Electromagnetic Components of Auroral Hiss and Lower Hybrid Waves in the Polar Magnetosphere
H. K. Won2
Department ot Space Science. J-outhwest Research Institute. Sail Antonio. rems
DE-1 has trequentlv observea waves in the whistler and lower nvbnd ireauencies nngc. Besides the electrostatic componenrs. these waves also exhibit electromagneuc components. It IS gencrailv believed that these waves are excited by the electron acousuc instabilitv and the electron-beam- driven lower hybrid instabdity. Because the elecrmn acoustic and the lower hybnd waves arc pmiominatelv electrostatic waves. they can not account for the observed electmmagnetic components. In this work. it is suggested that thcse electromagnetic components can be explained by waves that are generated near the resonance cone and that propagate away from the source. The role that these electromagnetic waves can play in particle acceleration processes at low altitude is discussed.
INTRODUCTION interest in VLF emissions. auroral hiss, and lighting-related phenomena. In the mid-altitude polar magnetosphere. the rlie Emh's polar magnetosphere has long been electron plasma frcquency oF'IS typically lcss than the recognized as an active region of wave acuvities. In the last electron cyclotron frequency Q. Under this condition. the two decades. numerous spacecnft have s'ampled this region whistler mode propagates between thc lower hvbnd ~t and have idenutied a large vanety ot wave modes. frequency. which is approximately the ton plasma trequency ~riostrwranlv. the auroral hiss and the auroral kilometnc w7,,and the electron plasma trcquency. The frequently radiauoii I tor a reuew, see Shawhan. 1979). These waves observed whistler mode auroral hiss IS believed to be .I~Cbcftc~cd to play an imponant role in vmous plasma generated ncx the resonancc cone by either precipitaung processes in the magnetosphere through wave-parucie eleLmns or upward moving electron beams (Gurnett et al., interacuons. Examples of such processes include the 1983). Another wave mode that has received considerable diffusion ot auroral electrons by electrostauc clecmn :ittenuon is the lower hybrid wave. Tlic lower hybnd wave cydotron w;ives (Ashour-AbdaIla and Kennel, 1978); the 113s also been observed in die auroral zone and the polar generation of the auroral kilomeulc radiation through cusp and can be generated near the resonance cone by either rciauvisuc cyclouon resonance (Gutnett, 1974: Wu and Lee. 'in elecuon beam or an ion nng disuibuuon thlaggs, 1976; 1079); .uu the acceleration and heating of ions and electrons Roth and Hudson, 1985). One distinct feature 01 the lower by WNCS in the auroral region, leading to the formation of hybrid wave is that it can accelerate both electrons and ions um mu dectron conical distributions (Cliang et al.. 1986; .mi might be die source for auroral precipitaung electrons I.ysak. 10%: Wong et al.. 1088; Crew et al.. 1990; Tcmenn .ind ion conics (Bingham et al.. 1984 Cllmg and COppl, :ind (k.tixi1s. 1990). 1981). ()ne 01 the wave modes that has been extensively studied The whistler and lower hybnd waves excited near the 111 the P;:SI IS the whistler mode. This is manly due to the rcsonancc cone are quasi-ciectrostatic waves with negligible
magneuc componena. However, thc " funnel-shaped"auroral hiss observed by DE-1 signlties that the auroral hiss has Space Pixmas: Coupling Between Small I al.. ,mu &tedium scale Processes considerable magnetic component.. Gumett et 1983). (;t.lmI?~IGU Monograph 86 Subsequent stability analyses using the observed particle <.cl~\rig: iVSC by the- Amencan Geophysical Union distnbuuons havc revealed that electron acoustic wave.
