Low-frequency electromagnetic waves near and below the proton cyclotron frequency at the AMPTE Ba release: Relevance to comets and Mars

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 14, NO. A4, PAGES , APRIL 1, 1999 Low-frequency electromagnetic waves near and below the proton cyclotron frequency at the AMPTE Ba release: Relevance to comets and Mars K. Sauer, i E. Dubinin, i M. Dunlop, K. Baumgartel, 3 and V. Tarasoy 4 Abstract. "Nongyrotropic massloading" is a characteristic signature of the AMPTE barium release in the solar wind because of the large pickup radius compared with the size of the interaction region. In the fluid picture this is equivalent to a beam-plasma configuration where the solar wind protons have a large relative velocity to the heavies, which can be considered as unmagnetized. Dispersion analysis on the basis of bi-ion Hall-MHD equations gives that two types of low-frequency electromagnetic waves are excited. The "upper" instability occurs in the branch of whistler/magnetoacoustic waves with the remarkable feature that the frequency in the beam (spacecraft) frame of the most unstable wave is close to the proton cyclotron frequency. Its growth rate strongly depends on the electron plasma beta (/,) which preferentially determines the whistler damping at oblique propagation. In consequence, the increase of the growth rate with the propagation angle is limited and an "instability gap" results at intermediate angles (4 ø < < 7ø). For / 1, only sm 11 r nges near p r llel nd transverse propagation remain. For a high beam velocity the "lower" instability is determined by the cutoff frequency of the bi-ion system. Both lines were clearly seen in the spectra of magnetic field data of the AMPTE Ba release during the transition phase when the Ba ion density decreases below the solar wind density. These results may be relevant for other situations in space' comets with low gas production rates (q <_ 18s-1), nonm gnetized planets (Venus, M rs) nd ctive moons (Phobos, Deimos) including their gas tori. 1. Introduction butions are always expected for solar wind mass loading over distances smaller than the gyroradius of the The excitation of low-frequency electromagnetic waves "pickup" ions. Weak comets like Grigg-Skjellerup and (LEFW) at the interaction of the solar wind with cometa- also unmagnetized planets such as Venus and Mars are ry heavy ion sources has been studied over many years; typical candidates for this type of interaction. Waves especially since the insitu measurements near comet near the heavy ion gyrofrequency are mainly described Giacobini-Zinner by ICE in 1985 [Smith et al., 1986]. in the recent papers [Motschmann et al., 1997], how- Predominantly, waves at the water group ion cyclotron ever. Only a few observations concerning wave emisfrequency have been observed and their excitation by sion at the proton cyclotron frequency upstream of pickup ring-beam distributions is mainly discussed by comet Halley [Mazelle and Neubauer, 1993], as mag- [Brinca, 1991; Tsurutani, 1991]. After the Giotto en- netic pulses near comet Giacobini-Zinner [Tsurutani et counter of comet Grigg-Skjellerup, studies went in a new al., 1989] and as left-handed waves in the upstream redirection by looking to LEFW excitation for uncomgion of Mars [Russsell et al., 199] have been reported. pletely filled rings as consequence of the limited sizes of the interaction region. Such nongyrotropic ion distri- Max-Planck Institut fiir Aeronomie, Katlenburg-Lindau, Germany. Imperial College, London, England. 3Astrophysikalisches Institut, Potsdam, Germany. 4Centre d'etude des Environement Terrestre et Planetedres, V lizy, France. Copyright 1999 by the American Geophysical Union. Paper number 1998JA /99/1998JA In this paper, the spectrum of low-frequency waves at the AMPTE Ba release in the solar wind is investigated by wavelet analysis. It is shown that in the transition phase of the release, when the Ba ion density becomes comparable with the solar wind density, strongly ex- cited waves at the proton cyclotron frequency Qp were observed, which mainly propagate transverse to the magnetic field, in addition to another wavemode at a lower frequency of about one fifth of Qp. Dispersion analysis, in which Hall-MHD and kinetic descriptions are combined, was used to study the relevant mechanism of wave excitation. It is found that newly gener-

