Abstract. On board the four Cluster spacecraft, the Cluster Ion Spectrometry (CIS) experiment measures the full, threedimensional ion distribution of the major magnetospheric ions (H + , He + , He ++ , and O + ) from the thermal energies to about 40 keV/e. The experiment consists of two different instruments: a COmposition and DIstribution Function analyser (CIS1/CODIF), giving the mass per charge composition with medium (22.5 • ) angular resolution, and a Hot Ion AnalCorrespondence to: H. Rème (Henri.Reme@cesr.fr) yser (CIS2/HIA), which does not offer mass resolution but has a better angular resolution (5.6 • ) that is adequate for ion beam and solar wind measurements. Each analyser has two different sensitivities in order to increase the dynamic range.

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fields, static randomness, etc. Thus, it is expected to become an important theoretical tool for studying many-body systems, especially narrow-band materials 8 (e.g.,NiS) and impurity bands in crystalline semiconductors. 9

We report a new type of spatially coherent plasma structure that is associated with quasistatic, magnetic-field-aligned electric fields in space plasmas. The solitary structures form in a magnetized plasma, are multidimensional, and are highly supersonic. The size along B 0 is a few l D and increases with increasing amplitude, unlike a classical soliton. The perpendicular size appears to be influenced by ion motion. We show that the structures facilitate ion-electron momentum exchange and suggest that an aggregate of structures may play a role supporting large-scale, parallel electric fields.[S0031-9007(98)06705-2] PACS numbers: 94.30. Kq, 52.35.Mw, 52.35.Sb, 94.30.Tz

Ion thermalization mechanisms downstream of the quasi-perpendicular Earth's bow shock are examined by means of plasma and magnetic field data from the AMPT•/IaM spacecraft which include three-dimensional ion distributions, plasma fluid parameters derived every ~ 4.3 s, and spectra of transverse and parallel magnetic fluctuations up to 16 Hz. The objects studied in detail are low-Mach number, low-f/shocks in which reflected-gyrating ions are present and contribute to the downstream ion temperature but where processes beyond the ramp take place slowly, so that the basic phenomenology becomes apparent. In MHD terms, most of these shocks qualify as marginally critical. Downstream of the ramp, the initially separated core and ring ions slowly merge into a joint, less anisotropic distribution possessing a high-energy tail. The ion temperature ratio, Ti/Tii, is high not only in the shock foot and ramp but also within some distance downstream; its speed of decline rises and the residual level lessens with increasing f/. The ions diffuse about equally fast in energy and in pitch angle. An asymmetry of the distributions with respect to the field direction is present when the shock is slightly oblique. It decays only slowly, which might indicate that the pitch angle diffusion rate near zero pitch angle is reduced. Low-frequency electromagnetic waves are present below the proton gyrofrequency; they are characterized by strong left-hand-polarized emissions and a low level of parallel fluctuations except very close to the shock. The left-hand emissions are often concentrated into a narrow frequency band but sometimes they exhibit a double-humped structure. Waves and ion distributions approach a slowly varying equilibrium some distance downstream of the shock. After extending the analysis to one supercritical shock representing the majority of bow shock encounters, we conclude that our deductions are more generally valid, although the thermalization is faster, usually, and appears to involve nonlinear processes which tend to obscure most of the features noted. 1. sipated at the bow shock. In quasi-perpendicular shocks, the dominant dissipation process is ion heating. For shocks above a certain Mach number, a consistent model of the ion heating mechanism has evolved in the course of the last 20 years from the results of laboratory experiments, theoretical studies, computer simulations, and observations in space. But several details of the downstream thermalization still are unsupported by direct observations. It is the purpose of this paper to present and discuss such observations.The essential element of the ion heating mechanism in higher-Mach number, quasi-perpendicular shocks is the initial reflection of some of the incident ions off the shock surface. The majority of the ions (• 80 %) are directly transmitted through the shock. Their temperature rises by more than the adiabatic rate that corresponds to the magnetic field compression. A similar, or even higher contribution to the total ion temperature is provided by the refl...