Indian Journal of Radio & Space Physics Vol. 28 , October 1999, pp. 216-220

Extremely small dispersion whistlers and VLF emissions recorded during daytime at Jammu

Lalmani , Madhu Kaul Babu & Rajou Kumar

Physics Department ,Regional Engineering College, Srinagar,Camp Jammu,Old University Campus, Canal Road, Jammu 18000 I

and

Rajesh Singh

Atmospheric Research Laboratory, Physics Department, Banaras Hindu University, Varanasi 221 005

and

AKGwal

Space Pl asma Laboratory, Physics Department, Barkatullah University, Bhopal 462 026

Received 30 March 1999; revised 31 May 1999; accepted 15 June 1999

Usi ng an improved system, a unique type of whistlers in addition to the usual types of VLF emissions has been recorded, fo r the fi rst time in India, in February 1998 during daytime at Jammu (geomagn. lat. , 22° 26 'N; geomagn. long., I 47°1O'E; L- l. 17 ). From the di spersion analysis of the daytime whistlers recorded at Jammu it is found that all the whist lers have extremel y small di spersion (ESD) in the range of 5-10 s 112, which clearly supports the nonducted propagation of daytime whistlers at low lati tudes, completel y in contrast with the earlier findings of ducted propagation of daytime whist lers in the presence of equatorial anomaly.

1 Introduction It is ('-well known that lightning discharges are

accompani ed by the generation of electromagnetic waves in a wide frequency range I.2. The broadband very low frequency (VLF, 0 .3-30 kHz) radiation from lightning propagates in the earth-ionosphere cavity as impulsive signals (spherics), and in the dispersive plasma regions of the ionosphere and magnetosphere, it propagates as tones of descending or ris ing frequency (whistlers)'. The VLF radio waves propagating in the magnetospheric plasma scatter energeti c elect rons by whistler mode wave-particle interactions (cyclotron resonance) into the atmos­phere4

.7

. Its consequences are the linear wave amplification and also the wave-induced pitch angle scattering of magnetospheric particles and the assoc iated precipitation into the lower ionosphere8

.

2 Observational records At low lati tudes, the whistler occurrence rate is

low and sporad ic. But once it occurs, as during disturbed periods of so lar activity, its occurrence rate becomes comparable to that at mid-latitudes9

. Similar behaviour has also been observed at the present low lat itude ground station Jammu (geomagn. lat. ,

22°26'N; geomagn. long. , 147°10'E; L-1.17) . During a routine recording in February 1998, the VLF spectra of lightning-generated daytime whistlers along with VLF emissions in large numbers were recorded . On 14 Feb. 1998, the spurt of activity started at around 1230 hrs 1ST (Indian Standard T ime) and lasted fo r about 5 hours, ending at 1730 hrs 1ST. During this period, about 100 whistlers and 25 events of VLF emissions were recorded .

In Figs I and 2 are presented four representative sonograms out of a huge collection of similar events record.ed on this day . Such extremely small d ispersion (ESD) whistler events have not been previously reported from any of the low latitude ground-based VLF observation stations. Most of the VLF emissions are rising tones, inverted hooks (ri ser followed by falling tones) and hiss . The recorded sonograms (Figs 1 and 2) show least noise with very distinct causati ve atmospherics. The daytime whistler acti vi ty at Jammu showed highest occurrence rate during the month of February 1998 and generally, occurred in the very restricted hours of late afternoon . Figure I (a) shows a single trace of a short whistler (W I) of di spersion 5s1l2

with distinct causative atmospherics. Figure I (a) also shows two inverted hook VLF emissions (E I and E2)

LALMAN I ('u li.: WHISTLERS & VLF EMISS[ONS DUR ING DAYTIM E AT JAMM U 217

(a) 1240 hrs 1ST

8 -

6 -

4-

.~

2 -

N J: ..::.: >' o -u

i z (b) 1300 hrs 1ST UJ :::) 8 -

" UJ II: LL

6 -

4 -

2 -

0-

TIME, s

Fig, 1- SOll ograllls as recorded a t Jamlllu Oil 14 Feh. , 1998 at (a ) 1240 hrs 1ST and (b) 1300 hrs 1ST

>­() z W ::J a !J.J a: !.:..