2 6764 SAUER ET AL- BRIEF REPORT ated heavy ions in the solar wind, which in the initial phase of the pickup process are nearly at rest, act as an effective source of electromagnetic waves at the proton cyclotron frequency. This instability is caused by res- onant interaction of whistler/ magnetoacoustic waves with the heavy ion beam, generally not aligned to the magnetic field. Doppler shift generates the proton cyclotron frequency in the beam frame. The influence of whistler kinetic damping on the parameter regimes (propagation angle, wavenumber k and / e) in which the instability may occur is presented. It is suggested that earlier observations of proton cyclotron emission at comets and Mars may be explained by the same mechanism.. LF Waves at the AMPTE Ba Release During the active part of the AMPTE mission in December 1984, an artificial comet was created by release of barium into the solar wind [Haerendel et al., 1986], 8O [ 6 4 AMPTE/UKS Ba release December 7, 1984 [Gurnett et al., 1986]. The solar wind parameters during the release were following: V o kin/s, ne 8 cm -a, T -- 5 ev, B -- 1 nt. After sudden injection of about 1 neutrals they become quickly ionized and form a high-dense heavy ion cloud, which expands and interacts with the solar wind. This "articial comet" ex- periment is thought here to be especially favorable for testing theoretical concepts of wave generation under nongyrotropic mass loading conditions. Evidence for this was discussed from the point of view of momentum balance using the magnetic field measurements by [Dunlop et al., 1987]. The event was just analyzed in terms of its ULF signatures by [Gleaves et al., 1988]. Only the magnetic field measurements from the UKS satellite, shown in the upper panel of Figure 1, are discussed in the present paper. About 1 s after the release, a field compression up to about 1 nt was measured. Then, strong shock-like oscillations with period of 5 s occur. Near t, 5 s they go over in amore regular pattern which is plotted in the bottom panel of Figure I with higheresolution. As clearly seen, it represents a superposition of oscillations with two different periods, one of about 6 s, and another one of of about 3 s. This pronounced wave structure is revealed in the wavelet analysis, shown in Plate l a, as two separated bands. In the interval between t 5 s and t 4 s after release the upper band precisely coincides with the proton gyrofrequency fp, mhz which is marked by the red curve. The lower band is near f* =3 mhz. The significant variations in the magnetic field magnitude indicate the strongly compressional character of both wave types. In the further analysis both types of emissions are considered. o O lo I Time 4 ter release in Time after release in s Figure 1. The magnetic field magnitude during barium release on December 7, 1984, in the solar wind. 3. Dispersion of Ion-Beam Excited LFEW It is known that an addition of a new ion population to a hydrogen magnetoplasma modifies the dispersion of hydromagnetic waves. In this section a dispersion relation is derived for low-frequency electromagnetic waves in a bi-ion plasma composed of the solar wind as the core plasma and a minority of unmagnetized heavy ions. The appropriate dispersion relation for LFEW can be derived in two ways, either from the general dispersion theory [Sauer et al., 1998] or by linearizing the bi-ion Hall-MHD equations [Baumgartel et al., 1998]. Here we adopt the general dispersion relation AN 4 + BN + C = (N = kc/w) to our specific situation. In the limit of massless electrons and unmagnetized heavy ions, which is relevant for heavy barium ions the coefficients A, B, and C are given by A - cos B - -(e +?yy COS ), C - with the dielectric tensor components

3 SAUER ET AL.- BRIEF REPORT 6765 a) AMPTE\UKS, Ba release 1984!1/7 lo 18 6½- 16 %. o Time after release in s b) PWS, PHOBOS-, 1989// :) :: 17:4: 17:6: 17:8: 17:1: 17:1: Plate 1. a) The magnetic field magnitude and the wavelet diagram of the magnetic field in AMPTE Ba release. Red line marks the proton gyrofrequency. Black line gives the oxygen gyrofrequency for the reference.b) Wavelet time-frequency diagram of emissions observed in the upstream region of the Martian plasma environment. Red line marks the proton gyrofrequency and the wave vector k (in the x- z plane)). Because of = i[ w f p _ + cop ne the asumption that the heavies are unmagnetized, no L (left-hand) mode resonating at the heavy ion cyclotron cop, w,, and we are the plasma frequencies of protons, frequency fh occurs. However, even then, the bi-ion heavy ions and electrons determined by the respective