International audienceThe Equator-S magnetometer is very sensitive and has a sampling rate normally of 128 Hz. The high sampling rate for the first time allows detection of ELF waves between the ion cyclotron and the lower hybrid frequencies in the equatorial dawnside magnetosphere. The characteristics of these waves are virtually identical to the lion roars typically seen at the bottom of the magnetic troughs of magnetosheath mirror waves. The magnetospheric lion roars are near-monochromatic packets of electron whistler waves lasting for a few wave cycles only, typically 0.2 s. They are right-hand circularly polarized waves with typical amplitudes of 0.5 nT at around one tenth of the electron gyrofrequency. The cone angle between wave vector and ambient field is nearly always smaller than 1°

Auster, H. U.

Baumjohann, W.

Fornacon, K.-H.

Georgescu, E.

Haerendel, G.

Treumann, R. A.

HAL-INSU

Magnetospheric lion roarsW. Baumjohann, E. Georgescu, K.-H. Fornacon, H. U. Auster, R. A.Treumann, G. HaerendelTo cite this version:W. Baumjohann, E. Georgescu, K.-H. Fornacon, H. U. Auster, R. A. Treumann, et al.. Magne-tospheric lion roars. Annales Geophysicae, European Geosciences Union, 2000, 18 (4), pp.406-410. <hal-00316613>HAL Id: hal-00316613https://hal.archives-ouvertes.fr/hal-00316613Submitted on 1 Jan 2000HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.Magnetospheric lion roarsW. Baumjohann1, E. Georgescu1,2, K.-H. Fornacon3, H. U. Auster3, R. A. Treumann1, G. Haerendel11Max-Planck-Institut fu¨r extraterrestrische Physik, Garching, Germany2 Institut for Space Sciences, Bucharest, Romania3 Institut fu¨r Geophysik und Meteorologie, TU Braunschweig, GermanyReceived: 6 September 1999 / Revised: 6 December 1999 /Accepted: 7 January 2000Abstract. The Equator-S magnetometer is very sensitiveand has a sampling rate normally of 128 Hz. The highsampling rate for the first time allows detection of ELFwaves between the ion cyclotron and the lower hybridfrequencies in the equatorial dawnside magnetosphere.The characteristics of these waves are virtually identicalto the lion roars typically seen at the bottom of themagnetic troughs of magnetosheath mirror waves. Themagnetospheric lion roars are near-monochromaticpackets of electron whistler waves lasting for a fewwave cycles only, typically 0.2 s. They are right-handcircularly polarized waves with typical amplitudes of0.5 nT at around one tenth of the electron gyrofrequency.The cone angle between wave vector and ambient field isnearly always smaller than 1°.Key words: Magnetospheric physics (magnetosphericconfiguration and dynamics; MHD wavesand instabilities; plasma waves and instabilities)1 IntroductionMore than 20 years ago, Smith and Tsurutani (1976)published the classic paper on detailed search-coilmagnetometer observations of lion roars inside magnet-ic troughs in the dayside magnetosheath. They foundthat the lion roars are narrow-banded right-handpolarized waves, basically electron cyclotron waves,that are relatively short-lived, about 2 s and that theyhave typical frequencies of about 120 Hz. Some yearslater, Tsurutani et al. (1982) studied the same phenom-enon with the plasma wave instrument on ISEE andfound these waves at somewhat lower frequencies (50–100 Hz). More recently, Zhang et al. (1998) usedGeotail waveform capture measurements and foundmagnetosheath lion roars between 20–300 Hz andBaumjohann et al. (1999) studied the waveform ofmagnetosheath lion roars below 64 Hz, using thefluxgate magnetometer on board Equator-S.However, until very recently, this type of electronwhistler waves had not been observed inside the Earth’smagnetosphere. Pickett et al. (1999) were the first toobserve a case of narrow band whistler mode waves inthe cusp region with the Polar satellite. These waves hadfrequencies near 200–300 Hz, had a clear packet struc-ture and resembled lion roars. Using the Equator-Sfluxgate magnetometer, we searched for these waves inthe equatorial plane of the dawnside magnetosphere andfound that they are abundant whenever the backgroundmagnetic field is low enough. In the following we willshow that the characteristics of these electron whistlersresemble closely those obtained by Baumjohann et al.