339 340 AURORAL HISS AND LOWER HYBRID WAVES
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he. 1. 11 rcptesenmuve spectrogram ot the eiectnc ftcld intcnsitics tor a nightside crossing ot thc auronl ticld lincs. llic pl;lsrna densitv over the polar region IS rclativelv low. with t7 rather tli;in the whistler wave, is the more unstable mMfe manly interested in ekCtrOmagfict1C wavcs with frcquencrcs ,]riven nv tile electron beam tLin et al.. 1984, 1985; Tokar between the ion cyclotron :uid the electron cyclotron :md Gary. 1984). nie electron acoustic wave is elecmstauc frequencies arid the corrciation ai’ these waves with the 111 tiaiure .and rhus might account for the elecmstlluc plasmas. As shown below. we have identitied broadband. zompcnent of auroral hiss. The ongin of the elecmmagneuc low-tiequency electromagnetic WiAVCS in the vicinity of the component ot the hiss. however, still remmns unanswered. lower hybrid frequency. We liave also found examples of hi n ditfercnt context, Benson et al. (1988) have reported the clectromagneuc waves in the whistler frcouency range, ?round-hased detectton of waves in the frequency range of which is quite different trom rhc funnel-shaped auroral hiss. 1%-300 kIIz. which indicates the generauon of fieid-aligned WC bciieve that these ektromagnctic waves ;uc tirst waves 111 rhe auroral zone. rlie field-aligned waves observed genefated either by electron beams or ion rings as the lower by tkison et al. are elecuomgneuc and fall into the hybrid or whistler mode near the resonance cone and acqulre trcuucticv range of the whistler mode. Motivated by these clrctromagneuc componciits wiicn thcV propagate away from \)osemauons. Wu et al. (1989) showed that an energeuc the sources. clectron population with a temperature anisotropy or a trapped rype distribution IS capable of generating OBSERV AfIONS tield-:iligned waves in the observed frequency range. In tiin paper. we present dm obtained from the Plasma Figure i shows a frequency-umc specuogm (from fig. Wave iiisuument (PWI) aid the High Altitude Plasma -4 of (hrnett et al.. 19x3) during a nightside crossing of the Iiicmimcnt tkIAPI) onboard the DE-1 spacecraft. We arc ;iuroral ticld lines under the condiuon that the electron plasma Ircquency. w,. is less dian the electron cyclotron t requeiicv. !L. The correspondin? elecmc dcld intensity " zpectrum is shown in Fizure 2 (from Fig. 5 oi Gurnett et al.. -= LO?- 1983). Phcse tipures display some common reatures ot high- .,E I I requeiicv cicctromagneuc waves frequently observed in the luw-density auroral region. At or above thc electron RAMIT(ON. cyclotron t rcquency is the auroral kilometnc radiation. which (FREE SPKf R-Xi is believed to be in the fast extraordinary (R-XImode. The Z mode radlauon is observed at or below the local electron cyclotron trequency, with some possible overhp of ordinary (L-0) mode radiauon (the upper cutoff of the 2 mode is at the upper hvbnd frequency. which is very close to the clectron cyclotron frequency for a low-density plasma). The auroral hiss is hemost commonly observed and is bounded hv the eiccuon plasma frequencv (for oF < ILL Auroral hiss into i:lIls rhc whistler mode range and exhibits a funnel- C542-4 UT 4i;tpe centered on a region ot intense electron precipitauon ,971 I iil 103 IO4 I+ 806 cGumat et al.. 1983). FRfQIENCY. HZ Besides the high-frequency elecuomagneuc waves, low- Fig. 2. An elecmc field intensity specwm selected from the pass frequency elecuomagneuc waves with frequencies at or in Figure A showing the relative intensiues of the aurorai kilomemc below the ion cyclotron frequencies have also &en observed radiation. the 2 mode radiation. and the auroral hiss. The shvp upper cutoff of the aurorai hiss 3t t, and the cutoff of the Z mode in the polar region (see Gurnett et (1984) for a detluled ai. radiation at