cutoff-frequency appears. For magnetized heavy ions densities np, n,, and ne. p, and e are the correspond- the bi-ion cutoffrequency is we! = np/ne qh+n /ne qp. ing cyclotron frequencies. Figure shows the dispersion In case of heavies with rnn - oo protons can also perof low-frequency electromagnetic waves in cold bi-ion form gyration with cutoff frequency giving rise to the plasma of protons and unmagnetized heavies for two mode near we! = f pnn/ne. It is seen that the L-mode cases of oblique propagation ( = 3 ø, 8 ø, where is evolves from the cutoff frequency and becomes coupled the angle between the magnetic field B (in z direction) to the upper R (right-hand) mode which goes into the

4 6766 SAUER ET AL.' BRIEF REPORT 1..8 Dispersion relation of cold bi-ion waves der the assumption of weak slowing down of the solar wind, a large relative streaming between protons and heavy Ba ions is admissible even transverse to the magnetic field. Below we consider mechanism of excitation of both wave modes in more detail. 3.1 Beam-Generated Whistlers / o = 3 ø ZR R L (b) At first, we discuss briefly the main features of the observed emissions near the proton gyrofrequency. Figure 4 shows the variation of the magnetic field components in the LMN frame over 4 s interval starting at t = 3 s after release. Minimum variance analysis indicates that the wave is left-hand polarized and propagates at an angle of = /(kn, B) = 89 ø to the magnetic field. In the other intervals (to = 34 s, to = 38 s) the propagation angle is also large = 7 ø and = 6 ø, respectively. The ratios of the maximum to intermediate and intermediate to minimum eigenvalues œ/ M and M/ N have the values œ/ M=1., M/ N =4.8, respectively. In a presence of a beam the given above dielectic tensor components are modified: To simplify the subsequent algebra, the heavy ion velocity v is taken transverse to the magnetic field B. Then, + --]. -- i [ p + p n where w* = w- kva is the Doppler-shifted frequency in the beam reference frame, which is almost the space- K=kVAp/t p craft frame. Combination of the equations leads ag ter straightforward algebra to a polynomial of sev- Figure. The dispersion of low-frequency electromagnetic waves in cold hi-ion plasma of protons and enth order in the complex variable w = w(k), which unmagnetized heavies (ran/top - 136, nn/np -.1, is solved by standard numerical procedures (see also o = 3 o, 8 o) Sauer et al. [1998]). For a case of a beam velocity at arbitrary angle to the magnetic field the equations become more complicated and are not given here. Results of the dispersion analysis for whistlers are plotted in Figwhistler branch at higher frequencies. Figure 3 makes ure 5 which shows the real and imaginary parts of the a hint about a source of free energy for wave excitation frequency of the relevant wave modes versus the wave in two frequency bands in such hi-ion plasma configu- number k for a beam velocity of M = va /v p = 3 at ration. Two separated wave bands at frequencies close angle 45 ø with respect to the magnetic field and oblique to cutoff frequency and a frequency which is in whistler wave propagation ( = 85 ø, na/n =.1, ma/mp = 136 frequency range may be generated in the plasma refer- ). Figure 5a depicts the w - k relation in the solar wind ence frame for a beam with a velocity larger than Alven frame. This pattern clearly arises from the intersection velocity because of two intersections between the L/R of the beam mode (wm kva) with the whistler (R) mode and the beam dispersion curves. Indeed, barium mode of the background plasma. As seen in Figure 5c, ions which are in relative motion to the core solar wind there is a sharp threshold in the wave number k for the plasma may act as a beam. Such plasma configuration onset of the instability, and the maximum growth rate takes into account that the newly created Ba ions near is attained just adjacent to it. The w - k relation of unthe center of the neutral Ba cloud have not enough space stable mode in the beam frame (w* = w - kva), where and time to be picked-up by the solar wind. Thus, un- the heavy ions are nearly at rest, is shown in Figure