(1999) for the magnetosheath lion roars and willconclude that lion roars are not only abundant in themagnetosheath, but also in the magnetosphere, at leastthe dawnside equatorial outer region.2 InstrumentationThe Equator-S magnetic field instrument is fully de-scribed in Fornacon et al. (1999). In short, it consists oftwo units with a pair of three-axes fluxgate magneto-meters each. The sensors of the primary and theredundant units are mounted on two rigid booms, withthe main sensor located at the end of the 1.8-m boomand the other 50 cm further inboard. During the intervalused in the present study, the sampling rate was128 vectors/s, at a resolution of about 10 pT.3 DataBefore proceeding to the full data set, we first presentsome typical examples. Figure 1 shows the waveform(high-pass filtered, f  4Hz, component perpendicularto the main field) of a magnetospheric electron whistlerobserved on 11 April, 1998. Only one component isCorrespondence to: W. BaumjohannAnn. Geophysicae 18, 406–410 (2000) Ó EGS – Springer-Verlag 2000shown, but the other transverse component is nearlyidentical, just phase-shifted by 90, indicating right-handcircular polarization of the wave.Since the lion roar signals are rather short-lived, weuse a wavelet analysis to determine spectrograms of thesignals. The wavelet used is the Morlet wavelet, which isbest suited for this type of analysis (Lui and Najmi,1997; Torrence and Compo, 1998). By transforming thecomplex signal (Br;l  Bx  iBy), we calculate the right-hand and left-hand polarization spectrograms. Since thesignals are all right-hand circularly polarized, Fig. 2shows the spectrogram of the right-hand polarizedsignal of Fig. 1 (upper panel) plus two more examplesobserved on 22 April (middle panel) and 11 April, 1998(lower panel).Figure 1 and the upper panel of Fig. 2 show a ratherlong-lived signal, about 80 wave cycles with an ampli-tude of up to 0.6 nT. The signal is nearly monochro-matic with a center frequency near 20 Hz. A packetstructure is clearly visible. The signal in the middle panelof Fig. 2 is centered around a frequency of about 35 Hzand also exhibits packet structure. That the electronwhistlers can be observed down to about 10 Hz and maycome in longer packets is visible in the lower panel ofFig. 2.Since by visual inspection we found that electronwhistlers were often observed by Equator-S in thedawnside magnetosphere, whenever the backgroundfield was weak, we automatically scanned all our dataobtained in the 02–06 magnetic local time (MLT)sector of the magnetosphere, whenever the ambientfield strength was below 20 nT. The result is thatabout 25% of these periods contained right-handedcircularly polarized waves between 8 and 64 Hz, whichwe analyze in this section. In particular we look at thefrequency and its dependence on the mirror troughbackground field, the duration of the lion roar wavepackets, and the cone angle of the wave vector withthe ambient field.But before going to the characteristics of the waves,let us first look where they occur. Figure 3 clearly showsthat there is no preferred region, neither radial distance(for R  8RE), nor magnetic local time or distance fromthe model neutral sheet of Zhu and Xu (1994). Thewaves are evenly distributed in the dawnside equatorialmagnetosphere, wherever the ambient field is low. Ascan be seen in the upper panel of Fig. 4, the waves areoften found inside magnetic troughs which resemble themagnetic troughs of magnetosheath mirror modes.However, as evident from the lower panel of this figure,they can also occur outside ambient magnetic field de-pletions, like the type-B lion roars of Zhang et al.(1998).Figure 5 shows the occurrence distribution of the lionroar frequencies, normalized to the electron gyrofre-quency, fce, for the more than 27 000 wave packetsfound and analyzed. The normalized frequencies arepeaked near 0.1 fce, with most of the values in the range0:05  f =fce  0:15. Case distribution as well as averagevalue are similar to those found by Zhang et al. (1998)and nearly identical to those presented by Baumjohannet al. (1999), both for magnetosheath lion roars.The packet structure of the waves visible in most ofthe cases presented is also found in the statisticalanalysis. Figure 6 gives the occurrence rates of theduration of the lion roar wave packets in number ofwave cycles. Close to two thirds of all cases have a wavepacket length of less than five wave cycles and about90% last less than ten wave cycles, very similar to theresults of Baumjohann et al. (1999) for the magneto-sheath lion roars.We have also determined the wave normal directionas the direction of the minimum variance, i.e., along theeigenvector having the smallest eigenvalue in a mini-0.60.3-0.6-0.30 1 2   3  0.0secB   [ nT]^Fig. 1. Lion roar waveform perpendicular to the mean field000.00.0 0.5 1.0 1.50.2 0.4 0.6 0.8 1.01 2 3              sec204060Frequenzy  [ Hz]0204060Frequenzy  [ Hz]0204060Frequenzy  [ Hz]         sec          secFig. 