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Advances in Space Research 41 (2008) 1695–1698

An explanation of the observation of pulsing hiss at low latitude

A.K. Singh a,*, Rajesh Singh b, Kalpana Singh c, R.P. Singh a,d

a Atmospheric Research Laboratory, Physics Department, Banaras Hindu University, Varanasi 221005, Indiab Indian Institute of Geomagnetism, New Panvel, Navi Mumbai 410 218, India

c Radboud University, Nijmegen, The Netherlandsd V.C., V.K. S. University, Ara, Bihar, India

Received 31 August 2006; received in revised form 21 November 2007; accepted 17 January 2008

Abstract

In this paper we report pulsing hiss emissions observed at the low latitude station, Jammu (geomag. lat. 22�260N, L = 1.17) in whichintensity decreases with the increase in frequency. The entire dynamic spectra contain somewhat irregular structure. To explain these wepropose that the hiss emissions are generated through Doppler-shifted cyclotron interactions near the equator and propagate to the earthin the whistler-mode. Further, ULF waves present in the generation region modulate the intensity of the emission resulting in the pulsingnature. The growth rates are computed and discussed in the light of recent works.� 2008 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: VLF emissions; Hiss emission; Whistler-mode waves; Magnetospheric physics

1. Introduction

Whistler-mode VLF emissions having periodic struc-tures have been reported from mid and high latitudes,which are divided into three main groups: periodic emis-sion, hisslers and pulsing hiss (Sazhin and Hayakawa,1994). Good correlations between the pulsing hiss periodswith magnetic pulsations were reported (Sato, 1980; Ward,1983). Comparing the mid latitude whistler data and Pc3pulsation data, Vero et al. (1997) suggested that whistlerducts and geomagnetic field line shells may be connectedwith each other. Marcz and Vero (2002) have analyzedmicropulsations and whistler data recorded at mid lati-tudes and concluded that certain structures in the magneto-sphere, such as field line shells, whistler ducts, paths forwave packets and particle precipitation appear together,suggesting some link between them.

In order to interpret dynamic spectra of pulsing hiss oneshould know the generation and propagation mechanism

0273-1177/$34.00 � 2008 COSPAR. Published by Elsevier Ltd. All rights rese

doi:10.1016/j.asr.2008.01.008

* Corresponding author. Tel.: +91 542 2313431.E-mail addresses: abhay_s@rediffmail.com (A.K. Singh), rajeshsing03@

gmail.com (R. Singh), kalpana.lofar@gmail.com (K. Singh).

of the emissions, the source region and the energy source.In order to get pulsations in the dynamic spectra, one ofthe above mechanisms should have a pulsing nature. Thegeneration mechanism for both hiss and pulsing hiss is con-sidered to be a Doppler-shifted cyclotron resonance inter-action in which pitch-angle anisotropy in the distributionfunction plays a significant role (Gendrin, 1975; Ward,1983). Singh et al. (2005) showed that the ULF wavespropagating along dipolar field lines modulate the geomag-netic field and produce pulsation in the hiss structurethrough changing growth rates.

In this paper we present the observation of pulsing hiss atthe low latitude ground station Jammu. To explain theobserved dynamic spectra we consider that the generationmechanism of the hiss emission is controlled by ULF wavespropagating along geomagnetic field lines, which modulatethe generation process by modulating the local magneticfields.

2. Experimental observation

The reported events were recorded during routine obser-vations of VLF whistler-mode waves using the usual

rved.

1696 A.K. Singh et al. / Advances in Space Research 41 (2008) 1695–1698

recording setup at the low latitude station Jammu. Fig. 1shows dynamic spectra of pulsing hiss recorded on March10, 2002 at around 23.00 h IST. The event continued forabout 30 min. The dynamic spectra are rather irregular instructure and intensity. The upper cut-off frequency variesfrom pulse to pulse. Because of this irregular structure it isdifficult to determine the period, although during the 2 sshown we have about 15 pulses of hiss emissions in the fre-quency range 2.8–6.5 kHz. Pulsing hiss emissions duringthe entire period have approximately the same frequencyrange and periodicity. The event was observed when thedaily R Kp value was 14+.

Fig. 1. Typical examples of the dynamic spectrum of pulsing hiss emissions rec23:20 h IST.

3. Generation mechanism

The observation of pulsing hiss at Jammu station clearlysuggests that these emissions could have been generatednear the equatorial region of L-value corresponding tothe recording station (L = 1.17) and may have propagatedin the whistler-mode. However, in the absence of arrivaldirection measurements it is not possible to state exactlyon which field line the event originated.

