Magnetic ~uctuations close to Io] ion cyclotron and mirror mode wave properties
\ PERGAMON
Planetary and Space Science 36 "0888# 032Ð049
Magnetic ~uctuations close to Io] ion cyclotron and mirror mode wave properties C[ T[ Russe...
Magnetic ~uctuations close to Io] ion cyclotron and mirror mode wave properties C[ T[ Russell \ M[ G[ Kivelson\ K[ K[ Khurana\ D[ E[ Huddleston Institute of Geophysics and Planetary Physics\ University of California\ Los Angeles\ CA 89984!0456\ U[S[A[ Received 16 January 0887^ revised 00 May 0887^ accepted 18 June 0887
Abstract As Galileo approached Io on December 6\ 0884 and passed through the Io wake it passed from a region of ion cyclotron waves at the SO¦ 1 gyrofrequency into a region of strong compressional waves at the edge of the wake region and through a region of waves linearly polarized transverse to the magnetic _eld in the center of the wake region[ As the Io wake was approached the properties of the ion cyclotron waves changed as if there was a change in the wave generating region[ The cyclotron waves were also signi_cantly weaker outbound than inbound[ We interpret the compressional waves to be mirror mode waves generated by the anisotropic pick! up distribution of SO¦ 1 [ The appearance of the mirror mode waves at the edges of the wake suggests that the magnetic _eld or plasma gradient plays a role in limiting the growth rate of the ion cyclotron instability in this region\ in addition to the disappearance of the isotropic warm torus plasma that stabilizes the mirror mode outside of the wake region[ Þ 0888 Elsevier Science Ltd[ All rights reserved[
0[ Introduction Magnetic ~uctuations are important phenomena in planetary magnetospheres as they are both agents for change in the plasma and energetic particle distributions and they are diagnostic of the processes occurring therein[ Despite the passage of _ve previous spacecraft through the Jovian magnetosphere we still know little about the wave processes occurring in the inner magnetosphere and especially in the Io torus\ the region we believe is respon! sible for supplying the mass of the Jovian magnetodisk\ a unique feature of the solar system|s largest planetary magnetosphere[ Previous measurements have shown Jupiter to have an unsteady magnetosphere[ Khurana and Kivelson "0878# reported 09Ð19 min period waves in the middle mag! netosphere\ at a distance of 09Ð24 RJ\ from Voyager 1 magnetometer measurements[ Glassmeier et al[ "0878# found 02Ð19 min waves in the Io torus from about 4[4Ð 6[4 RJ using Voyager 0 magnetometer measurements[ Balogh et al[ "0881# also reported an unsteady magnetic _eld using the Ulysses data[ All authors _nd that the waves have transverse and compressional components[ Other than these approximately 04 min oscillations\ the only other activity reported are ion cyclotron waves in Corresponding author[ Tel[] 990 209 714 9899^ fax] 990 209 195 2940^ e!mail] ctrusselÝigpp[ucla[edu
the middle magnetosphere near the current sheet "Dough! erty et al[\ 0886#[ On December 6\ 0884 the Galileo spacecraft passed through the Io torus returning particles and _elds data from 0410Ð0719 UT as the spacecraft moved from 6[6 RJ to 4[4 RJ and a local time of 0939Ð0139 h[ The trajectory had been chosen to remain close to the midplane of the Io torus[ The magnetometer returned magnetic measure! ments at a rate of 3[4 05!bit vectors per s using the tape recorder for later transmission[ As shown in Fig[ 0 0634]29 UT the Galileo spacecraft passed through the center of the Io wake at a distance of 9[4 Io radii above the surface of Io and almost precisely through the geometric center of the wake[ The initial observations of the mag! netic _elds investigation have been described by Kivelson et al[ "0885#[ As Io was approached by Galileo a relatively narrow band of ion cyclotron waves near the SO¦ 1 gyr! ofrequency was encountered[ The amplitude of these waves grew as Io was approached and diminished as Galileo left Io\ but in the vicinity of the wake they dis! appeared\ only to be replaced by new wave phenomena a strong compressional wave on the edges of the wake and a linear transverse wave in the center of the wake[ The source of these ion cyclotron waves have been dis! cussed by Warnecke et al[ "0886# and by Huddleston et al[ "0886#[ It is the purpose of this article to provide a more detailed description of the properties of the waves close to Io and in the vicinity of Io|s wake[