fs are clearly evident. he dashed line indicates a discussion]. However. not much attenuon has &en paid to region where the spectrum IS being modified by receiver distortion electromagnetic waves between the ion cyclotron and the erfects caused by the very intense kilometnc radiation tfrOm lower hvbnd frequennes. One of the main purposes of the Curnett et al.. 1983). present work is to show that electromagneuc waves in this trcqueiics range are a common occurrence in the polar Plate 2 shows an example of parucle and wave rnaenetospiiere and to explore the roles that these waves can observauons in the nrghtside auroral zone. nie upper panel play in Darucie acceierauon processes at low altitude. As a shows tlic electron data while the middle and lower panels IN CuInoie. Plate 1 (adapted from Sharber et al.. 1988) Jispiay the electric and magnetic ticld dam I'hc electnc tield 4iows [tic pauucie and wave observations in the dayside data showed 3 rather weak auroral lulometnc radlauon. .iuromt mne. I'he upper panel shows the electron data. The which lies above the electron cyclotron frequency t the upper rriiddlc aiiu lower panels display the electnc and magneuc Jotted line), suggesung that the AKR is generated below tlie bpecua. It IS cia from the eleculc and magneuc field satellite. The funnel-shaped auroral hiss emission shows a +natures that the most intense electromagneuc waves strong correlation with the auronf precipitaung electrons. ~~urrcdbetween 0650 UT and 07:15 UT. TIiese which is centered at approximately 07:33 UT.There are two clccuornamcuc waves have frequencies between 200 Hz and distinct bands of emissions just outside the prenpitaung 700 IIL. whereas the local electron cyclotron frequency region, started from 07:43 UT. The first band of emssion ( represented by the dotted Line) is approximately 25 kHz and has a frequency range from 200 Hz to 300 Hz. 'Illis band of (he cmesponding hydrogen cyclotron frequency is emission is essenualiy the same type of emission that we ,ipproxirnately 13.5 Hz. Thus. these electromagneuc waves just discussed with frequencies exten&ng from above the .ire above the ion cyclotron trequency and are in the vicinity ion cyclotron frequency (the lower dotted line) to the lower ot the lower hybrid frequency. rwo features of these waves hybnd frequency. The second band of emission has a xc worm menuoning. First. tliese waves occurred outside frequency range from 500 Hz to 2 kHz. 'This band of [lie rerion ot intense electron precipitation t- from 07:45 UT emission has a frequency above the lower hybrid frequency to ox40 (:TI in which broadband elecuostauc waves arc and falls into the whistler mode nnge. An exmmauon ot' observed. Sccond. the elecuomagneuc waves are observed die magneuc field data suggested that these two bands of at the higner magneuc field side ofthe electron precipitauon. emissions arc eiectmmagneuc. 'niese two bands of I'licse Ic.itures provide some clue of how these waves are clecuomagneuc emissions also have the same features as the xiier:itcu. n tiich we will discuss later. case that we discussed above. 1.e.. they arc observed outside 342 AURORAL HISS AND LOWER HYBRID WAVES rhc region and at the higher magnetic field side ot intense which the parucle precipimuon occurs and propagates clcctron prcapitation. downwxd. Lower hybrid waves can be excited either bs In addition to the two examples shown above. our electron bems in the auroral zonc or by ion nng dismbution rrclrminary survey of the DE plasma wave data has found 111 the poiar cusp. As the lower hybnd waves propagate iiurnerous examples of these electromagneuc waves in the toward higher magneuc tieid strcngth. the waves become .umral mnc. polar cusp, and polar cap. Thc lower band of more electromagnetic and eventually transit into fast rhese clectromagneuc waves typicailv has frequencies magnetosonic waves. As the waves propagate toward lower hctween 100 Hz and 1 kHz, falling between the ion magnetic fieid strength, the waves become moce elecuostatic cyclotron and lower hybrid frequenaes. The upper band and arc absorbed by the background plixixnas. 