Available online at www.sciencedirect.com Advances in Space Research 45 (2010) 790–797 www.elsevier.com/locate/asr Simultaneous observations of some unusual whistlers and VLF hiss emission at a low L-value (L = 1.17) K.K. Singh a, J. Singh b, M. Altaf c, Ashutosh K. Singh a, S. Kumar a, A.K. Singh a,*, Shyampati d, Lalmani c a Physics Department, Banaras Hindu University, Varanasi, India Physics Department, G.G.M. Science College, Canal Road, Jammu, India c Physics Department, National Institute of Technology, Hazratbal, Srinagar, Kashmir, India d Physics Department, Udai Pratap Autonomus College, Varanasi, India b Received 12 January 2009; received in revised form 16 October 2009; accepted 21 October 2009 Abstract The purpose of this paper is to describe some unusual whistlers (doublets and triplets) and VLF hiss emission recorded simultaneously on February 18, 1998 during nighttime at a low latitude Indian ground station Jammu (geomag. lat., 22° 260 N; L = 1.17), and to make some discussions about their origin. The detailed structures of the observed VLF hiss emission clearly show that these emissions confined to a narrow continuous frequency band. Some times the frequency of hiss band oscillates and subsequently touches the upper edge of the first whistler component of the doublet. Detailed structures of the dynamic spectra of whistler and VLF hiss emission are briefly presented. From the dispersion analysis of the whistler doublets and triplets, it is found that the individual whistlers of the doublets and triplets simultaneously observed on the same day are one-hop whistlers having propagation path along higher and closely spaced L-values. Our result also shows that VLF hiss has been generated in the equatorial region of higher L-values. Generation and propagation mechanisms are briefly discussed. Ó 2009 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: VLF emissions; Whistler doublet and triplets; Plasmaspheric parameters 1. Introduction Whistlers and emissions in very low frequency (VLF) ranges are considered to be invaluable tools in probing the plasma of ionosphere and magnetosphere. In particular whistler mode waves (VLF waves) and their interactions with energetic particles has been a subject of interest since the discovery of radiation belts. The wave–particle interactions occurring in the magnetosphere generate a variety of * Corresponding author. Address: Atmospheric Research Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221 005, India. Tel.: +91 9793181266; fax: +91 0542 2368390. E-mail addresses: krishna23singh@rediffmail.com (K.K. Singh), altafnig@rediffmail.com (M. Altaf), sk_itvns@yahoo.co.in (S. Kumar), abhay_s@rediffmail.com (A.K. Singh), drlalmani@yahoo.com ( Lalmani). emissions in the extremely low frequency (ELF) and very low frequency (VLF) ranges. ELF/VLF emissions from the Earth’s magnetosphere in the range of few hertz to 30 kHz, both continuous or unstructured and discrete or structured in nature are very fascinating, challenging and interesting natural phenomena. Helliwell (1965) has classified these emissions into hiss, discrete, periodic and quasiperiodic, chorus, hook and inverted hook, and triggered emissions. Of particular interest among, these are the steady, incoherent hiss emissions that were identified as early as a dominant contributor to the loss of radiation belt particles (Kennel and Petschek, 1966; Lyons et al., 1972). On the other hand triggered emissions exhibit a bewildering variety of dynamical spectral forms and follow their apparent source e.g. a whistler, a discrete emission, a signal from transmitter or radiation from World’s power line 0273-1177/$36.00 Ó 2009 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2009.10.016 K.K. Singh et al. / Advances in Space Research 45 (2010) 790–797 (Kennel and Petschek, 1966; Helliwell, 1965). Even some types of chorus are known to arise from the upper boundary of the hiss (Hattori and Hayakawa, 1994; Singh et al., 2000). Many observations support the idea that strong VLF emissions may be triggered even by very weak signals. In extreme cases the triggered source may be invisible. The dynamic spectra of these triggered emissions are complex. The observed facts that (a) triggered emissions are non-stationary transient phenomena, (b) rate of variation of frequency differs from one event to another, and (c) the frequency of these emissions bears no relation to that of triggering