8 -

6-

4 -

2 -

0 -

8 -

6 -

4 -

2-

0-

INOlAN J RADIO & smCE PHYS. OCTOB ER 1999

(a) 1350 hrs 1ST

(b) i 1425 hrs !ST

I o i 1.0 2.0

TIME, s

Fig. 2 - Same as Fig. I. but for (a) I ~50 hrs 1ST ;lIld (b) 1425 hrs 1ST

LALMANI el at.: WHISTLERS & VLF EMISSIONS DURING DAYTIME AT jAM~U 219

with a VLF hiss band (1.8-2.02 kHz) . Figure l(b) depicts two multi flash whistlers (W 2, W 3) having same dispersion of IO s 112 and a rising tone VLF emission (EJ) with hiss band (1.56-2.03 kHz). Figure 2(a) shows a ~;ngle short whistler (W4) of dispers ion 5 S "2 and an inverted hook (E4) with a hiss band ( 1.74-2.01 kHz). The spectra in Fig. 2(b) show a si ngle short whistler (W 5) of dispersion IO s 1/2 and an in verted hook (Es) with hi ss band ( 1.66- 2.06kHz).

3 Discussion

The measured dispersion values of all the recorded whistlers arc found to be extremely small lying in the range of 5- 10 S1/2 . The observation of such ESD whistlers prov ides an indirect and strong evidence in sllpport of nonducted propagation of dayt ime whistlers in contrast to the earlier finding of ducted propagation of daytime whist lers at low latitudes. The ESD whi stlers recorded at Jammu are found to obey strictly the Ec kersley Law (dispers ion being constant with frequency), thereby indicating that the whistlers had a quasi -longitud inal or pro-longitud inal whistler mode of propagati on. The normal dispersion value of whistlers observed at Jammu should be about 22 SII2

. based on the min imum critical frequency of the F2-layer and the e lectron number densi ty at the equatorial he ight of the geomagnetic line of force correspondin g to Jammu, and from the regression line given by Hay akawa and Tanaka lo and also based on the Allcock 's formula ll.

Two different interpretati ons have been proposed to ex plain ESD whi stlers observed during daytime at Jammu. The first is the field-aligned propagation suggested by Tsuruda et at .12 who speculated a hybrid propagati on in wh ich they assumed that the source region is around the magnetic equator and the field­aligned mec hanism is present at latitudes as low as 10°. However, Tsuruda et at. 12 have not observed any traces of hybrid whistlers. This Problem was re­examined by Okuzawa and HoritaU and concluded that the field-a ligned propagation model does not apply to ESD whistlers. The second afld very promi­sin g mechani sm was originally given by Ohtsu 14 who has suggested that a small dispersion of 5 SI I2 could be due to the oblique propagation effec t. The dayti me ESD whi stlers observed at Jammu could be well expitlined on the basis of oblique propagat ion effect arising from the large angle between wave norm~l and geomagnetic field direction at low latitudes as suggested by Ohtsu l4 . Similar mechanism has also

been proposed by Singh et at.15

, who have carried out ray tracing of nonducted whifitlers in the realistic ionospheric model to explain the small dispersion whistlers observed simultaneously at two stations of Gulmarg and Nainital 16

. The present results are in agreement with the conclusion of Singh and Tantry l7 drawn on the basis of their ray tracing study similar to that of Hayakawa and Iwai 18 on the trapping conditions that the ducted propagation of low latitude daytime whistlers appears to be improbable. From the satellite measurements, Cerisierl9-21 and Smith and Angerami22 have also found that nonducted propa­gation occurs on lower L-shells. The daytime ESD whistlers here are thus consistent with the suggestion of nonducted propagation at low latitudes given by

. k 14-22· I h II varIOUS wor ers m comp ete contrast to t e we established earlier findings of ducted propagation of daytime whistlers in the presence of equatorial

I 10 anoma y . The recent work reported so far on the propagation

characteristics of low lat itude daytime whistlers shows only the ducted propagation lo supported by si multaneous measurements by the multi-station networkB and rocket measurements of wave normal d· . 2425 H h b ' lrectlons ' . owever, t e present 0 servatlOns regarding extremely small dispersion whistlers, wh ich , to. our knowledge, have not been reported earlier from any of the higher or lower latitude stations, could be taken as a strong evidence in support of nonducted propagat ion of daytime whistlers at low latitudes . T hus, it is concluded that daytime whistlers observed at the low latitude ground stati on Jammu are attributed to nonducted propa­gation .