2. Right-hand polarized wavelet spectrograms for three mag-netospheric lion roar eventsW. Baumjohann et al.: Magnetospheric lion roars 407mum variance analysis (see, e.g., Smith and Tsurutani,1976; Zhang et al. (1998). The wave normal direction,i.e., its k-vector, was clearly aligned with the back-ground magnetic field. The occurrence distribution ofthe cone angle, hkb, is given in Fig. 7. Except for about50 cases, all cone angles are <1°. In fact, the cone angledistribution drops o sharply beyond, say, 0.2°.These values are again virtually identical to thosefound by Baumjohann et al. (1999). That they are muchsmaller than the typical cone angles of 10–30 found byZhang et al. (1998) and Smith and Tsurutani (1976) hasalready been explained in Baumjohann et al. (1999): Wesuspect that the authors of the earlier papers on lionroars have used too long stretches of data for theirminimum variance analysis and, thus, have not takeninto account the temporal variability of the ambientfield, even on the scale of seconds.98-04-1119:1501:33 01:36 01:39 01:42 01:45 01:4819:16 19:17 UTUT25201581216B [nT]B [nT]98-04-01Fig. 4. Ambient magnetic field strength and occurrence of electronwhistlers in the shaded intervals1500100050000.00 0.05 0.10 0.15 0.20 0.25300020002500FrequencyNumber of Wave PacketsNormalized Frequency, f/fceFig. 5. Occurrence distribution of normalized lion roar frequencies100000 5 10 15 20 252000300040005000Number of  Wave PacketsNumber of Wave CyclesNumber of cyclesFig. 6. Occurrence distribution of lion roar wave packet duration4030201008 9 10 11 12Radial Distance in REOccurence Rate in Percent25101520502 3 4 5 6Magnetic Local Time in HoursOccurence Rate in Percent2510152050-4 -2 0 2 4 6Distance from Neutral Sheet in REOccurence Rate in PercentFig. 3. Occurrence rate of electron whistlers for jBj < 20 nT withrespect to radial distance (upper panel), magnetic local time (middlepanel), and distance from a model neutral sheet (lower panel)408 W. Baumjohann et al.: Magnetospheric lion roars4 SummaryThe present study shows that the characteristics ofelectron whistlers in the dawnside equatorial magneto-sphere, i.e., (1) near-monochromatic right-hand circu-larly polarized, (2) typical frequencies of 0.1 fce,(3) packet structure with often less than 5 wave cycles,(4) wave cone angles of less than 1, are virtuallyidentical to those of magnetosheath lion roars. Acomparison of average numbers for frequency, packetduration, and cone angle, hkB, for magnetospheric andmagnetosheath electron whistlers are given in Table 1,with the latter values taken from Baumjohann et al.(1999).5 ConclusionSince the Equator-S electron plasma instrument failedand the ion data is not yet available with an accuracyneeded, we cannot draw any new conclusions on thenature of these waves other than those discussed inBaumjohann et al. (1999) at this stage.The whistler waves found in the dawnside equatorialmagnetosphere have the same characteristics as thosefound in the same frequency range by Baumjohannet al. (1999) in the magnetosheath. We believe that theycan rightfully be called magnetospheric lion roars. Asfirst shown by Tsurutani et al. (1982), magnetosheathlion roars are caused by a perpendicular electronpressure anisotropy in the magnetic troughs of mirrormode-like structures. Such electron pressure anisotro-pies have been inferred by Baumjohann et al. (1999)from lion roar observations in the magnetosheath. Therethe typical resonant electron energies have been explic-itly determined. As the present observations fall into thesame frequency band and the bandwidths are similar, wemay conclude that it is the same electron energy rangethat is responsible for the excitation of the magneto-spheric lion roars. Whenever mirror eects causedepressions in the magnetic field, trapped electrons canexcite