5 SAUER ET AL.' BRIEF REPORT s o.1, K = kv /.,' beam,,,," /," / R mode K-kVAp/fp Figure 3. Two intersection points of a "beam-mode" and œ/r mode in co - k diagram indicates a possible source of two wave emissions in plasma-beam configuration. 5b. It is evident that the wave near maximum growth the Bessel function, respectively. Higher harmonic conis Doppler shifted to the proton cyclotron band, that is tributions can be neglected because of the low phase co- kvh, -f p or co*, -f p, leaving behind a left- velocity of the waves under consideration. As expected, hand polarized wave in the beam frame. The analysis the inclusion of electron kinetic damping has serious shows that both the growth rate and the related wave consequences on the ranges of in which instabilities number increase as increases. On the other hand, it may occur. The grey-coded plots in Figure 6 show how is known that a damping due to finite temperature ef- the growth rate 7/f p varies in dependence of both the fects is also essential. According to Gary and M½llot propagation angle and the wave number k. The lower [1985], as increases, the damping increases, reaching a diagram is representative of negligible whistler dampmaximum near 6 ø, and vanishing again near, 9ø; ing in the case of relatively cold electrons (fie - 5). where the (fast) wave changes its character to a purely Both the growth rate and the related wavenumber inmagnetoacoustic mode. crease with increasing propagation angle. For higher In order to take into account the whistler wave damp- electron temperature, however, the onset of electron ing in our analysis, electron kinetic corrections must be damping at oblique wave propagation leads to a gap included into the expressions of the cold theory. Fortu- in the - k space (upper diagram), leaving behind two nately, in the frequency range of interest ( f p _ co _ unstable ranges: one near parallel and another at alco ~ f e), there is no need to use the complex apparatus most transverse propagation. The gap broadens with of the Vlasov description. From the general expressions increasing fie. In both cases, related frequency in the [Gary and Mellot et al., 1985; Baumgartel et al., 1998] beam frame is spread around the proton cyclotron freit can easily be estimated that the main contribution quency. This means Re(co*)/1p 1. For fie _> 1, the comes from the electron term in (yy alone, which for only instability which remains is near ø (whistler our conditions, reduces to branch) and ~ 9 ø (magneto-acoustic branch). &,, - -(o: ^'o 3. Low-Frequency Wave-Band where (e- colkllve, ye - (k. velfe), Ao(x) - e- Io(x). Low-frequency line (~. lqp) is also clearly observed Z(() and Io(x) are the plasma dispersion function and in the wavelet diagram. Minimum variance analysis of

6 6768 SAUER ET AL.' BRIEF REPORT to=3s z M o o -... 'A'... 'A'... - I i -5 z i i t(s) - -I L -- Figure 4. V ri tions of m ximum (g), intermediate (M), nd minimum (N) v ri nce components of the m gnefic field in the spectral interval near fly nd their hodogr m. emissions in the band (5-68 mhz) shows that the wave which is near the rest, co* ~ -.1ftp; that is the wave is right-hand polarized and propagates at the angle of is R-polarized. However, it is worth noting that co* ap- - /(k,b)- 19 ø to the magnetic field. The ratios pears to be less sensitive to the ratio no/ne and falls of the maximum to intermediate and intermediate to into a band of frequencies near.1 p in a broad range minimum eigenvalues Aœ/AM and AM/Air have the values AL/AM--1.8, AM/Air --3.7, respectively. Figure 7 of plasma parameters. The frequency value co* is determined by a width of a gap which separates the unstable shows the L, M,N components of the field perturba- mode and the beam mode in the co- k diagram. It tions in the given frequency range and a hodogram of is worth noting that the wavelength of unstable modes L- M variations. Figure 8a gives the co - k dispersion relation in the ( 15YApimp) occurs larger the characteristic scale of the system and the effects of inhomogeneous plasma frequency range below ftp in the cold plasma approach may be essential and shift the frequency to the observed with a beam of heavies (MA -- 3, n /n -.1, m /mp - band near. p. 136) for the angle - 15 ø. The pattern contains several modes which arise due to the intersection of the beam mode with low frequency modes in the background plasma. Figure 8b shows co- k relation for the mode with maximum growth rate in the beam reference frame. Figure 8c presents the growth rate of this mode versus k. The frequency of unstable L 4. Discussion and Summary As described above, electromagnetic waves at the proton cyclotron frequency ftp propagating transverse to the magnetic field and emissions at lower frequency (co ~.f p) were observeduring the last stage of the mode in the plasma frame depends on beam parame- AMPTE barium release. Over the observation interval ters (na, ma, va) and occurs between cutoff frequency of about 15s the propagation angle of waves near ftp coc!~ nh/neflp ~ O.lfip and proton gyrofrequency changes from nearly 9 ø at the beginning to about 65 ø _( tip where a splitting takes place. In the beam frame, at the end. To our knowledge, this observation is the