9921Ð9522:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved[ PII] S 9 9 2 1 Ð 9 5 2 2 " 8 7 # 9 9 9 8 9 Ð 6
033
C[T[ Russell et al[ : Planetary and Space Science 36 "0888# 032Ð049
Fig[ 0[ Sketch of the trajectory of Io for the 11 min surrounding the crossing of the Io wake[ The co!ordinate system is such that the ordinate is increasingly positive in the direction of the corotating plasma[ The abscissa increases toward Jupiter[ Indicated along the trajectory are the times corresponding to the panels of Fig[ 3 "L\ LC\ RC\ R# and Fig[ 4 "a\ b\ c#[
1[ Observations The measurements during the Io ~yby were made with the inboard magnetometer in its 05\999 nT range[ Each of the three orthogonal sensors is sampled at 29 samples per s with a 01!bit analog!to!digital converter that is accurate to one!eighth of the least signi_cant bit "Kiv! elson et al[\ 0881#[ The digital noise associated with this process is 9[25 nT1:Hz and is independent of frequency "Russell\ 0861#[ The data are then digitally _ltered to prevent aliasing and resampled to provide 3[4 05!bit sam! ples per s[ The digital noise level introduced by this 05!bit resampling is negligible relative to the noise introduced during the _rst digitization[ The rms noise associated with the 9[25 nT1:Hz digital noise is 9[8 nT[ Thus during the Io encounter the magnetometer should be able to resolve waves with amplitudes down to about 0 nT[ Figure 1 shows dynamic spectra of the waves in the frequency band 9[90Ð1[14 Hz for the 19 min surrounding closest approach for each of the three components[ This _gure covers almost all of the portion of the trajectory shown in Fig[ 0[ In these plots the data were Fourier analysed in 145!point samples overlapped by 21[ This corresponds to samples of length 46 s\ each one taken 6 s later[ The frequency estimates have been averaged in bands of 04[ The vertical scale displays the logarithm of the frequency[ The color scale goes from 79 nT1:Hz "blue# to 7999 nT1:Hz "red#[ The co!ordinate system in which the data are displayed has BR radially outward from Jupiter\ Bu southward and Bf in the direction of coro! tation[ During the ~yby the magnetic _eld was pre! dominantly in the u direction so that compressional ~uctuations are seen most clearly in the Bu measurements and transverse ~uctuations most clearly in the Br and Bf measurements[
These spectra clearly show the transition on either side of the wake region from the transverse ion cyclotron waves seen in BR and Bu at about 9[1 Hz "log f of about −9[6# both inbound and outbound from Io and to the compressional waves seen in Bu[ The strong burst of activity seen in BR and Bf from 0631Ð0632 UT signals the end of the transverse wave activity[ This activity appears to have begun at 0639 UT at a somewhat higher frequency "about 9[3 Hz# than the pre!Io cyclotron waves "at about 9[2 Hz at 0639 UT#[ The SO¦ 1 ion cyclotron frequency here is 9[3 Hz[ Since the magnetic _eld is prin! cipally in the Bu direction\ this component records the compressional activity and is fairly insensitive to the transverse waves[ The Bf component\ like BR is mainly transverse to the main _eld and generally mirrors the activity in BR[ We note that the burst of waves at 0630 is much stronger in Bf than BR and thus is not circularly polarized[ The region of compressional activity extends from about 0632Ð0638 UT or from 0[54 RIo before the center of the wake to 0[06 RIo post wake[ "Note that the spacecraft is crossing the wake at 9[36 RIo per min and thus there is some spreading of the wave energy due to the _nite width of our time window#[ On exit from the Io wake the compressional ~uctuations and the transverse ~uctuations are both weaker than upon entry[ Moreover\ there seems to be a clear gap centered at 0638]04 between the two types of activity[ Finally\ we note the weakening of activity in all three components in the center of the wake centered on 0635]04 UT[ The Bf component weak! ens the least in the center of the wake due to the presence of a linearly polarized signal almost completely restricted to Bf in the region[ Dynamic spectra of the coherence of the waves shown in the upper two panels of Fig[ 2 reveal a strikingly di}erent pattern[ The cross spectrum of the two trans!