signal, testify to this signal. All these type of emissions are frequently observed at high and mid latitudes (Helliwell, 1965; Parrot, 1990). Although the ELF/VLF emissions of different types are often observed at different times at low latitude ground stations in Japan and India, but there is no evidence of the simultaneous occurrence of unusual whistler doublet and triplet, and VLF hiss emission at low latitudes during night hours. Whistler studies in India, which have been in progress since 1963, have made significant contribution to the propagation of low latitude whistlers and VLF/ELF emissions, structure and dynamics of the low latitude plasmasphere (Somayajulu et al., 1972; Hayakawa and Tanaka, 1978; Singh, 1993). Under All India Coordinated Program of Ionosphere Thermosphere Studies (AICPITS) we have conducted initial observations of whistlers and VLF/ELF emissions at our Indian ground-based station Jammu and obtained unique and very interesting result of the some unusual simultaneous occurrence of whistler doublets and triplets, and hiss emission in the early morning local time sector during magnetically highly disturbed periods. These observations at Jammu indicate that lightning generated whistlers may be an important embryonic source for magnetospheric hiss. In all the measurements known to the authors, there is no report of simultaneous occurrence of VLF hiss along with whistler doublets and triplets at low latitudes. In the present paper, we provide a preliminary description and analysis of these whistlers and VLF hiss emissions. Possible interpretations are given. The dispersion analysis of the whistler doublets and triplets recorded simultaneously with the hiss emission shows that they have propagated along higher propagation path with L-values lying between L = 4.01 and L = 4.39. Thus, these reported events could be a part of mid/high latitude phenomena and after exiting from the duct they may have propagated through the Earth–ionosphere waveguide towards the equator to be observed at Jammu. 791 RC network, to avoid any possible ringing effect. The gain of pre/main amplifiers is varied from 0 to 40 dB to avoid any overloading of the amplifier at the time of intense VLF activity. The observations were taken continuously both during day and night times and whistlers/emissions, in large numbers, were observed during quiet and disturbed magnetic conditions. The data were analyzed using digital sonograph with digitization at 16 kHz sampling frequency. The inbuilt software for the spectrum analysis provides dynamic spectrum, which updates in real time typically covering 8 kHz in frequency and 2.54 s in time. The frequency range may be varied from 100 Hz to 40 kHz. Among the VLF data acquired during the span of three years from December 1996 to December 1999 some unique and unusual whistlers and emissions are observed on 18 February, 1998. We report the characteristics of VLF hiss emission, whistler doublets and triplets simultaneously recorded at Jammu during a high geomagnetic activity period (RKp = 33_) on February 18, 1998 in the early morning local time sector (0300-0400 IST). These emissions were recorded during the strong magnetic storm period from 16 to 20 February 1998 with minimum Dst-index-100 nT and maximum Kp – index 7_ on 18 February. Fig. 1(a and b) shows the variation of Kp – index during storm period. The observation period of emission events at Jammu 2. Experimental observations and data analysis The VLF waves are recorded by T-type antenna, amplifiers and tape recorder having bandwidth of 50 Hz–15 kHz. T-type antenna with 25 m vertical length, 6 m horizontal length, 3.2 mm diameter (impedence 1 MX) has been used to record vertical component of wave electric fields. The antenna is rendered a periodic with the help of suitable Fig. 1. The Kp index and Dst index variation during magnetic storm on 16–20 February 1998. The duration of VLF events is also marked. 