Abstract. The magnetic field experiment on WIND will provide data for studies of a broad range of scales of structures and fluctuation characteristics of the interplanetary magnetic field throughout the mission, and, where appropriate, relate them to the statics and dynamics of the magnetosphere. The basic instrument of the Magnetic Field Investigation (MFI) is a boom-mounted dual triaxial fluxgate magnetometer and associated electronics. The dual configuration provides redundancy and also permits accurate removal of the dipolar portion of the spacecraft magnetic field. The instrument provides (1) near real-time data at nominally one vector per 92 s as key parameter data for broad dissemination, (2) rapid data at 10.9 vectors s -1 for standard analysis, and (3) occasionally, snapshot (SS) memory data and Fast Fourier Transform data (FFT), both based on 44 vectors s -I. These measurements will be precise (0.025%), accurate, ultra-sensitive (0.008 nT/step quantization), and where the sensor noise level is < 0.006 nT r.m.s, for 0-10 Hz. The digital processing unit utilizes a 12-bit microprocessor controlled analogue-to-digital converter. The instrument features a very wide dynamic range of measurement capability, from :E4 nT up to • 536 nT per axis in eight discrete ranges. (The upper range permits complete testing in the Earth's field.) In the FTT mode power spectral density elements are transmitted to the ground as fast as once every 23 s (high rate), and 2.7 rain of SS memory time series data, triggered automatically by pre-set command, requires typically about 5.1 hours for transmission. Standard data products are expected to be the following vector field averages: 0.0227-s (detail data from SS), 0.092 s ('detail' in standard mode), 3 s, 1 rain, and 1 hour, in both GSE and GSM coordinates, as well as the FFT spectral elements. As has been our team's tradition, high instrument reliability is obtained by the use of fully redundant systems and extremely conservative designs. We plan studies of the solar wind: (1) as a collisionless plasma laboratory, at all time scales, macro, meso and micro, but concentrating on the kinetic scale, the highest time resolution of the instrument (=0.022 s), (2) as a consequence of solar energy and mass output, (3) as ~n external source of plasma that can couple mass, momentum, and energy to the Earth's magnetosphere, and (4) as it is modified as a consequence of its imbedded field interacting

[1] Using magnetic field data from the four Cluster spacecraft between February and May 2001, a detailed statistical analysis of interplanetary discontinuities is accomplished. In order to find the surface normals, we apply three different methods:(1) minimum variance analysis (MVA), (2) cross-product method, and (3) triangulation, the latter being the only one that needs information from all four spacecraft. Whereas the cross-product normals are always almost the same at the position of the four spacecraft, strong deviations between the MVA normals are observed. The crossproduct normals also agree fairly well with the triangulation normals, which mostly point in a direction approximately perpendicular to the magnetic field. The scatter of the MVA normals around the triangulation normal can be reduced by triggering the parameters controlling the MVA accuracy, i.e., l 2 /l 3 L , the lower limit of the MVA eigenvalue ratio, and w L , the lower limit of the spreading angle. Our analysis allows to compare the impact of these two parameters on the MVA normals. The error analysis shows that both parameters have strong impact on the MVA accuracy; however, l 2 /l 3 L is more important. In order to ensure reliable normal estimates, MVA should only be applied if w L > 60°. The MVA error analysis also enables us to provide a new criterion to distinguish rotational discontinuities (RDs) from tangential discontinuities. However, the triangulation results show that there are no clearly identified RDs in our data set. We will discuss implications for the theory of solar wind discontinuity generation processes. A practical consequence is that the cross-product method presumably yields better normal estimates than MVA when B n is small. The colinerarity of the cross-product normals at the positions of the four spacecraft demonstrate that the discontinuities are one-dimensional structures on the Cluster separation scale.

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The four‐satellite array of the Cluster mission will allow the separation of spatial and temporal variations in the Earth's magnetosphere and magnetosheath and in the solar wind. We are using a general linear filter formulation to construct a mathematical formalism to determine not only the frequency spectra but also directional distribution and mode separation in a general wave field. This formalism is completely model independent and requires knowledge of the basic dispersion and polarization properties of the participating waves only. The four‐satellite array combined with a suitable formalism is called a wave telescope. We show that at a given frequency the transmission properties expressed by a transmission function can be shaped by 72 independent parameters for a three‐component vector output of the wave telescope filter. Considering the periodicity of the transmission function in wave vector k space, we are led to the concept of an elementary cell in k space given by the geometric properties of the array such that outside the elementary cell the k ‐ vectors are subject to spatial aliasing analogously to aliasing in time at frequencies above the Nyquist frequency. We show that for MHD waves the fast mode is particularly benign as far as spatial aliasing is concerned. The aliasing properties of other wave modes are also discussed. As an example a filter is constructed which suppresses waves propagating in the magnetic field direction but allows passage of waves propagating antiparallel to the magnetic field.