'nie same emissions have frequencies above the lower hybrid mechanism can also apply to the electromagnetic whistler lrcquciicv and are in the whistler mode range. However. waves except whistler waves can only he genentcd by rlicir charnctenstics are very different from the auroral hiss ciccvon beams since the frequency of the waves is too high xhistlcr mode. In the following, we try to identify these for ions to respond. This sccnano explaiiis naulnlly why ~.\;ives:ind explore the possibilities ot how these waves arc these waves arc mamly observed outside the region and at 2 c I icratcd . Ihe higher magnetic ticld side ut iiitense clcctron precipitation. IXSCUSSION AND SUMMARY We now divert our attcntion to what role Ihc tast magnetosonic waives can play in parucle acceleration ( )ur initial interpretation of the electromagnetic waves processcs at low altitude. -nit acceleration of ionospheric with frcqucncics between the ion cyclotron nnd the lower ions and thc formmon of ion conics have been extensively hybrid frequencies are fast magnetosonic waves since fast studied in the past. Despite the fact that elecuosmuc ion magnetosonic waves arc the only electromqneuc waves in cyclotron arid lower hybrid heaung are regarded as the this frcqucncy range that exhibit such charactenstics (w out promising ion acceleration mechanisms and these waves best knowledge). The other band of electromagnetic liave frequently been observed in the polar regions. thcre is emissions with frequencies above the lower hybrid frequency 110 conclusive evidence that these waves are correlated with (hut f:u below tlie electron cyclotron frequency) are probably ion conics (Kimer and Gurney, 1984). Also. tlicre is t'lccuomagneuc whistler waves. One immediate question to considerable doubt whether thcse processes are opemve :it .I& is whether these waves are locally gcneratcd and by low alutudes. probably 25 low as 400 km. where ion conics .ikit mcc1i:misms. It seems highly unlikely that these waves have been observed (Ynu et nl.. 1983). Tlic electrostatic ion !rc ioeillv generafed based on tlie following consideratlons. cyclotron W:IVCS are difficult to excite at low altitudes since ' :rv. uic t:ist magnetosonic wave has a phase speed at or it requires :i very large crittcai current. rhc lower liyhnd hvciiic Alfven speed. which is very high for a low density wwc: Iins too hish a phase ve~ocitvto resonate with the cold i)~;t,mat ;it lest of the order of 0.1 c, with c being !lie speed ioiiospheric io^. One way to get around this difticulty is to OI light). L'hus hdauresonance can not easily be saustied assume that the lower hybrid w:ives uiidergo a pmmculc Iwwccii [tic waves and the parucles, except for very deav process. The resultin8 dmghter lower hybrid waves cnerectic ciectrons (10's of keV or higher). However. have lower phase velocity and can resonate wid1 the thermal clcctrms with energies LO keV or higher are seldom ions (Koskincn. 1985; Rettcrer et al.. 19x6). IIowever. liils oheervcd in the dayside polar region. Second. when wave process requires intense lower hybnd pump waves. but there I requciicics m far above the ion cyclotron trequency but far is no strung ohservational support of intense lower hybrid hclow rlic electron cyclotron frequency. it is very difficult wivcs at very low altitudes. As was just mentioned. fast \or cyclotron resonance to occur. This might he the main rnalgnetosoiiic w;ives have a very high phase velocity when I c:Lsoii rliat little attention has been pad to electromapetic the w;ive ficquency is not too close to lower hybrid v.;tvcs til this frequency range since there is no obvious frequency. 