) C[T[ Russell et al[ : Planetary and Space Science 36 "0888# 032Ð049
034
Fig[ 1[ Dynamic spectrum of the power observed in the radial "top#\ azimuthal "middle# and u "bottom# directions from 0625Ð0645 as Galileo traversed the Io wake[ The radial direction points radially outward from Jupiter^ the azimuthal direction points in the direction of the corotating ~ow^ and the u direction is southward[ See text for the details of the construction of the dynamic spectrum[
The periods over which these spectra were obtained are shown in Fig[ 0[ The upper curve shows the transverse power and the lower curve the compressional power in each panel[ While there is a band of frequencies present\ the wave amplitude outside the wake clearly peaks right at the ion cyclotron frequency[ However\ inside the wake the spectrum is totally featureless and very steep[ The transverse power always dominates over the com! pressional power except for frequencies below 9[1 Hz when the mirror mode waves are present[ We note that the ion cyclotron waves have a signi_cant compressional component[ This is consistent with the combined ellip! tical polarization of these waves and their non!parallel propagation[ We emphasize that the compressional waves in the wake are quite strong[ This can be seen in the power spectra shown in Fig[ 3 but we make this point again in Fig[ 4 that shows the time series of the total _eld[ The bottom trace here shows the overall _eld pro_le across the wake while the upper three panels illustrate sections of the traversal[ These intervals are also illustrated in Fig[
verse components shows strong coherence in the fre! quency band around 9[4 Hz on either side of the wake but little coherence at any frequency within the wake[ The low amplitude waves "barely visible on the color scale of Fig[ 1# from 0625 ] 0628 UT and from 0644Ð0645 are quite coherent while the strongest "compressional# waves from 0632Ð0638 UT do not appear in Fig[ 2 at all[ The bottom panel of Fig[ 2 shows the phase spectrum between Br and Bf with a mask covering any estimates for which the coherence is less than 9[4[ The waves that have been identi_ed as transverse ion cyclotron waves are clearly at 89> as expected for cyclotron waves[ However\ there is a small burst of signal at 0639 UT that has a 019> phase di}erence[ This is a real e}ect\ not an artifact[ The dynamic spectra in Fig[ 1 illustrate the trends in the data well but do not illustrate clearly the spectral shape or the absolute amplitudes[ Figure 3 shows slices through the spectrum prior to the wake from 0639Ð0633 "L#^ during the wake passage from 0634Ð0635 "LC# and 0635Ð0636 "RC# and after the wake from 0644Ð0647 "R#[
CMYK Page 034
)
) 035
C[T[ Russell et al[ : Planetary and Space Science 36 "0888# 032Ð049
Fig[ 2[ Dynamic spectrum of the coherence and phase between pairs of components during the Io ~yby] BrÐBu coherence "top#\ BrÐB8 coherence "middle#\ BrÐB8 phase "bottom#[ See text for the details of the construction of the dynamic spectrum[
average frequency of the peak[ This technique di}ers from that employed by Kivelson et al[ "0885# and by Warnecke et al[ "0886#\ who calculate an instantaneous frequency whose power is averaged over 29 s[ The column labeled ukB gives the direction of propagation determined from the quadrature portion of the signal and hence is appropriate for nearly circularly polarized waves as we have here[ The column labeled o indicates how circular\ o 20\ or linear\ o 9\ are the waves[ A positive sign indicates right!handed polarization[ Perhaps surpris! ingly\ the waves|s phase velocities are most closely aligned with magnetic _eld lines and are most circularly polarized about 0629 UT\ not closer to Io[ After Io closest approach the waves have a lower percentage polarization than before closest approach\ the waves are more linearly pol! arized and the angle of propagation less _eld aligned[ These properties and the greater wave amplitude suggest that the mass loading was stronger on the inbound leg over the sunlit hemisphere of Io[ In the outbound region the minimum variance direction di}ers from the direction of the wave normal calculated from the Means|s quad!
