THE CLUSTER MAGNETIC FIELD INVESTIGATION A. BALOGH, M. W. DUNLOP, S. W. H. COWLEY, D. J. SOUTHWOOD and J. G. THOMLINSON The Blackett Laboratory, Imperial College, London, U.K. K. H. GLASSMEIER, G. MUSMANN, H. L ¨ UHR and S. BUCHERT Institut f ¨ ur Geophysik und Meteorologie, Technische Universit¨ at Braunschweig, Germany M. H. ACU ˜ NA, D. H. FAIRFIELD and J. A. SLAVIN Goddard Space Flight Center, Greenbelt, MD., U.S.A. W. RIEDLER and K. SCHWINGENSCHUH Institut f ¨ ur Weltraumforschung, Graz, Austria M. G. KIVELSON Institute for Geophysics and Planetary Physics, University of California, Los Angeles, CA., U.S.A. THE CLUSTER MAGNETOMETER TEAM Abstract. The Cluster mission provides a new opportunity to study plasma processes and structures in the near-Earth plasma environment. Four-point measurements of the magnetic field will enable the analysis of the three dimensional structure and dynamics of a range of phenomena which shape the macroscopic properties of the magnetosphere. Difference measurements of the magnetic field data will be combined to derive a range of parameters, such as the current density vector, wave vectors, and discontinuity normals and curvatures, using classical time series analysis techniques iteratively with physical models and simulation of the phenomena encountered along the Cluster orbit. The control and understanding of error sources which affect the four-point measurements are integral parts of the analysis techniques to be used. The flight instrumentation consists of two, tri-axial fluxgate magnetometers and an on-board data-processing unit on each spacecraft, built using a highly fault- tolerant architecture. High vector sample rates (up to 67 vectors s 1 ) at high resolution (up to 8 pT) are combined with on-board event detection software and a burst memory to capture the signature of a range of dynamic phenomena. Data-processing plans are designed to ensure rapid dissemination of magnetic-field data to underpin the collaborative analysis of magnetospheric phenomena encountered by Cluster. 1. Introduction: Overview of Objectives The four-spacecraft Cluster mission (Escoubet et al.; Credland et al., this issue) will provide the first opportunity to determine the three-dimensional, time-dependent characteristics of small-scale processes and structures in the near-Earth space plasma, both in the magnetosphere and in the nearby interplanetary medium. Small- scale phenomena, such as localised, transient magnetic reconnection (flux transfer events) or turbulent diffusion, which operate at the boundaries between plasmas of different origin, are largely responsible for determining the nature and geometry of the interactions. The objectives of the Cluster mission and of the magnetic-field Space Science Reviews 79: 65–91, 1997. c 1997 Kluwer Academic Publishers. Printed in Belgium.