Pulsing hiss observed on the ground may have propa-gated along a geomagnetic field line in either the ductedor non-ducted mode. The source of energy could be

orded at Jammu on 10 March, 2002, at (a) 22:54 h IST, (b) 23:10 h IST, (c)

Fig. 3. Variation of the growth rate (s�1) of the VLF wave with time forL = 1.17.

A.K. Singh et al. / Advances in Space Research 41 (2008) 1695–1698 1697

charged particles spiraling along the field lines. Whistler-mode waves propagating along geomagnetic field linesscatter electrons into the loss cone, which may drive highlylocalized field-aligned currents leading to the generation ofAlfven waves that may setup ULF waves along the fieldlines. Or these would be generated at higher latitudes,and propagate across L shells as illustrated in Fig. 2. Thus,the equilibrium conditions breakdown when consideringsuch fast variations and the interaction between the wavesand the electrons becomes a function of time. However, thecondition of resonance interaction remains the same, butthe physical parameters involved become a function oftime. A schematic diagram to illustrate the generation ofpulsing hiss is shown in Fig. 2.

Considering the parameters appearing in the wavegrowth as time dependent functions, separating the vari-ables and differentiating the basic equation for the wavegrowth coefficient, c (Coroniti and Kennel, 1970), weobtain

d ln cdt¼ d ln X

dtþ d ln g

dtþ d ln A

dtð1Þ

where X is the local electron gyrofrequency, g is fraction ofelectron near resonance, and A = (T\ � Tk)/ Tk is the tem-perature anisotropy factor. In the presence of micropulsa-tions of frequency x0, the local magnetic field is modifiedand can be written as

B ¼ B0ð1þ b cos x0tÞ; ð2Þ

where B0 is the background dipolar magnetic field, and b isthe normalized amplitude of the propagating micropulsa-tion/ULF wave. Solving Eqs. (1) and (2), the growth rateis written as (Singh et al., 2005)

c ¼ 1þ b cos x0t1þ b

� � 1þ2AA �

3mX20

kTkK2x

� �

� exp2mX2

0b

kT kK2x

ðcos x0t � 1Þ" #

ð3Þ

where m is mass of electron, k is Boltzmann’s constant andKx is the wave vector of the interacting wave which is ob-tained by the relation Kx = x1/2xpc�1/(X0 � x)1/2 rad/m.This shows that the hiss amplitude varies at the fundamen-tal frequency of the micropulsations. We have numerically

Fig. 2. A schematic diagram to show the generation of pulsing hiss. TheVLF wave propagating in the flux tube shown is amplified and modulatedby ULF waves in the small region (shaded) near the equatorial plane. TheV is the velocity of the interacting ULF waves.

evaluated Eq. (3) for the parameters relevant to L = 1.17 asshown in Fig. 3. In the computation we have used b = 0.05,x0 = 2pf0, f0 = 7.38 Hz, A = 1.5 (Burton, 1976),Tk = 4.6615 � 107 K (Kennel and Petschek, 1966). X0 andxp are the electron cyclotron frequency and the plasma fre-quency, respectively, chosen corresponding to the equato-rial value for L = 1.17 (Carpenter and Anderson, 1992).The wave frequency is taken as 5 kHz as in the middle ofthe band from 2.8 to 6.5 kHz. In computation of growth

rate the value of function 1þ2AA �

3mX20

kT kK2x

comes out to be equal

to 2.66 and then computed growth rate found to oscillatebetween 0.87 and 1.13 s�1 which is clearly seen fromFig. 3. Singh et al. (2005) have also shown that the growthrate oscillates and the amplitude of the oscillation decreasesas L-value increases. However, the absolute value of c islarger at larger L-values.

In the present case the pulsing hiss time period is about0.1355 s, which corresponds to continuous pulsations Pc1.Pc1 forms a standing wave pattern along the geomagneticfield line and thus produces oscillations in the fluxes oftrapped electrons bouncing back and forth along the fieldlines. Thus, the wave–particle interaction is also modified.Considering the hiss emission intensity to be the result ofthe amplification of the waves, we expect correspondingpulsations in the hiss intensity. Davidson and Chiu(1991) have discussed a non-linear mechanism for auroralpulsations, which may provide some more indication onthe possible origin of pulsing hiss.

4. Conclusion

In this paper the observation of pulsing hiss at the lowlatitude station Jammu is reported. An attempt is madeto explain the observed dynamic spectra by consideringthe modulation of the ambient magnetic field by ULFwaves propagating through the source region of the VLF

1698 A.K. Singh et al. / Advances in Space Research 41 (2008) 1695–1698

hiss. The modulated VLF hiss propagates in the whistler-mode to reach the ground station.

Acknowledgements

The work is partly supported by DST, New Delhi underSERC project and partly by UGC, New Delhi under MajorResearch Project. The authors thank the reviewers for theirvaluable comments.

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