0 by the levels a\ b\ c corresponding to the top three panels of Fig[ 3[ The dips in the _eld strength at the two edges of the wake are about 499 nT inbound and 199 nT on exit[ Even in the center of the wake there are 29 nT peak!to!peak compressional variations[
2[ Wave properties The dynamic spectra shown in Fig[ 1 illustrate the presence of ion cyclotron waves on either side of the wake and both compressional and transverse waves within the wake region[ However\ these spectra\ per se\ do not pro! vide the quantitative measurements of wave properties that one would need to compare with theory or simu! lations of the wave growth process[ The properties of the ion cyclotron waves are sum! marized in Table 0 using the wave analysis technique of Means "0861# that is optimum for circularly polarized waves such as these[ The wave period shown is given in proton gyro periods derived from the power weighted
CMYK Page 035
)
C[T[ Russell et al[ : Planetary and Space Science 36 "0888# 032Ð049
036
Fig[ 3[ Spectra of the transverse "thin trace# and compressional "thick trace# wave power at four times during the Io encounter from] 0639Ð0633 just prior to the wake encounter^ from 0634Ð0635 during the _rst period of mirror mode waves^ from 0635Ð0636 during the period when mirror mode waves were not present in the wake^ and from 0644Ð0647 UT just after the wake encounter[
rature technique "Means\ 0861#[ The minimum variance direction remains close to the _eld line[ Thus in order to obtain the direction of the k!vector for the cyclotron waves in this region it is necessary to use the Means technique[ The amplitudes shown are the square root of the total power summed over the three orthogonal directions[ The compressional amplitude is usually about one!quarter the transverse and is clearly non!zero[ In fact immediately post encounter the compressional amplitude is one!half the transverse\ when the wave amplitudes are near their largest values[ The last column shows the ratio of the wave amplitude to the background _eld strength[ Even though these are intense ion cyclotron waves\ they remain in the linear regime at all times with amplitudes no greater than about 1) of the background magnetic _eld strength[ The growth of these waves is examined by both Huddleston et al[ "0886# and Warnecke et al[ "0886#[ Basically\ the damping by the Io torus plasma dominates over the wave growth stimulated by the newly added ions at all frequencies except near the SO¦ 1 gyrofrequency because the SO¦ in the torus has dissociated[ However\ 1 very close to Io the newly added plasma eventually domi! nates over the torus plasma and a new class of waves appears[ Table 1 presents a summary of the wake wave proper! ties using the technique of Rankin and Kurtz "0869# patterned after optical methods[ This technique is opti! mum for linearly polarized signals[ We begin the analysis at 0633 UT where the compressional waves in the wake
_rst begin and end the analysis at 0638 UT where ion cyclotron waves appear again[ The table shows the fre! quency band analysed in Hz\ the angle between the mini! mum variance direction in that frequency band and the magnetic _eld direction\ the eccentricity of the ellipse made by the magnetic ~uctuations\ the percentage pol! arization of the signals\ the rms amplitude of the waves adding the power of all three components\ the amplitude of the compressional part of the wave\ and the ratio of the wave amplitude to the background _eld strength[ Examining _rst the column labeled\ ukB\ we _nd that the minimum variance direction at the edges of the wake is close to orthogonal to the background magnetic _eld[ However\ in the middle of the wake region at all fre! quencies the minimum variance direction is along the _eld[ In other words the waves have become transverse[ This can be con_rmed by inspecting the second and third last columns of the table that show the compressional amplitude and total amplitude of the waves[ Entering and exiting from the wake the compressional amplitude is a major fraction of the total wave amplitude but in the center of the wake the compressional amplitude is about 09) of the total wave strength[ The large wave normal angle implied by the large ukB of the compressional waves can be checked using what amounts to an application of the coplanarity theorem applied to the fast mode shock wave "Russell et al[\ 0876#[ The magnetic perturbations of compressional waves are perpendicular to the direction of propagation and lie in the plane containing the back! ground magnetic _eld and the propagation vector[ Thus
037
C[T[ Russell et al[ : Planetary and Space Science 36 "0888# 032Ð049
Examining the column labeled o\ we see that the waves in the wake region are generally quite linearly polarized[ Finally\ we note that the percent polarization of the waves remains high throughout the wake passage indicating that the waves are due to a single coherent source[
3[ Discussion and conclusions
