity (Vx), interplan- 17 whistlers on 4th January at low latitude Varanasi etary magnetic field (IMF) Bz and solar wind pressure during winter season but this time the Southern hemi- (Psw) during the whistler observation period from 3Ð5 sphere (Indian Ocean) conjugate point of Varanasi has January 2017. The variation of Dst index, IMF Bz, Psw summer season, where the Sun rises earlier than at and Vx during January 3Ð5, 2017 is shown in Figure 2. Varanasi. Lightning discharge may illuminate a large It is clear from Figure 2 that the minimum Dst index area in the ionosphere through which wave energy may occurs during the previous day (3rd Januuary 2017) Ð enter the magnetosphere to appear as a whistler wave in 27 nT at 18:00 UT, the same day (4th January 2017) Ð21 the conjugate region (Chum et al. 2006) and it has been nT at 21:00 UT and the next day (5th January 2017) Ð33 suggested that stations situated within 1,000 km could nT at 23:00 UT. It shows that solar wind speed increases record the same whistlers. Observation of whistlers is during these days. The solar wind speed has a maximum possible when the lightning occurs within a few hun- value of 703 km/s on 5th January 2017 at 23:00 UT and a dred kilometers at the conjugate point of the ground minimum value of 367 km/s at 00:00 UT on 3rd January receiver (Varanasi) during the day time (sunrise), oth- 2017. IMF Bz was northward with a maximum value of erwise observation of whistlers has seldom occurred. 4.7 nT on 3rd January 2017 at 08:00 UT and suddenly It has been reported by many authors that the maxi- turns to southward direction with a value of Ð4.9 nT at mum occurrence activity of whistlers is found in the 09:00 UT on the same day. The whistlers were recorded winter months (JanuaryÐApril) for the Indian low lat- at 01:00 UT on 4th January 2017. The solar wind pres- itude (Singh et al. 2007, 2014; Srivastava et al. 2013; sure reaches a maximum up to 8.55 nPa at 05:00 UT Gokani et al. 2018). The statistical occurrence feature on 4th January 2017, after that it dropped to a mini- of whistlers at low latitudes during the sunrise period mum value of 2.39 nPa at 08:00 UT. All the above data in winter has been reported by Chen et al. (2017). After of solar and geomagnetic activity shows that the day local midnight time, the whistler’s activity was found time whistlers were recorded during a quiet geomag- to be intensified and peaked around sunrise, and then netic period. Recently, Gokani et al. (2018) reported decreased quickly (Chen et al. 2017). The observed day time whistlers during the quiet geomagnetic period. ducted whistlers at ground low latitudes depend on various factors such as the geomagnetic conditions, the 3.3 Computation of ionospheric/plasmaspheric receiver location and the lightning activities in both parameters hemispheres. The less observations of the whistlers at Varanasi may be due to poor match between the The dispersion is a kind of property of whistlers, which causative sferic of the whistlers and WWLLN detected is widely used to yield information about the medium lightning strikes. It cannot strongly support the prop- parameters. The constant unit dispersion (s1/2)isthe agation mechanism of the whistlers during day time group delay of whistler waves propagating at differ- (sunrise). The sensitivity of the ground observation’ ent frequency from the source to the receiver which is receiver, conditions of Earths’ magnetosphere, loca- expressed as (Helliwell 1965) tions of lightning source and ducts in relation to the 1 fo ground observation receiver are the main factors of the D = t f = √ ds. (1) 2c f whistler’s observation and also affect the number of path H observations as well as characteristic intensity of the Dispersion of a whistler depends on the propagat- whistlers. ing path and involving their parameters of the medium which is indicated by Equation (2). The parameter t 3.2 Whistlers and geomagnetic activity represents the group delay travelling time about the propagation path, fo and fH represent plasma fre- Many authors have investigated the whistler’s quency and electron gyro-frequency. In Equation (2), occurrences activities with respect to the geomagnetic the integral over ds is over a dipole magnetic field storm for the low latitude and found that occurrence line from a particular initial latitude. It is difficult to J. Astrophys. Astr. (2019) 40:49 Page 5 of 10 49