whistler mode waves in the correspondingfrequency range.The modes that occur outside of magnetic troughs, i.e.those indicated in the lower panel of Fig. 4 and thetype-B lion roars of Zhang et al. (1998), may have beenexcited in a similar way and have travelled along theambient magnetic field to the observation point. Unfor-tunately, the lack of plasma data does not allow for acheck of this assumption. However, since whistler wavespropagate at a fractionof the electronAlfve´n speed,whichfor the outer magnetosphere typically is of the order of104 km/s, they can leak from their low-field excitationregion and propagate to another observation site.It is of course not unknown that whistlers can beexcited in the outer magnetosphere. The most typicalexamples are the equatorial chorus emissions frequentlyobserved during substorms (Tsurutani and Smith, 1974)and associated with tail plasma injections. The newlyinjected plasma destabilizes the whistler mode in arelatively broad frequency band (50–100 Hz wide), incontrast to our observations of a very narrow band.Most of the whistlers forming the lion roars discussedhere are generated locally in small-scale decreases of themagnetospheric magnetic field. The narrow-bandednessindicates that they are caused by a small anisotropy ofthe electrons trapped in these mirror-like structures.Thus they seem clear relatives of the magnetosheath lionroars (Baumjohann et al. 1999). The much lower num-ber of whistlers we see at magnetic field gradients maylikely be lion roars that have travelled there fromneighbouring troughs, because they are also narrow-banded. But we cannot exclude the possibility that atleast some of them are related to chorus-like emissions.Acknowledgements. Equator-S was financially supported via grant50OC94024 by the German Space Agency, DARA (now DLR).The wavelet software was provided by C. Torrence and G. Compo,and is available at http://paos.colorado.edu/research/wavelets/.Topical Editor G. Chanteur thanks N. Cornilleau-Wehrlin andanother referee for their help in evaluating this paper.ReferencesBaumjohann, W., R. A. Treumann, E. Georgescu, G. Haerendel,K.-H. Fornacon, and H. U. Auster, Waveform and packetstructure of lion roars, Ann. Geophysicae, 17, 1528–1534, 1999.0.0 0.2 0.4 0.6 0.8 1.0 1.2100001000100101Number of Wave PacketsCone Angle between k and B0Cone angleFig. 7. Occurrence distribution of cone angle between lion roar wavevector and ambient magnetic fieldTable 1. Average frequency, wave packet duration, and wave coneangle for magnetospheric and magnetosheath electron whistlersQuantity Magnetosphere MagnetosheathCentre frequency f/fce = 0.10 f/fce = 0.09Packet duration Dt = 0.19 s Dt = 0.25 sWave cone angle hkB = 0.15° hkB = 0.33°W. Baumjohann et al.: Magnetospheric lion roars 409Fornacon, K.-H., H. U. Auster, E. Georgescu, W. Baumjohann,K.-H. Glaßmeier, J. Rustenbach, and M. Dunlop, The magneticfield experiment onboard Equator-S and its scientific possibil-ities, Ann. Geophysicae, 17, 1521–1527, 1999.Lui, A. T. Y., and A.-H. Najmi, Time-frequency decomposition ofsignals in a current disruption event, Geophys. Res. Lett., 24,3157–3160, 1997.Pickett, J. S., et al., Plasma waves observed during cuspenergetic particle events and their correlation with Polarand Akebono satellite and ground data, Adv. Space Res.,24(1), 23–33, 1999.Smith, E. J., and B. T. Tsurutani, Magnetosheath lion roars,J. Geophys. Res., 81, 2261–2266, 1976.Torrence, C., and G. P. Compo, A practical guide to waveletanalysis, Bull. Am. Meteorol. Soc., 79, 61–78, 1998.Tsurutani, B. T., and E. J. Smith, Postmidnight chorus: a substormphenomenon, J. Geophys. Res., 79, 118–127, 1974.Tsurutani, B. T., E. J. Smith, R. R. Anderson, K. W. Ogilvie,J. D. Scudder, D. N. Baker, and S. J. Bame, Lion roars andnonoscillatory drift mirror waves in the magnetosheath,J. Geophys. Res., 87, 6060–6072, 1982.Zhang, Y., H. Matsumoto, and H. Kojima, Lion roars in themagnetosheath: the Geotail observations, J. Geophys. Res., 103,4615–4626, 1998.Zhu, M., and R.-L. Xu, A continuous neutral sheet model and anormal curved coordinate system in the magnetotail, Chin.J. Space Sci., 14, 269–278, 1994.410 W. Baumjohann et al.: Magnetospheric lion roars

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Magnetospheric lion roars

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