7 SAUER ET AL.' BRIEF REPORT Dispersion relation of beam-excited whistlers al., 1986], one gets fie.1, just the value which guarantees the necessary weak damping. The observation of the unstable waves under different angles in the late phase of the release may be caused by temporal changes of the draped magnetic field configuration around the Ba cloud. In the limit of cold electrons the growth rate is larger, the larger propagation angle is. Electron kinetic damping, however, restricts the propagation angle of unstable waves to small angles and near 9 ø. For a plasma with fie 1, in agreement with the observations, only whistler waves propagating nearly parallel to the magnetic field survive. Emissions at co -..f p are probably also caused by beam-plasma instability. As well as emissions near f r the low-frequency waves are elliptically polarized, but evolve from the L mode in the range between cutoff frequency and proton gyrofrequency giving rise to right-hand polarization in the spacecraft frame. Frequencies of both modes in this reference frame are determined by a frequency gap between unstable and beam modes, and are rather stable with respect to variations of plasma parameters ø ø 45 ø ø.5 - (c) 1 3 K=kVAp/fi p Figure 5. (top to bottom) Real part of wave frequency (whistler mode) in the plasma and beam reference frames versus the wavenumber k for beam-plasma con- figuration (MA -- 3, /)- 84 ø, nh/np --.1, mh/mp ); the growth rate. Dashed and solid curves correspond to the cases without and with damping due to electron temperature effects (/ e-.1). most precise example for electromagnetic emission at - f p which obviously is caused by beam-plasma interac- 1 3 tion. From Figure 6 it is evident that wave emission at K=kvAp/fi p /) 7 ø requires/ e _.1. Otherwise, kinetic damping would drastically reduce the growth rate of instability Figure 6. Dependence of growth rate 7 on the propato nearly zero. For the solar wind parameters of the gation angle O and the wavenumber for two regimes of undisturbed state prior to the Ba release [Gumerr et fie (5-.15). 15 ø fie = 5 75 ø 1. 6 ø 45 ø 3 ø ½ 15 ø J li i

8 ß,, 677 SAUER ET AL' BRIEF REPORT to=65s - o o 1 oo M o 5 1 oo L 6 4. o so oo so :oo t(s) L Figure 7. Variations or maximum (L), intermediate (M), and minimu m(n) variance components of the magnetic field in the low-frequency spectral interval near.qp and their hodogram. Furthermore, we suggest that the dispersion analysis given above allows a direct link between the proton cyclotron emission seen at the AMPTE Ba reheavy ion beam, seem to be a competitive mechanism, because the waveform of the magnetic field fluctuations also contains evidently lower frequency variations with lease and waves in the same frequency range observed the period close to min. These tones at frequencies 7- in the plasma environment of Mars [Russsell et al., 199; Sauer et al., 1998; Baumgartel et al., 1998] and 14 mhz indicate the presence in the solar wind of heavy ions (e.g. O+), which could form a beam in proton comets [Mazelle and Neubauer, 1993], which mostly plasma. The efficiency of this mechanism is supported propagate parallel to the magnetic field. We believe by distinct observations of wave emissions at f p during that the waves are generated by the same mechanism AMPTE barium release when the alternative mechaof instability is supplied by the relative motion between newly generated heavy ions at rest and the solar wind. nism of wave excitation by pickup protons can be excluded. Another examples of emissions near f p, which Plate lb shows, for example, wavelet time-frequency could be also attributed to a heavy ion beam, are related diagram of low-frequency emissions observed in the upstream region of the Martian plasma environment. A to the so-called "Phobos events". Outgassing from the Phobos moon, micrometeorite impacts on its surface chain of spot-like emissions near f p is clearly observed. and accretional interaction with the hot oxygen mar- Another evident wave line is seen at w ~.f p The minimum variance analysis shows that the waves near tian corona may produce a neutral gaseous torus [Ip and