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[1] Magnetic field fluctuations in the frequency range [0.02-12.5] Hz are studied with the four Cluster satellites in the Earth magnetosheath downstream of a quasi-perpendicular bow shock. The turbulent spectrum presents a spectral break accompanied by a broad maximum usually interpreted as due to Alfvén ion cyclotron waves. In this paper we establish that this spectral knee corresponds to space-localized coherent magnetic structures in the form of Alfvén vortices. The Alfvén vortex is a nonlinear cylindrical Alfvén wave, quasi-parallel to the mean magnetic field B 0 and propagating in a plane perpendicular to B 0 . In this plane the observed vortices are localized within 20c/w pi . The frequent observations of such structures indicate their stability in the plasma. Therefore the Alfvén vortices can be an important element in the magnetosheath turbulence. The possible origins of these vortices, such as a strong turbulence or the filamentation instability of an Alfvén wave, are discussed.

[1] On March 18, 2002, under northward interplanetary magnetic field (IMF) and high ($15 nPa) solar wind dynamic pressure conditions, Cluster observed reconnection signatures and the passage of an X-line at the large ($175°) magnetic-shear high-latitude magnetopause (MP). The observations are consistent with the occurrence of a reconnection site tailward of the cusp and in the vicinity of the spacecraft. At the same time IMAGE observed a bright spot poleward of the dayside auroral oval resulting from precipitating protons into the atmosphere. The intensity of the proton spot is consistent with the energy flux contained in the plasma jets observed by Cluster. Using the Tsyganenko-01 magnetic field model with enhanced solar wind pressure, the Cluster MP location is mapped to the vicinity of the IMAGE proton spot. Mapping the auroral spot out to the MP implies an X-line of at least 3.6 R E in y GSM . In addition to confirming the reconnection source of the dayside auroral proton spot, the Cluster observations also reveal sub-Alfvénic flows and a plasma depletion layer in the magnetosheath next to the MP, in a region where gas dynamic models predict super-Alfvénic flows.

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Abstract. The time domain sampler (TDS) experiment on WIND measures electric and magnetic wave forms with a sampling rate which reaches 120 000 points per second. We analyse here observations made in the solar wind near the Lagrange point v1. In the range of frequencies above the proton plasma frequency f pi and smaller than or of the order of the electron plasma frequency f pe , TDS observed three kinds of electrostatic (e.s.) waves: coherent wave packets of Langmuir waves with frequencies f 9 f pe , coherent wave packets with frequencies in the ion acoustic range f pi f`f pe , and more or less isolated non-sinusoidal spikes lasting less than 1 ms. We con®rm that the observed frequency of the low frequency (LF) ion acoustic wave packets is dominated by the Doppler eect: the wavelengths are short, 10 to 50 electron Debye lengths k h . The electric ®eld in the isolated electrostatic structures (IES) and in the LF wave packets is more or less aligned with the solar wind magnetic ®eld. Across the IES, which have a spatial width of the order of 925k h , there is a small but ®nite electric potential drop, implying an average electric ®eld generally directed away from the Sun. The IES wave forms, which have not been previously reported in the solar wind, are similar, although with a smaller amplitude, to the weak double layers observed in the auroral regions, and to the electrostatic solitary waves observed in other regions in the magnetosphere. We have also studied the solar wind conditions which favour the occurrence of the three kinds of waves: all these e.s. waves are observed more or less continuously in the whole solar wind (except in the densest regions where a parasite prevents the TDS observations). The type (wave packet or IES) of the observed LF waves is mainly determined by the proton temperature and by the direction of the magnetic ®eld, which themselves depend on the latitude of WIND with respect to the heliospheric current sheet.