792 K.K. Singh et al. / Advances in Space Research 45 (2010) 790–797 are also marked in this figure. These events were recorded in the post mid-night period during the recovery phase of the storm. The simultaneous occurrence of this type of events seems to be rare and unique in the sense that such events have not been reported earlier from any of the low latitude ground stations. Typical examples of ground based observations of hiss emission, whistler doublets and triplets recorded simultaneously at Jammu are presented in Figs. 2–4. Whistler doublet (WD) is a pair of closely spaced whistler traces, whereas a whistler triplet (WT) consists of three closely spaced whistler traces. Whistlers of doublet and triplet are one hop multipath whistlers generated by the lightning discharges. On February 18, 1998, the VLF activity started around 0300 h IST (Indian Standard Time) and continued for about an hour. Fig. 2 shows the well-defined spectrograms of simultaneously observed hiss emission, whistler doublet occurred at 0325 h IST. The hiss has bandwidth of about 1 kHz in the frequency range of about 3.5– 4.5 kHz and it tends to rise upto a frequency of about 7.5 kHz. We note that the hiss endures for 5min. The date and time of the observations of each spectrogram of whistlers and emission are mentioned on the top of each figure. The whistlers W1 and W2 of the doublet WD1 are separated by about 0.15 s at 4.0 kHz frequency. The lower and the upper cut-off frequencies of the traces W1 and W2 lie in the range of 2.0 kHz–7.0 kHz and 2.5 kHz– 6.5 kHz respectively. The diffuseness in the traces of the doublet indicated that they might have propagated through the ducts having diffused boundary. Fig. 3 shows another interesting example of oscillating tone VLF hiss and whistler doublet recorded simultaneously at Jammu at 0330 h IST. The bandwidth of the oscillating tone hiss is about 0.5 kHz in the frequency range of 3.5–6.5 kHz with varying dispersion along with a whistler doublet WD2. Oscillating tone hiss emission touches almost the upper edge of the first whistler component W1 of whistler doublet WD2 as if whistler starts from the end of this hiss band. The whistler components W1 and W2 of the whistler doublet WD2 occurred in the frequency range 3.0–5.6 kHz and 5.3–7.4 kHz respectively are separated by about 0.12 s at a frequency of 5.0 kHz. Fig. 4 shows an interesting event of whistler triplet (WT), consisting of three whistler traces separated by about 0.4 s at 5.0 kHz frequency, recorded simultaneously at 0355 IST during the same period of observations of hiss emission on February 18, 1998. The dynamic spectra of the first (W1), the second (W2) and the third (W3) whistlers of the triplet lie in the frequency range of about 2.0–7.0 kHz, 2.8–7.0 kHz, and 3.5–6.0 kHz respectively. All these three traces of this triplet are diffused and of longer periods. In all the three cases reported in this paper, from the dispersion analysis of the whistler components of whistler doublets (WD1 and WD2) and triplet (WT) using Dowden–Allcock method (Dowden and Allocock, 1971) we find that whistlers have propagated along the higher geomagnetic field lines and corresponding L-values are found to be of the order of 4 as shown in Table 1. It should be emphasized that the L-values of the path of propagation of whistlers (shown in Table 1) are derived entirely from the properties of the observed non-nose whistlers at Jammu and by considering a realistic model of electron density distribution along the field line path as discussed in detail by Dowden and Allocock (1971). They have shown that the error in fn is not significantly different between those observed dispersion and by considering the dispersion expected from a model of electron density distributions along the field line path, and the error (percent) is about ±5. Moreover, Helliwell (1965) has developed a nose extension method for the analysis of non-nose whistlers in which he has used the fact that the dispersion function is relatively Fig. 2. Temporal variation of frequency spectra of whistler doublet WD1 (W1 & W2 are whistler components) and hiss emission observed simultaneously at Jammu. K.K. Singh et al. / Advances in Space Research 45 (2010) 790–797 793 Fig. 3. Temporal variation of frequency spectra of whistler doublet WD1 (W1 & W2 are whistler components) and hiss emission observed simultaneously at Jammu. Fig. 4. Temporal variation of frequency spectra of whistler triplet (WT) observed simultaneously at Jammu. insensitive to the shape of electron density distribution model. Subsequently he has also obtained the relation between nose frequency and path latitude of whistlers using nose extension method by taking gyrofrequency electron density model and has shown that this relation is relatively insensitive to the assumed electron density distribution model. Further, based on the evidence from satellites Cahill and Amazeen (1963) have shown that the gyrofrequency electron density model and shape of the Earth’s magnetic field are virtually independent of magnetic disturbance out to distances of the order of eight Earth radii. Helliwell (1965) has measured the effect of magnetic storm on electron density in the magnetosphere using nose whistlers and nose extension methods and has found that the electron density typically drops by about 20% sometimes after the onset of a substorm that has a Kp level of 6 or more. It is worthwhile to mention here that dispersion of whistlers usually recorded in various low latitude Indian ground stations are almost less than 25 s1/2 (Somayajulu et al., 1972). High dispersion whistlers have also been recorded in Japanese, Chinese and Indian low latitude ground stations during the period of severe magnetic disturbances which have 794 K.K. Singh et al. / Advances in Space Research 45 (2010) 790–797 Table 1 Computed L-value and invariant latitude of unusual high dispersion whistler doublets and triplet recorded on 18 February 1998. Time of observation hrs (IST) Trace Dispersion (s1/2) L-value Invariant lat. (deg.) Doublets WD 1 0325 WD 2 0330 W1 W2 W1 W2 65.0 68.0 49.0 53.0 4.25 4.32 4.10 4.21 67.7 68.0 67.1 67.6 Triplet WT 0355 W1 W2 W3 82.0 87.0 89.0 4.35 4.37 4.39 68.1 68.2 68.3 Whistler been interpreted in terms of increase in f0F2 (Hayakawa and Tanaka, 1978; Bao et al., 1983; Chauhan and Singh, 1992). However, since the ionospheric contribution to dispersion is small, an increase in f0F2 is unlikely to increase the dispersion by so much. From the above results, it appears that highly dispersed whistlers (D = 49–89 s1/2) observed on 18 February 1998 at Jammu during strong geomagnetic storm periods are high latitude whistlers as evident from Table 1. Table 1 clearly shows that there exist different ducts (L  4.10– 4.39) in the high latitude ionosphere and magnetosphere. From this table it is also evident that the whistlers of doublets and triplet have propagated along the higher geomagnetic field lines in different ducts. Our whistler dispersion analysis shows that the whistler components of doublet and triplet have dispersion in the range of 49– 89 s1/2 and corresponding L-values found in the range of 4.1–4.4. It may be inferred that whistler doublets and triplet recorded on this day belong to mid/high latitudes and whistler components of these whistlers may have propagated in different ducts along higher L-values and after exiting from ducts, they penetrated the ionosphere and are trapped in Earth–ionosphere waveguide and there after propagating in the waveguide they are received at Jammu. The wave normal angle (lying in the range of 0.2–2.3°) at the entrance into the waveguide is such that they propagated towards the equator and are recorded at Jammu (Singh et al., 2004). The most frequent observations of VLF emissions by satellites near the geomagnetic equator support the idea that the sources of emissions most probably are localized near the equatorial region (Tsurutani and Smith, 1974; Burtis and Helliwell, 1976). The magnetic equator can be found well off the nominal position for dipole fields if the magnetosphere is compressed. Tsurutani and Smith (1974) showed that the equator can be quite elongated on the day side where the solar event pressure can form minimum B pockets. It is now commonly believed so far from the study of low latitude VLF/ELF emission observations that they originate in the equatorial magnetosphere and may have propagated along higher L-values and after exiting from the duct, they penetrated the ionosphere and are trapped in the Earth–ionosphere waveguide and propagated towards the equator and are received at our low latitude ground stations (Singh et al., 2003). The upper boundary frequency (UBF) method as developed by Smirnova (1984) has been generally used for locating the source of VLF/ELF emissions observed at low latitude ground stations (Singh et al., 1996). The upper boundary frequency (fUB) of the ground observed VLF/ELF emissions are determined on the assumption of dipolar geomagnetic field configuration, by the half electron gyrofrequency in the generation region irrespective of the observation station. The UBF method is valid for ducted mode propagation of VLF emissions as well as whistlers and hence can be used for the ducted hiss emissions observed at Jammu. The L-value of the VLF/ELF source is then computed with the help of the