Fig[ 4[ Time series of the magnitude of the magnetic _eld during the Io wake passage[ The bottom panel shows the full wake passage[ The top three panels show segments of the wake crossings on an expanded scale[ Temporal resolution is 9[111 s in all panels[
the triple vector cross!product of the _eld change\ the background _eld and the _eld change is parallel to the propagation direction[ Applying this formula to the com! pressional dips in the _eld strength we obtain angles of propagation of close to 69>[ Thus the direction of propagation implied by the minimum variance direction is qualitatively correct\ but not quantitatively correct[ The reason for this di}erence is that the minimum vari! ance direction aligns with the wave normal only for cir! cularly or elliptically polarized waves[ These waves are very linearly polarized[
The compressional waves on the edges of the wake resemble mirror mode waves seen in the coma of Comet Halley and in planetary magnetosheaths "Russell et al[\ 0876^ Tsurutani et al[\ 0871# and are quite distinct from the properties of the ion cyclotron waves seen before and after the ~y by[ The appearance of the dips and their derived directions of propagation are very similar to those previously observed mirror mode waves[ The plasma analyser "Frank et al[\ 0885# does not have su.cient temporal resolution to follow any density increase in these structures to con_rm the expected anti! phase relationship present between the magnetic _eld and the plasma density "Vaisberg et al[\ 0878# but the other properties of these ~uctuations are very similar to those seen in situations where we had that verifying infor! mation such as at Comet Halley "Vaisberg et al[\ 0878#[ This includes the steep spectrum\ and the depth of _eld minima and their random overlapping occurrence[ Mirror mode waves arise in the presence of a strong pitch angle anisotropy\ the same anisotropy as respon! sible for the growth of ion cyclotron waves[ We would not expect these waves to arise in the Io torus where the warm torus plasma is quasi!isotropic and will stabilize the mirror mode[ When these particles disappear in the wake it is natural that the mirror mode arises in the presence of the newly picked up ions gyrating per! pendicular to the _eld[ Thus\ the question these waves present to us is why do we not see ion cyclotron waves in the wake region[ What prevents the ion cyclotron waves from growing at the edge of the wake is not clear[ One clue is in the dimensions of the structures[ The gyro radius of SO¦ 1 gyrating in the Jovian _eld at Io with a velocity equal to the corotational velocity is about 19 km[ The velocity of Galileo as it passed through the Io wake was about 03 km:s and the plasma velocity several 09|s of km:s[ Thus the dips in _eld strength associated with the mirror mode waves\ that last 1Ð2 s are a few gyro radii in width with the precise thickness dependent on the orientation of the mirror mode wave fronts[ The gradient in _eld strength and by inference the gradient in the plasma density and pressure are clearly similar but per! haps several times larger as illustrated in the bottom panel of Fig[ 4[ If the gradients in the _eld strength are large\ the phasing of the cyclotron motion of the particles will be randomized and the coherent motion of the ions associated with ion cyclotron waves will be upset[ The mirror mode\ however\ does not require coherent ion
038
C[T[ Russell et al[ : Planetary and Space Science 36 "0888# 032Ð049 Table 0 Ion cyclotron waves properties Time
Notes] RIo is the distance of the observation point from the center of Io^ Vp:³f× is the observed wave period measured in terms of the proton gyro period^ ukB is the angle of the wave normal to the magnetic _eld direction^ o is the wave ellipticity^ )Po0 is the percentage polarization^ Amp is the rms wave amplitude^ CompAmp is the rms amplitude of the compressional ~uctuations^ dB:Bo is the amplitude of the wave normalized by the background magnetic _eld strength[
Notes] Here ukB is the angle of the minimum variance direction from that of the magnetic _eld[
gyro motion[ In the center of the tail the ion b drops as the temperature decreases and the mirror mode is stable even though a large anisotropy could be present[ The transverse linearly polarized waves in the center of the tail have no obvious explanation in terms of wave! particle instabilities[ There is no obvious source of free energy that would cause these waves to grow[ We note that this is the region in which _eld!aligned electron beams were seen "Williams et al[\ 0885# but we would not expect these waves to be resonant with because the electron velocities are much higher than the ULF wave velocity[ These waves may\ however\ be associated with unsteadiness in the mass!loading process[ The pol! arization observed would be expected if the bending or draping of the Jovian _eld associated with the slowing down of the material closest to Io varied with time[ This brief examination of the ion cyclotron waves clos!