'Thus. the w;ive c:iii propagate :i long dislance ,iircct means of generating electromagnetic waves in tliis downward pracucally undamped. until the wave frequency rccimc. Lhird, when these eiecnomagneuc waves i~e is very close to die local ion cyclotron rrcquency in which lthcerved. there is little acuvity among die plasmas. ion cyclotron damping OCCUTS. c-nie fast magnetosonic wave if one rules out the possibility ihat these waves arc consists of both riglit and left hnnd components. 'I'hc lett l(Ic;lllv ynerated. one can easily provide an explmauori for Iiand component conmbutcs to cyclotron damping). This I hc occurrence of these waves. We suspect that these waves process leads to the hcaung or acceleration of ions at low .KC ccncrated first as lower hybnd waves near the lower altitudes bv inst magnetosonic wavcs onginating ;it hieh 'lvhrid resonance cone above the satellite in tield liiics under .iltitudcs :ind also provides additional support that the tast WONG 343 3J4 AURORAL HISS AND LOWER HYBRID WAVES > w 1 a u1 I v) I c I w Q . .CW C. c N- 00 WONG 345 ixi;~gnetos(iiiicwaves xc otten observed at above. iiot helow. electric anu magnetic iioise along the auronl ticld lines. J. ilie io~icsclotron trequencv. Geophvs. Res.. 89. 807 1. 1984. hi surnmnry. it ins been shown tlirough exc:lmples trom Etncr. P. M.. mu D. J. (iorncv. A search for the plasma processes DE- 1 w;ivc arid plasma data that elccuomagncric waves wirli associmxi witn perpendicular ion heating. J. I~eopiry.Hec.. 89. 937. 1984. trcquencics between the ion cyclotron and Uilc elccuon Koskinen. tI. E. J.. Lower hybnd paramctnc processes on auroral c~clotrontrcquencics are common occurrcnccs in Uic htli's lieid lines in the topside ionospnerc. J. C;ropIivs. Kcs.. YO. polar rIiacnetosphere. It is suggested that these 8361. 1985. cfcaromayicuc W:IVCS are first generated either hy electron Lin.. C. S.. J. L. Burch. S. D. Shawhan. and L). A. (iuniett. beas or ion rings as the lower hybrid or whistler mode Comlatioii of auroral hiss and upward electron beams iiex the nears [tic rcsonmcc cone :ind then tticv acquire polar cusp. J. Grophvs. Res.. 89. 925. 1984. clccuomngncuccomponents when propagated aWiiV trom tlie [.in. C. S.. D. Winske. and R. L. Tokar. Simulation 01 thr electron sources iow;ird higher magnetic field strengths. The fast acoustic instability in the polar cusp. J. Geophys. Res.. 90. ma~nccosonicwaves might play an Important role in the 8269. 1985. xcelerauon ot ions at low altitudes as they propagate toward Lysak. K. L.. Ion acceleration by wave-particle interaction. it1 loti the ionosphere. Further stu&es which include snbiiity Acceleratrurr in [lie Magnetosphere nnd ionosphere. edited by analvsis using the observed pmcie disutbutions. ray tracing, T. Chang. p. 261. A. G. U.. Washington. D. C.. 19x6. Maggs. J. E.. Coherent generation of VLF hiss. 1. Geuph,hvs. Kes.. :ind quasilinear and ntmiinear analysis desirable to arc 81. 1707. 1976. cxmi~icquainuratmlv the onprn of these W:IVCS. how tlicv Ketterer. J. M..T. Chang, and J. R. Jasperse. fon acccieration hv propagmc down to die tonospllcre. mid thcu subsequent lower hvbna waves in the suprauroral region. J. (;eoph\*s. Hrs.. I1catiIie o[ cold ionosphcre ions through cyclotron dampi~ig. 91. 1609. 1986. Roth. I.. and M. K. Hudson. Lower hybrid heating 01 ionospheric ,\ckrio,~,lEdqetiietifs. 'Ilic author would lie to thank Doug ions duc to ion ring distribution in the cusp. J. C;eoplivs. Res.. Menietu and Chin Lin for valuable discussions. This work was YO. 4191. 1985. supported by NASA grants NAGW- 1620 and NAGS-1552. Sharber. J. R.. J. D. Menietti. H. K. Wong. J. L. Rurch. I). A. (iurnett. 1. D. Winningham. and P. J. Tanskancn. Plasma waves REFERENCES associated with diffuse auroral electrons at mid-altitudes. Ad. Ashour-Abdalla. 