relation (Smirnova, 1984) 1=3 L ¼ ð440=fUB Þ ð1Þ where fUB is in kHz. Such an approach is called UBF method. Making use of Eq. (1) and the observed parameters, the values of source region of the hiss observed at Jammu are found to be L = 3.88 for fUB = 7.5 kHz (Fig. 2). Our spectrum analysis clearly shows that the source of VLF hiss observed at Jammu is in the auroral region. Thus, we find that hiss emission observed at Jammu may have been generated in the equatorial region of the geomagnetic field at L  4. After generation, the hiss emission along with whistlers propagated along the field lines in ducted mode and after exiting from the ionosphere must have excited Earth–ionosphere waveguide and propagated towards the equator, so that they could be received at a low latitude station like Jammu. The details of the path of the propagation can be obtained if we could measure polarization and arrival direction of the VLF wave. But these measurements are not available for the reported events. The simultaneous occurrence of hiss along with whistler doublets and triplet gives experimental evidence that VLF hiss generated in the equatorial region of L  4 has propagated in ducted mode along higher L-values and are received at Jammu through Earth–ionosphere waveguide propagation. 3. Results and discussion Whistler and VLF emission occurrence rate is normally quite low at low latitudes but gets enhanced during magnetic storm periods. Because of low occurrence rate for whistlers, the VLF emissions are also rare at low latitudes. From the detailed analysis of considerable volume of VLF data collected at Jammu, we found some well-defined and unique events of whistlers (doublets and triplets) and hiss emissions during magnetically disturbed periods recorded simultaneously on 18 February 1998. Here, we now discuss thoroughly the propagation characteristics of whistler doublets and triplet recorded simultaneously along with VLF hiss on 18 February 1998 at Jammu during strong storm period. It is well-known that K.K. Singh et al. / Advances in Space Research 45 (2010) 790–797 the majority of nighttime whistlers recorded on the ground are interpreted in terms of ducted mode, the non-ducted whistlers are not transmitted to the ground on account of large wave normal angles associated with them and hence they are recorded mostly in satellites (Somayajulu et al., 1972; Hayakawa and Tanaka, 1978; Singh, 1993). Our dispersion analysis clearly shows that whistler doublets and triplet recorded at Jammu have propagated in ducted mode along higher L-values and after exiting from the ducts, they penetrated the ionosphere and are trapped in the Earth– ionosphere waveguide. The wave-normal at the entrance into the waveguide is such that they propagated towards the equator and are received at Jammu. A very valid question on the propagation mechanisms of ducted whistlers (doublets and triplet) and VLF hiss suggested by us above is that since the wave normal of the down coming whistler waves are generally oriented polewards at their exit points from the ionosphere, how these whistlers were recorded at Jammu station whose geomagnetic latitude is much lower than those of the exit latitudes of the mid and high latitude whistlers. In order to answer this it may be mentioned that the ionospheric irregularities existing in the Eregion may scatter the wave normal to be oriented towards the equator and hence their propagation to lower latitudes is possible (Kimura, 1966). The effect of F-region irregularities on whistler propagation to ground has been discussed by James (1972) also. Although, we do not have experimental data to support our argument, the unusual simultaneous occurrence of whistler doublets and triplets, and VLF hiss within just an hour suggests that an ionospheric irregularity did exist during the time of observation. From the dispersion analysis of the whistlers observed at ground station Jammu it is found that they have propagated in ducted mode along the high geomagnetic field lines. Whistler doublets observation by SAS equipment on broad satellite ISIS 2 were interpreted in terms of prolongitudinal (PL) propagation (Thomoson, 1977) whereas whistler doublets observed in active satellite have been interpreted in terms of ducted mode of propagation (Lichtenberger et al., 1991). Singh et al. (1997) has reported the observation of whistler triplets of very low dispersion (14 s1/2) in a low latitude