K. Meziane1, C. Mazelle1, C. d'Uston1, H. Reme1, R. P. Lin2, C. W. Carlson2, D. Larson2, J. P. McFadden2, R. E. Ergun2, K. A. Anderson2, G. K. Parks3, D. Berdichevsky3, and R. P. Lepping4
1 Centre d'Etude Spatiale des Rayonnements, 9 Avenue du
Colonel Roche, 31400 Toulouse,
2 Space Sciences Laboratory, University of California, Berkeley, CA 94720.
3 Geophysics Program, University of Washington, Seattle, WA 98195.
4 NASA Goddard Space Flight Center, Greenbelt, MD 20771.
Several upstream ion distributions having a gyrating signature have been identified with the 3DP/PESA-High analyser on board the WIND spacecraft. These distributions are observed at distances greater than 20 RE from the Earth's bow shock. The distributions are observed in association with low frequency waves propagating quasi-parallel to the background magnetic field. By estimating the bulk velocity of the gyrating ions, we have found that the waves resonate with the particles. The observation of gyrating ions at large distances from the shock suggests their local production, probably from field-aligned beam disruption.
The Earth's foreshock has been investigated extensively in the last two decades. The main features of suprathermal ions including the studies of last decades are reviewed in Fuselier (1994 and references therein). It is established that several upstream ion distributions exist in the Earth's foreshock region: field-aligned beams, intermediate and diffuse ions, and gyrating ions (gyrophase-bunched and gyrotropic). The reflected ion beams and diffuse ion population represent the two limits of the distribution functions. Accelerated in the quasi-perpendicular region of the shock, the beams travel a large distance in the foreshock and then are destroyed by growth processes leading gyrophase-bunched or intermediate ions (Fuselier, 1995). Gyrophase-bunched ion distributions can also be produced by processes at the shock in a specular reflection of a fraction of incoming solar wind ions. These latter ion distributions can suffer a gyrophase-mixing leading to nearly gyrotropic ion distributions. However, it seems that the shock-issued gyrating ions cannot propagate above 4 RE distance from the shock (Fuselier et al. 1986). Such distributions will evolve to diffuse ions observed in quasi-parallel shock. On the other hand, the propagation of both ion beam and gyrating ions in the solar plasma is unstable. These ions can excite right-handed resonant and non resonant MHD-like waves propagating in both parallel and oblique direction with respect to the background magnetic field (e.g. Gary, 1991). This explains why intermediate and gyrating ion populations are often associated with low frequency waves (Paschmann et al. 1979; Hoppe et al. 1981). The excited waves trap the ions, and further isotropize the initial ion distribution. The waves then modify as well the ion distribution accelerated at the shock as the temporal ion distribution in the foreshock. Thus, the observation of backstreaming ion distribution functions and their association with the MHD-like waves at large distances from the shock gives indications on the evolution of upstream ion distribution functions.
The Wind spacecraft crossed the Earth's bow shock many times and its orbit allowed to investigate the foreshock region up distances larger than ISEE and AMPTE apogees. The WIND-3DP experiment consists of one pair of ion electrostatic analysers (PESA-Low and PESA-High), one pair of electron electrostatic analysers (EESA-Low and EESA-High), and one pair of Solid State Telescope for high energy particle measurement (up to 300 keV). A detailed description of the 3DP experiment can be found in Lin et al. (1995). We are concerned with the PESA-High detector which covers energy from 3 eV to 30 keV with a resolution (E/E 20%. The detector have 4 FOV and a full distribution is obtained in one-half spin period (3 sec) of the spacecraft. The distribution functions sampled below are obtained from 48-sec integration time (16 spin periods). The magnetic field data used here are provided from the WIND-MFI experiment (Lepping et al. 1995). To investigate the association of backstreaming ions with low frequency waves, the 3-sec averaged magnetic field components were used. We report here the observation of gyrating ion distributions at large distance from the shock and their association with low frequency waves. The characteristics of these distributions and their formation process are discussed.
The backstreaming ion distribution functions presented below have been identified when the mean direction of the IMF is quasi-steady over many minutes around the time of interest and close to the ecliptic plane (small BZGSE). It this case, the ecliptic plane contains the magnetic field; as the guiding center velocity of the backstreaming ions lies in the plane containing the IMF and solar wind directions, it is appropriate to sample the distribution function in the Vx-Vy GSE plane (Vz 0).