Figure 2. Variations of Dst index, IMF Bz, solar wind pressure (Psw) and solar wind velocity (Vx) during 3Ð5 January 2017. determine the arrival time of causative sferics of the magnetosphere’s regions. This propagating path and the whistlers (to) with the help of dynamic wave spectrum, magnetic field are specified by the equatorial gyro fre- and hence there is a chosen alternative set τ = t + to, quency fHeq, which is given as where the exact time to of the causative sferic can√ be = . / 1/3 . obtained by plotting the curve of total time t vs. 1/ f L 9 56 fHeq (2) from the interception of the axis of to and the slope of the curve. The results of the dispersion analysis of the The equatorial gyro frequency f Heq is measured in kHz. observed whistlers during day time have been found The ionospheric dispersion Di and the columnar elec- between 11.16 and 14.78 s1/2. The dispersion analysis tron content Ni is expressed as (Park 1972) shows that the observed whistlers have ducted mode 1/2 Di = 1.15 × (Ni ) , (3) propagation. For our low latitude station Varanasi, it 12 −2 has been observed that the whole propagation path of where Ni is expressed in 10 el-cm . The whistlers whistlers lies in the ionosphere (Singh et al. 2014). pass their propagation path twice in the ionosphere, The dispersion analysis of the signals suggests that so the columnar electron content along the complete whistlers come from distances corresponding to the whistler’s path is defined as (Singh et al. 2014) 49 Page 6 of 10 J. Astrophys. Astr. (2019) 40:49

Table 1. Parameters of observed day time whistlers at Varanasi on 4th January 2017.

Date Figure 1 Dispersion (s1/2) L-value Columnar ionospheric electron contents (TECU)

04-01-2017 (a) 12.50 ± 0.25 1.60 ± 0.129.56 (b) 11.91 ± 0.04 1.11 ± 0.126.83 (c) 12.66 ± 0.29 1.50 ± 0.130.31 (d) 13.85 ± 0.13 1.18 ± 0.136.30 (e) 11.16 ± 0.21 1.37 ± 0.123.57 (f) 13.85 ± 0.08 1.21 ± 0.136.26