Indian ground station Agra. The time separation between the consecutive whistler traces of the triplets was found to be 0.61 s. They have shown that whistler triplets recorded at Agra during night hours are one hop multipath whistlers which propagated to the ground station under the influence of equatorial anomaly. In our case the simplest explanation of these whistler doublets and triplets observed at Jammu is to assume that each of the successively occurring lightning flashes generated two/three whistlers which started their propagation in closely spaced ducts on the opposite hemisphere (Lichtenberger et al., 1991). Several factors may affect the observed cutoff frequencies of whistlers. Both upper and lower cutoffs are affected by the source spectrum, and by the properties of the Earth– ionosphere waveguide. In additional the lower cutoff fre- 795 quencies may be affected by ion-resonances, and the upper cutoff frequency by collisional absorption in the ionosphere, by duct properties, and by the thermal (Landau) damping near the top of the path. An important cause of whistler attenuation frequencies is the Earth–ionosphere waveguide, through which both the whistler energy and source energy usually travel (Helliwell, 1965). In general the upper cut-off frequency of the whistlers should correspond well to the half electron gyrofrequency at the equator (for the L-shell of duct). In order to verify this fact we have calculated the equatorial electron gyro-frequencies from the calculated L-values of the order of 3.75–4.39 of whistlers observed at Jammu on 18 February 1998 using Dowden Allocock (1971) nose extension method and it was found that the calculated values of the half of the electron gyrofrequency lies in the range of about 5.05– 8.25 kHz. These values do not match exactly with the observed upper cut-off frequency for all whistlers of the doublets and triplet as it varies and is different for different whistlers of the observed doublets and triplet. The unusual lower and upper cutoff frequencies of whistlers W1 and W2 of the doublet shown in Fig. 3 may be due to absorption in the large distance Earth–ionosphere waveguide mode propagation. Dikshit et al. (1971) have mentioned that a strong absorption band exists for whistler waves of frequency 2– 6 kHz in Earth–ionosphere waveguide mode propagation for distances 6000–7000 km. In our case, the distance traveled by whistlers is almost of same order and hence lower and upper frequencies of whistlers above 4 kHz were attenuated. It is worthwhile to mention here that the other possibility for the observed lower and upper cutoff frequency of whistlers could be due to the effect of source spectrum. It may be possible that energy in a source lies only in the frequency range of the observed whistlers W1 and W2. A more detailed investigation on this line is in progress. The occurrence of such whistlers at our station indicates that the waveguide model must have been very specific on that day of observation. The simultaneous occurrence of such type of whistler doublets and triplet along with VLF hiss which traveled a much longer distance in the Earth–ionosphere waveguide is quite incidental. Finally, the whistler triplet consisting of three closely spaced whistler traces shown in Fig. 4 is caused by multiple strokes lightning. Since the dispersion of the three whistlers of the triplet are 82, 87 and 89 s1/2 respectively having very small difference between them and are very close to each other, it is inferred that they are generated by three different closely spaced lightning strokes (Sferics). Unfortunately the lightning data are not available on this day. As mentioned earlier that three generated whistlers of the triplet from each of the successively lightning flashes have propagated in closely spaced ducts on the opposite hemisphere. Such a temporal fine structure in the whistler triplet has not been reported earlier from the ground observations at low latitudes. The reception of VLF hiss on the Earth’s surface clearly shows that the hiss may have been propagated along the geomagnetic field line either in ducted mode or in non-ducted 796 K.K. Singh et al. / Advances in Space Research 45 (2010) 790–797 pro-longitudinal mode. The simultaneous reception of whistler doublets and triplet along with VLF hiss at Jammu clearly shows that VLF hiss has also propagated along the field line in ducted mode similar to that of whistlers. The