Banaszkiewicz, 199] along the moon orbit. A pref p propagate at small angles to the B ( ~ 7 ø) and left- dicted enhancement of heavy ion population in the torus hand elliptically polarized. Lower-frequency emissions can give rise to beam-plasma instabilities and wave genare right hand polarized and propagate at ~ 7 ø. It is worth noting that the emissions at f p could be also exeration at f p. Indeed, evident emissions near f p were observed during all crossings of the hypothesized Phocited by pickup protons [Russsell et al., 199; Tarasoy bos torus [Baumgartel et al., 1998]. et al., 1998] which have the origi,' rom an extended Finally, the analysis of the electromagnetic wave sprechydrogen atmosphere of Mars. Waves, generated by trum at the AMPTE barium release revealed proton

9 . SAUER ET AL.: BRIEF REPORT 6771 Dispersion relation of beam-excited References ULF electromagnet:ic waves Baumgartel K., K. Sauer, E. Dubinin, V. Tarasoy,and M. 1..5, I O.lO - (b), i ' E, I (c) o K-kVAp/fl p Figure 8. top to bottom) Real part of wave frequency (low-frequency part) in the plasma and beam reference frames versus the wavenumber k for beam-plasma configuration (M t = 3, = 15 ø, rnh/m:136, nh/np=o.1 ); the growth rate. Dougherty, Phobos-events-signatures of solar wind interaction with a gas toms?, Earth Planets Space, 5, , Brinca, A. L., Cometary linear instabilities: From profusion to perspective, in Cometary Plasma Processes, Geophys. Monogr. $er., vol. 61 edited by A.D. Johnstone and A. J. Coates, pp. 11-1, AGU, Washington, D.C., Dunlop M.,D. A. Southwood, W. A., and W. Mier- Jedrzejowicz, On a magnetic source of southward motion of the AMPTE solar wind Barium release of 7 December 1984, Planet. Space Sci., 35, 493-5, Gary, S. P., and M. M. Mellot, Whistler damping at oblique propagation: Laminar shock precursors, J. Geophys. Res., 9, 99-14, Gumeft, D. A., et al., Waves and electric fields associated with the first AMPTE artificial comet, J. Geophys. Res., 91, , Gleaves, D. G., D. J. Southwood, M. Dunlop, and W. Mier- Jedrzejowicz, Low frequency magnetic wave spectra associated with the AMPTE Ba release of 7 December 1984, Adv. Space Res., 8(9) , Haerendel G., G. Paschmann, W. Baumjohann, and C. W. Carlson, Dynamics of the AMPTE artificial comet, Nature, 3, 7, Ip, W.-H., and M. Banaszldewicz, On the gas/dust tori of Phobos and Deimos, Geophys. Res. Lett., 17, , 199. Mazelie, C., and F. M. Neubauer, Discrete wave packets at the proton cyclotron frequency at comet P/Halley, Geophys. Res. Lett.,, , Motschmann, U. H. Kalemann, M. Scholer, Nongyrotropy in magnetoplasmas: Simultion of wave excitation and phasespace diffusion, Ann. Geophys., 15, , Russell, C. T., et al., Upstream waves at Mars, Phobosobservations, Geophys. Res. Lett., 17, 97, 199. Sauer, K., K. Baumgartel, E. Dubinin, and V. Tarasov, Lowfrequency electromagnetic waves and instabilities within the Martian bi-ion plasma, Earth Planets Space, 5, 69-78, Smith E. J., et al., ICE encounter with Giacobini-Zinner: Magnetic field observations, Science, 3, , Tarasov, V, E. Dubinin, S. Perraut, A. Roux, Sauer, K., A. Skalsky, and M. Delva, Wavelet application to the magnetic field turbulence in the upstream region of the martian bow shock, Earth Planets Space, 5, , Tsurutani, B.T., et al., Magnetic pulses with duration near the local proton cyclotron period: Comet Giacobini- Zinner, J. Geophys. Res., 9, 9, Tsurutani, B. T., Comets: A laboratory for plasma waves and instabilities, in Cometary Plasma Processes, Geophys. Monogr. Ser., vol. 61, edited by A.D. Johnstone and A. J. Coates, pp , AGU, Washington, D.C., cyclotron emission as an indicator for small-scale heavy ion beam sources in the solar wind. From this view point, the study provides an interesting diagnostic aspect for detecting heavy ion sources in flowing plasmas in space. Acknowledgments. One of the authors (E.D.) is grateful to MPAe where this work was done. Janet G. Luhmann thanks Uwe Motschmann and S. Peter Gary for their assistance in evaluating this paper K. Baumgartel, Astrophysikalisches Institut, Telegrafenberg A31, D ,Potsdam, Germany. (kbaumgartel@aip.de) E. Dubinin, and K. Sauer, Max-Planck Institut fiir Aeronomie, D Katlenburg-Lindau, Germany. (sauer@linmpi.mpg.de) M. Dunlop, Physics Department, Imperial College, London, SW7 AZ UK. (m.dunlop@ic.ac.uk) V. Tarasov, Centre d'etude des Environement Terrestre et Planetaires, V lizy, France. (tarrasov@balsa.cetp.ipsl.fr) (Received August 31, 1998; revised October 9, 1998; accepted November 5, )