When the stream of plasma emitted from the Sun (the solar wind) encounters Earth's magnetic field, it slows down and flows around it, leaving behind a cavity, the magnetosphere. The magnetopause is the surface that separates the solar wind on the outside from the Earth's magnetic field on the inside. Because the solar wind moves at supersonic speed, a bow shock must form ahead of the magnetopause that acts to slow the solar wind to subsonic speeds. Magnetopause, bow shock and their environs are rich in exciting processes in collisionless plasmas, such as shock formation, magnetic reconnection, particle acceleration and wave-particle interactions. They are interesting in their own right, as part of Earth's environment, but also because they are prototypes of similar structures and phenomena that are ubiquitous in the universe, having the unique advantage that they are accessible to in situ measurements. The boundaries of the magnetosphere have been the target of direct in-situ measurements since the beginning of the space age. But because they are constantly moving, changing their orientation, and undergoing evolution, the interpretation of single-spacecraft measurements has been plagued by the fundamental inability of a single observer to unambiguously distinguish spatial from temporal changes. The boundaries are thus a prime target for the study by a closely spaced fleet of spacecraft. Thus the Cluster mission, with its four spacecraft in a three-dimensional configuration at variable separation distances, represents a giant step forward. This 20th volume of the ISSI Space Science Series represents the first synthesis of the exciting new results obtained in the first few years of the Cluster mission.

Cluster was one of the two missions - the other being the Solar and Heliospheric Observatory (SOHO) - constituting the Solar Terrestrial Science Programme (STSP), the first `cornerstone' of ESA's Horizon 2000 Programme. After the catastrophic Ariane-5 accident on 4 June 1996 which destroyed the four Cluster spacecraft, the European Space Agency Science Programme Committee gave approval to refurbish the spare Cluster spacecraft and make it ready for flight. This new spacecraft, considered to be the first of a new fleet, is called Phoenix. In the meantime various options to repeat the Cluster four-point measurements are being studied. Since Phoenix, as the fifth Cluster spacecraft, will be equipped with the spare Cluster experiments, the instrumentation articles in this book are still appropriate to the new mission. Furthermore, the objectives of the recovery mission, the ground systems, the ground observation program and the theory and modelling efforts all remain unchanged. Thus this series of articles will continue to be essential to the Cluster community and to the general scientific community as the recovery mission is implemented.

8. 8 Boundary Layer Structure and Detached Plasma 305 8. 8. 1 Background 305 8. 8. 2 Structure inside the boundary layer 306 8. 8. 3 Observation of detached plasma 308 8. 8. 4 Summary 309 8. 9 Summary and Conclusions 310 References 312 9. CLUSTER AT THE MAGNETOSPHERIC CUSPS 321 9. 1 Introduction 321 9. 1. 1 Previous work 323 9. 1. 2 How Cluster investigates the cusp 325 9. 2 The High-Altitude Cusp 326 9. 2. 1 March 17, 2001 328 9. 2. 2 February 4, 2001 332 9. 2. 3 February 13, 2001 337 9. 2. 4 Statistical survey 340 9. 2. 5 Waves and turbulence 343 9. 3 The Mid-Altitude Cusp 352 9. 3. 1 Structure: Case study 352 9. 3. 2 Structure: Statistical survey 354 9. 3. 3 Ionospheric ions 354 9. 3. 4 Mid-altitude signatures of the LLBL 357 9. 4 Discussion 359 References 360 10. MAGNETOPAUSE PROCESSES 367 10. 1 Magnetopause Reconnection 368 10. 1. 1 Intermittent vs. quasi-steady reconnection 368 10. 1. 2 Component vs. anti-parallel reconnection 382 10. 1. 3 Tailward-of-the-cusp reconnection 385 10. 1. 4 Quantitative tests of reconnection occurrence 388 10. 1. 5 Summary 391 10. 2 Kelvin-Helmholtz Instability at the Flank Magnetopause 391 10. 3 Microphysics of Magnetopause Processes 396 10. 3. 1 Collisionless generalised Ohm's law 397 10. 3. 2 Ion di?usion region observations 398 10. 3. 3 High-frequency waves 402 10. 3. 4 Lower-hybrid waves 405 10. 3.

Cluster was one of the two missions - the other being the Solar and Heliospheric Observatory (SOHO) - constituting the Solar Terrestrial Science Programme (STSP), the first cornerstone' of ESA's Horizon 2000 Programme. After the catastrophic Ariane-5 accident on 4 June 1996 which destroyed the four Cluster spacecraft, the European Space Agency Science Programme Committee gave approval to refurbish the spare Cluster spacecraft and make it ready for flight. This new spacecraft, considered to be the first of a new fleet, is called Phoenix. In the meantime various options to repeat the Cluster four-point measurements are being studied. Since Phoenix, as the fifth Cluster spacecraft, will be equipped with the spare Cluster experiments, the instrumentation articles in this book are still appropriate to the new mission. Furthermore, the objectives of the recovery mission, the ground systems, the ground observation program and the theory and modelling efforts all remain unchanged. Thus this series of articles will continue to be essential to the Cluster community and to the general scientific community as the recovery mission is implemented.