source may lie in the equatorial region of low latitudes or in the auroral region. Our computations of L-source of VLF hiss by UBF method (Smirnova, 1984) clearly shows that VLF hiss observed at Jammu lies around L = 4. The simultaneous observation of whistler doublets, triplet and VLF hiss gives experimental evidence that VLF hiss has also propagated in ducted mode along higher L-values and after exiting from the duct, they penetrated the ionosphere and are trapped in the Earth–ionosphere waveguide and are recorded at Jammu after propagating through waveguide. The generation mechanism of nighttime VLF hiss at Jammu on 18 February, 1998 during geomagnetic storm could be Cerenkov radiation. An excellent review on auroral VLF hiss has been given by Sazhin et al. (1993). Singh et al. (1999) have shown that the Cerenkov radiated power at low latitudes is quite small. To explain the observed hiss emissions, the radiated wave should be amplified. They have suggested that the radiated wave may be subsequently amplified by the energetic electrons present in the medium through the process of cyclotron resonance mechanism. We now discuss the implications of our observations from several different perspectives. Firstly, our data suggest that the whistlers and broad band VLF hiss observed in the frequency range 3.5–7.5 kHz in the morning sector during magnetic storm period are a dominant whistler wave activity inside the plasmasphere between L = 3.88–4.39 Thus the whistlers and hiss emission observed at our low latitude ground station Jammu may play an important role in the loss of radiation belt particles with energies 645 keV. These waves observed at Jammu can also interact with ring current protons of tens of keV via anisotropic proton instability (Parady, 1974). Secondly we wish to consider the role of lightning which plays an important role in the generation and/or amplification of hiss. VLF hiss and whistlers both are common phenomena observed at Jammu and the typical frequency band of whistlers ranges from 2.0 to 7.0 kHz, whereas of hiss ranges from 3.5 to 7.5 kHz. Whistlers are known to interact with the radiation belt particles (Voss et al., 1984) whereas hiss plays a dominant role in the precipitation of energetic particles (Lyons et al., 1972). We note the fact that prior to the occurrence of reported events on 18 February 1998 there was no whistler and no hiss emission was observed, but a hiss band was still present. This observation implies that hiss can sustain itself in the absence of whistlers and is consistent with the view that lightning generated whistlers initially feed energy into the magnetosphere in the form of hiss which is further amplified enough to reach the observed levels. 4. Conclusion This paper presents interesting observations based on the long term data of whistlers and VLF hiss emission at a low latitude ground station showing that they are not limited to mid and high latitudes. These are observed during strong magnetic storm periods in post midnight sector. The simultaneous observations presented in this paper are unique and is reported for the first time during geomagnetic storm period from low latitudes. Much detailed experimental and modeling study remains to be done in this area, but our results naturally account for the essential features of whistler doublet, triplet and VLF hiss emission simultaneously observed during storm periods. This experimental study is unlikely to be the final word on the origin of these events and further experimental confirmation will, of course, be required at low latitudes. Nonetheless, the observation has the potential to be a ‘circuit breaker’ in our understanding of the generation mechanism of these events observed at low latitudes. However, further detailed mechanism (or process) of the data presented here is a challenging problem and this task will be left for further investigations. Acknowledgements K.K.S. acknowledges support from the Council of Scientific and Industrial Research (CSIR) New Delhi for Research Associate award. Lalmani and A.K. Singh acknowledge support from Department of Science and Technology (DST), New Delhi, India under SERC project. Lalmani, and M.Altaf are thankful to Prof. M. Mubashshir, Director, NIT Srinagar, Kashmir, India for his constant encouragement and support. References Bao, Z.W., Tingzhu, X., Jisheng, C., Songbo, Baixian, I. 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