= 2/ . . Ni Di 5 29 (4) model or GPS measurements. It is well-known that the whistlers propagate in the ducted mode from one hemi- Typical examples of the observed whistlers on 4th Jan- sphere to another hemisphere in a curved semi-circular uary 2017 at 01:00 UT have been shown in Figure 1.The path. During their propagation through the duct in a computed propagating parameters of these observed curved path they pass twice through the ionosphere, and whistlers (Figure 1) are listed in Table 1. These param- some propagation path is horizontal. It is also known eters of the observed whistlers have been calculated that the dispersion is dependent on the intensity of the with the help of Equations (2), (3)and(4). It is clear ambient magnetic ﬁeld, the electron density and the path from Table 1 that the dispersion varies between 11 and length along the propagation paths. Hence our com- 13 s1/2 and also their columnar ionospheric electron puted ionospheric total electron contents derived from content varies between 23.57 TECU and 36.26 TECU, whistler’s data at Varanasicompare well with other mea- where 1 TECU = 1012 el-cm−2. The variations of the surements made by other techniques. L-value (McIlwain L-shell) lie between 1.11 ± 0.1to For the whistler waves to escape the ionosphere and 1.60 ± 0.1. to propagate in the EarthÐIonosphere wave guide, their We have plotted the columnar ionospheric electron wave vector should be close to vertical and the hori- contents with the dispersion of all the observed whistlers zontal component of the refractive index (wave vector) during 01:00 UT on 4th January 2017 in Figure 3.The should be conserved during propagation. At low lati- results of the dispersion from Figure 3 has been found tudes, it is far from being vertical. Hence observation to be between 11.16 and 14.78 s1/2 which are lower of whistlers at low latitude is possible if there are than the reported day time whistler’s observation for strong horizontal gradients in the ionosphere (i.e. at the low latitude (Hayakawa & Tanaka 1978). Gokani the edge of equatorial anomaly) or the presence of et al. (2018) reported dispersion of whistlers recorded some irregularities like spread F irregularities at the at low latitude station Allahabad between 17.15Ð18.83 time of whistler observations. Using GPS-based mea- s1/2 for day time during 12:00 UTÐ13:00 UT. The dis- surements, it is found that Varanasi lies at the edge persion analysis shows that the observed whistlers have of the equatorial ionization anomaly region (Kumar ducted mode propagation and their whole propagation & Singh 2009; Rathore et al. 2018). We have also path lies in the ionosphere (Singh et al. 2014). For checked our GPS data for any possibility of obser- low-latitude duct formation, it is quite difﬁcult com- vation of some ionospheric irregularities or plasma pared to mid latitude, hence we observe less number bubbles. The rates of change of TEC during the time of whistlers as compared to the mid latitude (Licht- of observations of whistlers on 4th January 2017 are enberger et al. 2008; Rodger et al. 2009). Figure 3 showninFigure5. It can be seen from Figure 5 that shows the variation of columnar ionospheric electron PRN 7 shows two depletions at ∼1345 UT and 1430 contents between 23.57 TECU and 39.44 TECU, where UT. PRN 28 and PRN 30 gives gradient in TEC at 1 TECU = 1012 el-cm−2. ∼1700 UT and PRN 30 shows gradient at ∼2230 UT. Figure 4 shows the GPS TEC, IRI TEC as well as While the time difference in observation of deple- whistlers computed TEC for a particular day (times) on tions for all PRNs is about or more than 1 hour, it is 4th January 2017 over Varanasi for better comparison. interesting to note that the longitudinal location ◦ Note that the TEC values obtained from whistlers are (∼84 E) of the depletions seen by each PRN about three times as large as the ones obtained from IRI (particularly for PRN 7 and 30) is nearly the same. J. Astrophys. Astr. (2019) 40:49 Page 7 of 10 49

Figure 3. Variation of dispersion and columnar ionospheric electron contents of the observed whistlers during 01:00 UT on 4th January 2017.

Figure 4. GPS TEC, IRI TEC as well as whistlers computed TEC for a particular day (times) on 4th January 2017 over Varanasi. 49 Page 8 of 10 J. Astrophys. Astr. (2019) 40:49

Figure 5. The rates of change of TEC during the time of observations of whistlers on 4th January 2017.

This implies that the plasma bubble that produced these During day time, the ionosphere is weakened so that depletions may have sustained for at least an hour, the Earth-ionosphere waveguide (EIWG) is practically wherein it drifted from north to south, in nearly the same isolated from the magnetosphere. The whistler-wave longitude. This could also be conﬁrmed by the similar- ranges satisfy the geomagnetic optical conditions of the ity of the shape of the depletions observed by PRN 28 ionosphere and there are no rapid surges of the refrac- and PRN 30 that passes over Varanasi with time delay tive index during day time (Yonezawa 1966). During of an order of an hour. Figure 5 shows the TEC gradient magnetic quit conditions, passing of day time’s whistler of the order of 4 and 8 TECU indicating the presence mode waves through the ionosphere is a very interesting of small plasma bubbles at the time of observations of problem for us. After analyzing the received waves, we whistlers at Varanasi. The plasma bubble can cause the can get more information about the travelled path of the decrease of average background electron density and whistlers because waves carry the information about the shows the presence of strong horizontal gradients in medium through which they travel (different regions of the ionosphere. This supports the possibility of ducted space). More detailed analysis of the spatio-temporal propagation of whistlers at our low latitude stations variations of whistler’s observations, their propagation Varanasi during day time. properties and connection with the ionospheric and J. Astrophys. Astr. (2019) 40:49 Page 9 of 10 49 magnetospheric conditions during the day time will be Lichtenberger of Eötvös University, Budapest, Hungary more important in the study of whistler’s observations for his help and discussions in the preparation of the mechanism. manuscript.

4. Conclusion References

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