est to Io also reveals complexity in their generation process[ The most obvious feature is the inboun! d:outbound asymmetry signaling a strong source region of SO¦ 1 ion pickup on the dayside[ The ion cyclotron waves occur in bursts and those bursts have di}erent characteristics[ The phase di}erences between the com! ponents is not always 89>\ nor are the waves always circularly polarized[ In conclusion\ while the pick!up process seems to domi! nate the growth of waves in the vicinity of Io\ the nature of these waves di}ers markedly depending on the proper! ties of the background plasma[ In the torus proper ion cyclotron waves appear but the edges of Io|s wake pro! mote strong mirror mode growth[ Only 0[4 RIo down! stream of the center of Io the waves have grown to a large fraction of the background _eld[ The dimensions of these features are several ion gyro radii whether one uses the
049
C[T[ Russell et al[ : Planetary and Space Science 36 "0888# 032Ð049
spacecraft velocity or the observed plasma velocity to convert from the temporal signature to the spatial dimen! sions[ In the center of the wake a new wave mode appears[ This wave is linearly polarized transverse to the magnetic _eld and has a steep\ featureless spectrum[ Acknowledgements The authors wish to thank R[ J[ Strangeway for many discussions about wave dispersion relations and Steve Joy for his assistance with Galileo data acquisition[ This work was supported by the National Aeronautics and Space Administration through the Jet Propulsion Lab! oratory|s grant JPL 847409[
References Balogh\ A[\ Dougherty\ M[K[\ Forsyth\ R[J[\ Southwood\ D[J[\ Smith\ E[J[\ Tsurutani\ B[T[\ Murphy\ N[\ Burton\ M[E[\ 0881[ Magnetic _eld observations on the Ulysses ~yby of Jupiter[ Science 146\ 0404Ð 0407[ Dougherty\ M[K[\ Southwood\ D[J[\ Lachin\ A[\ 0886[ Ion cyclotron waves in the Jovian magnetosphere[ Adv[ Space Res[ 19\ 104Ð108[ Frank\ L[A[\ Patterson\ W[R[\ Ackerson\ K[L[\ Vasyliunas\ V[M[\ Coroniti\ F[V[\ Bolton\ S[J[\ 0885[ Plasma observations at Io with the Galileo spacecraft[ Science 163\ 283Ð284[ Glassmeier\ K!H[\ Ness\ N[F[\ Acuna\ M[H[\ Neubauer\ F[M[\ 0878[ Standing hydromagnetic waves in the Io plasma torus] Voyager 0 observations[ J[ Geophys[ Res[ 83\ 04952Ð04965[
Huddleston\ D[E[\ Strangeway\ R[J[\ Warnecke\ J[\ Russell\ C[T[\ Kivel! son\ M[G[\ Bagenal\ F[\ 0886[ Ion cyclotron waves in the Io torus during the Galileo encounter] Warm plasma dispersion analysis[ Geophys[ Res[ Lett[ 13\ 1032Ð1035[ Khurana\ K[K[\ Kivelson\ M[G[\ 0878[ Ultra low frequency MHD waves in Jupiter|s middle magnetosphere[ J[ Geophys[ Res[ 83\ 4130Ð 4143[ Kivelson\ M[G[\ Khurana\ K[K[\ Means\ J[D[\ Russell\ C[T[\ Snare\ R[C[\ 0881[ The Galileo magnetic _eld investigation[ Space Sci[ Rev[ 59\ 246Ð272[ Kivelson\ M[G[\ Khurana\ K[K[\ Walker\ R[J[\ Warnecke\ J[\ Russell\ C[T[\ Linker\ J[A[\ Southwood\ D[J[\ Polanskey\ C[\ 0885[ Io|s Inter! action with the Plasma Torus] Galileo Magnetometer Report[ Sci! ence 163\ 285Ð287[ Rankin\ D[\ Kurtz\ R[\ 0869[ Statistical study of micropulsation pol! arizations\ J[ Geophys[ Res[ 64\ 4333Ð4347[ Russell\ C[T[\ 0861[ Comments on the measurement of power spectra of the interplanetary magnetic _eld\ in Solar Wind NASA SP!297[ 254Ð263[ Russell\ C[T[\ Riedler\ W[\ Schwingenschuh\ K[\ Yeroshenko\ Ye[\ 0876[ Mirror instability in the magnetosphere of comet Halley[ Geophys[ Res[ Lett[ 03\ 533Ð536[ Tsurutani\ B[T[ et al[\ 0871[ Lion roars and non!oscillatory drift mirror waves in the magnetosheath[ J[ Geophys[ Res[ 76\ 5959Ð5961[ Vaisberg\ O[L[\ Russell\ C[T[\ Luhmann\ J[G[\ Schwingenschuh\ K[\ 0878[ Small scale irregularities in comet Halley|s plasma mantle] An attempt at self!consistent analysis of plasma and magnetic _eld data[ Geophys[ Res[ Lett[ 05\ 4Ð7[ Warnecke\ J[\ Kivelson\ M[G[\ Khurana\ K[K[\ Huddleston\ D[E[\ Rus! sell\ C[T[\ 0886[ Ion cyclotron waves observed at Galileo|s Io encoun! ter] Implications for neutral cloud distribution and plasma composition[ Geophys[ Res[ Lett[ 13\ 1028Ð1031[ Williams\ D[J[ et al[\ 0885[ Electron beams and ion composition mea! sured at Io and in its torus[ Science 163\ 390Ð392[
