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.

Eleven passes of the ISEE satellites through the frontside terrestrial magnetopause (local time 9 -'17 h; GSM latitude 2 0 -43 0 N) have been identified, where the plasma velocity in the magnetopause and boundary laver was substantially larger than in the magnetosheath. This paper examines the nature of°the plasma flow, magnetic field, and energeticparticle fluxes in these regions, with a view to determining whether the velocity enhancements can be explained by magnetic-field reconnection.

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A total of 21 passes of AMPTE/IRM through the dayside (0800–1600 hours local time) low‐latitude magnetospheric boundary region have been examined, all of which were characterized by large magnetic shear across the magnetopause. The purpose of the study was to use the improved accuracy and time resolution of the IRM plasma measurements to reexamine the occurrence of high‐speed flows as signatures of the reconnection process at the magnetopause. A total of 12 of the 21 passes showed magnetopause crossings with high‐speed flows. The duration of these flows was sometimes as short as 10 s and rarely more than 30 s. The occurrence of high‐speed flows is inversely correlated with β, the ratio of plasma to magnetic pressure. For β < 2 the ratio, ΔV*, of observed to theoretically predicted flow velocity changes is high (ΔV* = 0.74 on average), while for β > 2 the average ratio is low (ΔV* = 0.18). We tentatively interpret this to indicate that reconnection may occur preferentially for low β values. A total of nine high‐speed flows were subjected to quantitative momentum and energy balance test. The agreement between observations and theoretical predictions was generally quite good. The signs of the normal magnetic field, Bn, deduced from the momentum balance agreed with the sense of magnetic connection inferred independently from the measured proton heat flux direction. In one case the observations suggest that the satellite may have traversed the diffusion region of the reconnection process. The momentum balance results indicate that at times the effective mass of the ions in the magnetopause and boundary layer was of the order of twice the proton mass.