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.

Further Heos 2 plasma and magnetic field data obtained in the frontside boundary layers of the magnetosphere are presented. They reveal that the low‐latitude extension of the entry layer is of a somewhat different nature. The most pronounced difference with respect to the entry layer in the cusp region is the substantial density jump at the magnetopause. Furthermore, the low‐latitude boundary layer tends to be thinner and less turbulent, and the flow velocity inside the layer is always lower than that of the adjacent magnetosheath. This observation excludes large‐scale reconnection at the front of the magnetosphere as the origin of the layer. It is suggested that diffusive entry of magnetosheath plasma and/or heating of detached plasma from the plasmasphere leads to the formation of the layer. It appears likely that reconnection is dominantly occurring as a transient process in the cusp region and accompanies the eddy convection inside the entry layer. As a consequence, magnetic flux is being eroded from the front of the magnetosphere. This is in agreement with the signature of short‐term large‐amplitude magnetic perturbations observed in the low‐latitude boundary layer.

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Individual multispacecraft case studies confirm that the underlying current sheets are tangential discontinuities, but most I-t•As have relatively small jumps in field magnitude from before to after and thus would fail traditional identification tests as definite tangential discontinuities. The combination of our results suggests that HFAs should occur at a rate of several per day, and thus they may play a significant role in the solar-terrestrial dynamics.