11.1.. and C. F. Kennel. Diffuse auroral Space Res.. 8. no. 9. 447. 1988. precipitation. J. Geottiag. Geoelectr.. 30. 239. 1978. Shawhan. S. D.. Magnetosphenc plasma waves. in Solar Svstent Benson. R. F.. M. D. Desch. K. D. Hunsucker. and J. G. Romick. Pfasnta Phwics. vol. III. edited by L. J. Lanzerotti. C. F. Ground-level detection of low and medium-frequency auroral Kennel. and E. N. Parker. North-Hollaird. Amsterdam. 1979. emissions. J. Geophvs. Res.. 93. 277. 1988. Ternerin. M. A.. and D. Cravens. Production of electron conics by Dingham. K.. D. A. Bryant. and D. S. Hall. A wave model for the stochastic acceleration parallel to the magnetic iiold. J. aurora. tieophys. Res. Left.. 11. 327. 1984. Ceophvs. RES..YS. 4285. 1990. L'hans. I ind B. Coppi. Lower hybnd accelention and ion Tokar. R. L.. and S. P. Gary. Electrostatic hiss and the beam dnvcn cvoiution in the supnuronl regions. Geophvs. Res. Lerr.. X. electron acoustic instability in the davside polar cusp. (rrop1rv.s 1253. 1981. Res. Lerr.. 11. 1180. 1984. Chang. I . G. l3. Crew. N. Henhkowitz. J. R. Jasperse. J. M. Wong. H. K.. J. D. Menietti. C. S. Lin. and J. L. Burch. belieration Ketterer. and 1. D. Winningham. Transverse acceleration of of eleciron conical distributions by upper hybrid waves in the oxvgen ions by eteclromagnet~cion cycloeon resonance with Earth's polar region. J. Geophvs. Res.. 93. 10025. 1988. broadoand left-hand polarized waves. Geophys. Res. Left., 13. Wu. C. S.. and L. C. Lee. A theory ot the terresrnal kilomcvic 636. 1986. radiation. Astrophvs. J.. 230. 621. 1979. ('rew. ci B.. T. Chang. J. M. Retterer. W. K. Peterson. D. A. Wu. C. S.. P. €1. Yoon. and H. P. Freund. A theory ol electron (iurnett. and R. L. Huff. Ion cyclotron resonance heated conics: cyclotron waves generated along auroral field lines observed by Theory and observations. 1. Geophvs. Res.. 95. 3959. 1990. ground facilities. Geophvs. Res. Lett.. 16. 1461. 1989. tiurnert. I) .I.. Ihe Earth as a radio source: Terrestrial kilometric Yau. A. W..13. A. Whalen. A. G. McNamara. P. J. Kellopp. and radiation. J. Geophvs. Res.. 79. 4227. 1974. w.Bemstcin. Particle and wave observations of low altitude (iurnett. C, 11.. S. D. Shawhan. and K. R. Shaw. Auroral hiss. Z ionosphenc ion acceleration events. J. Geophvs. Res.. 88. 341. moae radiation. and auroral kilometnc radiation in the polar 1983. magnetospncre: DE-1 observations. J. Geophvs. Res.. 88. 329. 1983. tiurnert. I) .I.. K. L. Huff. J. D. Menietti. J. 1.. Burch. J. D )I. K. Wonp. Ucparunent ot Space Science. Souihwest Research Winninaiiam. and S. D. Shawhan. Correlated low-rrequency Institute. PO Ihwer 28510. San Antonio. TX 78228-0510. Reply Hung K. Wong Aurora Science Incorporated, San Antonio, Texas Charles W. Smith Bart01 Research Institute, University of Delaware, Newark Orlowski and Russell [this issue] claim that the en- We take as our base parameterization: np = ne = ergetic electron distributions used by Wong and Smith 6 ~m-~,Tp = T, = 10 eV, B = 5 nT, nb = 0.6 ~m-~, [1994] in a general theoretical study of instabilities at Tb = 100 ev,q,b = Tib, vb = 3.4 X lo8 CmfS. 'The whistler mode frequencies is irrelevant to the wave ob- resulting instability is maximum at parallel propaga- servations studied by Orlowski and colleagues. We tion with k = 1.6 x and w = 130 rad/s (21 Hz). never claimed to model specific electron distributions This yields spacecraft-frame frequencies greater than 11 ' or to account for particular magnetic wave observations. Hz (if the solar wind speed is 400 km/s and the wave We begin this reply by clarifying the OrZowski and Rus- propagates sunward along a radial magnetic field) and sell [this issue] description of our work, and we end it frequencies as high as 21 HZ if the wave propagates at by showing that ample justification can be