Further, the observations of daytime VLF emi ssions at Jammu provide a strong evidence of magnetospheric wave-partide interaction process (shown in Figs I and 2). These types of VLF emissions recorded duri ng daytime have also been reported earl ier by Singh et at?6.27 . The present observation of VLF emissions alongwith ESD whistlers at Jammu clearly suggest that these VLF emissions are generated in the magnetosphere due to whistler-mode wave in teraction with particles .

A detailed analysis of these sonograms is now being carried out to study various ionospheric and magnetospheric features and it is hoped that these results at low latitudes in combination wi th those of mid and high latitudes will contribute significantly towards a better understanding of the wave-particle

220 INDIAN J RADIO & SPACE PHYS, OcrOBER 1999

interaction processes at work in the inner magnetosphere. A further study of these properties is beyond the scope of this paper, and will be reported in a later paper.

Acknowledgements The authors are grateful to the Principal, Regional

Engineering College, Srinagar, Kashmir and to the Principal, Government College of Engineering and Technology, Old University Campus, Jammu, for their constant help and providing facilities to carry out the present study. The present work ~s supported by the Department of Science and Technology, Government of India, New Delhi , India under Grant No./Sanction No. ESSnSI028/93 dated 10103/95.

References I Pathak P P, Rai 1 & 'Varshneya N C, Ann Geophys (France ),

38 (1 982) 765 . 2 Prasad R P & Singh R N , 'Nuovo Cimento (Italy). C5 (1982)

462 . 3 Hel! iweil R A, Whistlers 'C1nd Related Ionospheric

Phenomena (Stanford University Press, Stanford), 1965. 4 Kennel C F & Petschek H E, J Geophys Res (USA) , 71

(1966) I. 5 Inan U S,Bel! T F & Chang H C, J Geophys Res (USA ), 87

(1982) 6243. 6 Chang H C, Inan U S & Bell T F, J Geophys Res (USA ), 88

(1983) 7037. 7 Voss H D, Imhof W L, Mobilia J, Gaines E E, Walt M, Inan

U S & Helliwell R A, Nature (UK) , 3 12 (1984) 740. 8 Brice N M, Tech Trep No. SEL 64088 (Radiosci . Lab.,

Stanford Univers ity, USA ), 1964.

9 Hayakawa M, Tanaka Y, Sazhin S S, Tixier M & Okada T, J Geophys Res (USA), 93 (1988) 5685.

10 Hayakawa M & Tanaka Y, Rev Geophys Space Phys tUSA) , 16 (1978) II I.

I I Allcock G Mck, IGY whistler results, Paper presented at \3 1h

General Assembly, Union Radio Sci . lnt. , London, 1960. 12 Tsuruda K, Kokubun S, Oguti T & Nagota T, Rep lonos &

Space Res (Japan) 18 (1964) 438. 13 Okuzawa T & Horita M, Rep lonos & Space Res (Japan), 28

(1 974) 111. 14 Ohtsu J, 'The effect of oblique p ropagation on whistler

dispersion,' Paper presented at 14'h General Asse:nbly, Union Radio Sci . Int. , Tokyo, 1963.

15 Singh B, Mishra S N & Tantry B A P, J Geomagn & Geoelectr Jpn (Japan ), 24 ( 1972) 277.

16 Dikshit S K, Lalmani, Somayaju1u V V & Tantry B A P, Indian J Pure & Appl Phys, 9 (1971 ) 580.

17 Singh 8 & Tantry B A P, Ann Geophys (France), 29 (1973) 56 1.

18 Hayakawa M & Iwai A, J Atmos & Terr Phys (UK) , 37 (1 975) 1211.

19 Cerisier'J C, Space Res (Netherlands), 11 (1971 ) 1313. 20 Cerisier 1 C, J Atmas & Terr Phys (UK), 35 (1973) 77. 21 Ceri sier J C, J Atmas & Terr Phys (UK) , 36 (1974) 1443. 22 Smith R L & Angerami J J , J Geophys Res (USA ), 73 (1968)

I. 23 Hayakawa M & Ohtsu J, J Atmos & Terr Phys (UK) , 35

(1973) 1685 . 24 Iwai A, Okada T & Hayakawa M, J Geophys Res (USA ), 79

(1974) 3870. 25 Iwai A, Okada T & Hayakawa M, Denki Tsushin Gakkai

Zasshi (Japan ), 598 ( 1976) 181 . 26 Singh U P, Singh R P, & Lal mani , Earth. Moon & Planets

(Netherlands), 73 (1993) 267. 27 Singh R p, Singh U P, Si ngh, A K & Singh D P, Can. J Phys

(Canada) , 72 (1 994) 73 .