Here we report the first three-dimensional spatial spectrum of the low frequency magnetic turbulence obtained from the four Cluster spacecraft in the terrestrial magnetosheath close to the magnetopause. We show that the turbulence is compressible and dominated by mirror structures, its energy is injected at a large scale kp approximately 0.3 (l approximately 2000 km) via a mirror instability well predicted by linear theory, and cascades nonlinearly and unexpectedly up to kp approximately 3.5 (l approximately 150 km), revealing a new power law in the inertial range not predicted by any turbulence theory, and its strong anisotropy is controlled by the static magnetic field and the magnetopause normal.

Abstract. The Spatio Temporal Analysis of Field Fluctuations (STAFF) experiment is one of the five experiments, which constitute the Cluster Wave Experiment Consortium (WEC). STAFF consists of a three-axis search coil magnetometer to measure magnetic fluctuations at frequencies up to 4 kHz, a waveform unit (up to either 10 Hz or 180 Hz) and a Spectrum Analyser (up to 4 kHz). The Spectrum Analyser combines the 3 magnetic components of the waves with the two electric components measured by the Electric Fields and Waves experiment (EFW) to calculate in real time the 5×5 Hermitian cross-spectral matrix at 27 frequencies distributed logarithmically in the frequency range 8 Hz to 4 kHz. The time resolution varies between 0.125 s and 4 s. The first results show the capabilities of the experiment, with examples in different regions of the magnetosphere-solar wind system that were encountered by Cluster at the beginning of its operational phase. First results obtained by the use of some of the tools that have been prepared specifically for the Cluster mission are described. The characterisation of the motion of the bow shock between successive crossings, using the reciprocal vector method, is given. The full characterisation of the waves analysed by the Spectrum Analyser, thanks to a dedicated program called PRASSADCO, is applied to some events; in particular a case of very confined electromagnetic waves in the vicinity of the equatorial region is presented and discussed.

The main goal of the Cluster mission, consisting of four identical spacecraft, is the spatial resolution of plasma structures. For the determination of the wave vectors of a wave field from four positions, classical Fourier analysis is inappropriate. We develop a generalized minimum variance technique which gives a high wave vector resolution though the spatial grid is restricted to only a few sampling positions. This technique uses the amplitude and phase information of the magnetic field from the four satellite positions and determines the optimum wave field corresponding to the measured data. The components of the magnetic field are assumed to be normally distributed. The divergence-free nature of the magnetic field is used as a constraint. Using the magnetic data measured at four positions allows up to seven different wave vectors at one frequency to be uniquely resolved.

The magnetosheath lies between the bow shock and the magnetopause and is formed mainly from decelerated and deflected solar wind, with a small contribution of plasma from the magnetosphere. The observed magnetosheath plasma parameters show both large scale spatial ordering, imposed by the shape of the magnetopause, and variability dependent on the solar wind input. Because the nature of the bow shock depends on the orientation of the interplanetary magnetic field with respect to the local bow shock normal (θ Bn ), the properties of the magnetosheath plasma just behind the bow shock depend also on whether the shock is quasi-perpendicular or quasi-parallel. In general, the magnetosheath tends to be in a more turbulent state