found in the right angles to the solar wind velocity. These frequen- work of Orlowski a:ld colleagues and elsewhere to jus- cies are well above the range of any magnetometers used tify the pursuit of a better theoretical understanding of by Orlowski and collaborators. - these instability mechanisms. From this starting point, we vary the above parame- Wong and Smith (19941 focuses primarily on the ex- ters and make use of an anisotropic electron beam. This citation at 1AU of parallel-propagating waves at whist- destabilizes the obliquely propagating waves through ler mode frequencies with plasma-frame frequencies in the same mechanism as Sentman et al. 119831 and shows ;he range 10 to 20 Hz. The spacecraft-frame frequen- that the generation of two simultaneous whistler mode cies tend to be similarly valued, so our paper addresses waves at distinct frequencies and propagation directions primarily waves that are observable in the lowest chan- is again possible at 1 AU as it is at 20 AU. We also nels of plasma wave experiments. Orlowski and Russell demonstrate that the beam-plasma system possesses an [1991] and Orlowski et al. [1990,1993, 19951 investigate unstable mode that is left-hand polarized at whistler waves at spacecraft-frame frequencies of N 1 Hz ob- mode frequencies. We examine the full range of oblique served by magnetometers upstream of Mercury, Venus, propagation using numerical codes and develop a simple and Earth. analytical treatment of the parallel-propagating ' The principal motivation for our paper is our own bilities. work with whistler mode waves upstream of the Uranian It is possible to produce parallel-propagating wavk bow shock [Smith et al., 19911. In that study we report with Doppler-shifted spacecraft-frame frequencies-Glow two instances of simultaneous whistler -mode waves ex- as 1 Ha if the beam speed is small. We-show on 17,141 17,142 WONG AND SMITH: COMMENTARY near the shock and propagate into the upstream region. References They also acknowledge that the electron distributions observed concurrent with 1-Hz waves in the upstream Feldman, W. C., R. C. Anderson, S. J. Bame, S. P. Gary, region may result from the interaction of the beam with J. T. Gosling, D. J. McComas, M. F. Thomson, G. Pasch- preexisting waves and may not represent the source of mann, and M. M. Hoppe, EIectron velocity distributions the 1-HZ fluctuations. The instabilities we discuss could near the Earth's bow shock, J. Geophys. Res., 88,96-110, be operating closer to the shock and may be a source 1983. of the 1-Hz waves seen further upstream. Fitzenreiter, R. J., A. J. Klimas, and J. D. Scudder, Detec- tion of bump-on-tail reduced electron velocity distribu- Orlowshi et al. [1990] report that the majority of 1- tions at the electron foreshock boundary, Geophys. Res. Hs waves upstream of Mercury and Venus are left-hand Lett., 11, 496-499, 1984. polzirized. Since only Newman et al. [1988] and Wong Fitienreiter, R. J., J. D. Scudder, and A. J. Kl+nas, Three- and Smith (19943 demonstrate the ability to produce dimensional analytical model for the spatial variation of left-haid pol-arked waves at whistler mode frequencies, the foreshock electron distribution function: Systemat- bility that this mechanism may explain the left; ic$ and comp&ons with ISEE observations, J. Geophya. Res., 95,4155-4173,1990. handd'waves..-*-1- - 2- *.::-. is worthy of further examination.' The Newman, D. L., R. M. Winglee, and M. ?. Goldman, The- -?elidice --r(rh-.. .aof .c-this- instability should not be minimized. . ory and simulation of $ectromagnetic beam mod- and OrlbWhi et al; [1990, p. 22951 write about 1-Hz waves whistlers, Phyg. Fluids, 91, 1515-1531, 1988. Orlowski, D. S., Bnd C. T. Russell, ULF waves upstream of the Venus bow-shijdi: Properties of the one-Herts waves, .. J. Geophys. Res., 96, 11,271-11,282, 1991. a& with sufficient group velocity to stand Orlowski, D. S., and C. T. Russell, Comments on the "Elec- are whistler mode waves. .. .the observed tron beam excitation of upstream waves in' the whistler must be in fact right-handed in the plasma "a -.