Fig. 1. Caption included with image.
Figure 1 displays several ion distribution functions registred in the upstream region of the Earth's bow shock. The distribution function values are deduced from the angular bin counts in which we have removed the solar wind and the background mean level counts. These last values are estimated on a long time interval relatively close to the instant of the measurement of the backstreaming ions, and when the spacecraft is outside the foreshock. The distributions are sampled in the solar wind plasma frame. The arrows indicate the IMF orientation. The contour plot centered around the origin correspond to the solar wind residue because of the difference between the instantaneous and the mean values of the solar wind parameters. However, the solar wind distribution is completly removed in the event labeled (a). The backstreaming ion distributions displayed in Figure 1 are highly symmetrical about the magnetic field direction. The suprathermal component appears as torus-like shaped around the magnetic field direction. It is an indication of the gyrotropic nature of the distribution, although a weak difference appears between the two peaks distribution values (about one-half order of magnitude) indicating a slight deviation from gyrotropy in the distribution (a). The distribution functions labeled (a), (b), (c), and (d) are observed repectivly at 31 RE, 22 RE, 37 RE and 27 RE from the bow shock; the angle BNthat makes the magnetic field direction with the bow shock-normal associated to each distribution is respectivly, 47°, 12°, 26° and 14°.
The above ion distributions are simultaneously observed with large enough magnetic fluctuations. Figure 2 displays the magnetic field components in the GSE coordinates for intervals including the time when the distributions (a) have been observed. The oscillations appear mainly on By and Bz components with peak-to-peak amplitude of 3 nT. We have studied the magnetic fluctuations by using the minimum variance technique, and we have characterized the waves by computing the direction of propagation with respect to the background magnetic field, the wave vector, the polarisation and the magnetic field compression ratio |B|/B0. The waves can be reasonably taken as plane waves (2/3 11, 1/2 1.3) and the propagation direction is nearly parallel with respect to the ambiant magnetic field (KB = 6± 6°). For the first two intervals the hodograms show that the waves are nearly circularly polarized, while the waves associated to distributions (c) and (d) reveal a certain degree of ellipticity. The waves are left-handed in the spacecraft frame.
Figure 2. Caption included with image.
In agreement with earlier investigations (Thomsen et al. 1985; Fuselier et al. 1986), the gyrating ion distributions reported here are always associated with highly transverse weakly compressive low frequency waves propagating at small angles relative to the IMF.
To investigate quantitatively the possibility of local resonance between the observed waves and gyrating ions, we have compared the observed wave period with the period one would expect for waves in cyclotron resonance with the observed distribution. For this aim, we have used the resonance condition expression: - k||V|| + p = 0, where is the wave frequency in the plasma frame, p is the proton gyrofrequency, k|| is the component parallel to the background magnetic field of the wave vector, and V|| is the parallel resonnant velocity. As the waves are convected by solar wind flow, the observed frequency pred must be Doppler-shifted, then pred = + k||VSWcoskV / coskB, where kV and kB are the angles that make the wave vector with the background IMF and the solar wind velocity respectively. With some algebra and using the experimental values, we have computed the predicted wave periods Tpred= /2 and compared it with the observed periods Tobs. The results are given in Table 1. Taking into account the experimental uncertainties, the values listed in Table 1 suggest strongly the possibility that local cyclotron resonance occurs for these events. Moreover, the observed variations of the wave field such as shown by the hodogram on figure 2 are indications for local interaction with particles of the plasma.
Table 1. Observed and Predicted waves periods in the Wave-Particle Resonance Case
The statistical study by Fuselier et al. (1986) suggests that the distance of 4 RE can be considered as an effective cut-of distance for shock specularly reflected distributions; indeed, only few ion distributions have been identified as gyrotropic above this latter distance. We have reported here several ion distributions which have a signature of gyrotropic-like distributions at distances larger than 20 RE from the shock. On the basis of the bulk velocity and for a given bow shock model, we have found that the bulk velocity of each distribution is not consistent with a specular reflection at the shock surface of a portion of incident solar wind ions. Moreover, the distribution (a) does not seem to result from a gyrophase mixing of a shock-issued gyrophase bunched distributions, because it is observed in the quasi-perpendicular region. These arguments suggest the hypothesis that the most probable source for the reported distributions is then a local production. One possibility often invoked is that the distributions result from field-aligned beam disruption (Hoshino and Teresawa, 1985).
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