-..- mode frequency range" by H. K. Wong and C. W. Smith, frame.. explains the apparent paradox of a Ieft- . This J. Geophys. Res., this issue. handed wave having a compressional component. Orlowski, D. S., G. K. Crawford, and C. T. Russell, Up- stream waves at Mercury, Venus, kid Earth: Comparison The _electron population responsible for the whistler of the properties of one Herb waves, Geophis. Res. I;&., waves upstream of Saturn [Orlowski et al., 19921 is not 17,2293-2296, 1990. resolved by observations. The implication by Orlowski Orlowski, I). S., C. T. Russell, and R. P. Lepping, Wave phenomena in the upstream region of Saturn, J. Geophys. ell that further discussion of possible source Rea., 97, 19,187-19,199, 1992. . amsmg for these waves is irrelevant seems prema- Orlowski, D. S., C. T. Russell, D. Krauss-Varban, and N. ture. ___ Omidi, On the source of upstream whistlers in the Venus --In-summary, Wong and Smith [1994] is a theoret- foreshock, Plasma Environments of Non-Magnetic Plan- i$yd-&.cussion of electron beam instabilities at 1 AU ets, edited by T. I. Gombosi, pp. 217-227, Pergamon, New that emphasized anisotropic temperature distributions York, 1993. in the generation of right- and left-hand polarized elec- Orlowski, D. S., C. T. Russell, D. Krauss-Varban, N. Omidi, and M. F. Thomsen, Damping and spectral formation of tromagnetic waves at whistler mode frequencies. That broadband upstream whistlers, J. Geophys. Res., in press, paper lays the groundwork for an improved theoretical 1995. description of these instabilities and provides an ana- Scudder, J. D., A. Mangeney, C. Lacombe, C. C. Harvey, lytical treatment of the instabilities. Waves at whist- T. L. Aggson, R. R. Anderson, J. T. Gosling, G. Pasch- ler mode frequencies upstream of planetary bow shocks mann, and C. T. Russell, The resolved layer of a colli- possess a wide variety of possible sources. We have no sionless, high-#, supercritical, quasi-perpendicular shock wave, 1, Rankine-Hugonoit geometry, currents, and sta- quarrel with arguments by Orlowski and coauthors that tionarity, J. Geophys. Res., 91, 11,019-11,052, 1986. whistler mode waves in planetary foreshocks may orig- Sentman, D. D., M. F. Thomson, S. P. Gary, W. C. Feldman, inate close to the shock and may not be excited by the and M. M. Hoppe, The oblique whistler instability in the more distant upstream energetic electron distributions Earth's foreshock, J. Geophys. Res., 88, 2048-2056, 1983. observed concurrent with the waves. In fact, we contend Smith, C. W., H. K. Wong, and M. L. Goldstein, Whistler that the broad class of energetic electron observations waves associated with the Uranian bow shock: Outbound observed close to the shock may provide the source for observations, J. Geophys. Res., 96, 15,841-15,852, 1991. Wong, H. K., and C. W. Smith, Electron beam excitation at least a class of upstream whistler waves. We believe of upstream waves in the whistler mode frequency range, that pursuit of these instabilities over a wide range of J. Geophys. Res., 99, 13,373-13,387, 1994. parameter space is worthwhile. C.W. Smith, Bartol Research Institute, University of Del- aware, Newark, DE 19716. (e-mail: chuckQbarto1.udel.edu) H.K. Wong, Aurora Science Inc., 4502 Centerview Dr., Acknowledgments. This work is supported by NASA Suite 215, San Antonio, TX 78228. (e-mail: swrkkit) grant NAGW-3445and Jet Propulsion Laboratory contract 959167 to the Bartol Research Institute as well as NASA (Received December 19, 1994; revised February 28, 1995; grant NAGW-1620 to the Southwest Research Institute. accepted April 3, 1995.)