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.

The present 15th volume of the ISSI Space Science Series is devoted to Auroral Plasma Physics. The aurora is arguably the most intriguing phenomenon in space plasma physics. Not only is it the most spectacular manifestation of the Sun-Earth connection chain, but the underlying plasma processes are expected to be ubiqui- tous in the plasma universe. Recognizing the enormous progress made over the last decade in the understanding of the physics of the auroral acceleration processes, it seemed timely to write a comprehensive and integrated book on the subject. Re- cent advances concern the clarification of the nature of the acceleration process of the electrons that are responsible for the visible aurora, the recognition of the fundamental role of the large-scale current systems in organizing the auroral mor- phology, and of the interplay between particles and electromagnetic fields. The project began in March 1999, as a natural follow-up of the project on Magnetospheric Plasma Sources and Losses that resulted in volume 6 of this se- ries, with a planning meeting by a core-group that coordinated the project. The group consisted of J. E. Borovsky, Los Alamos National Laboratory; C. W. Carl- son, University of California, Berkeley; G. Haerendel, Max-Planck-Institut fur ex- traterrestrische Physik, Garching; B. Hultqvist, Swedish Intitute ofSpace Physics, H. E. J. Koskinen, Finnish Meteorological Institute, Helsinki; W. Lotko, Kiruna; Dartmouth College, Hanover, New Hampshire; K. A. Lynch, University of New Hampshire, Durham and G. Marklund, Royal Institute ofTechnology, Stockholm. G. Paschmann, ISSI, Bern, was the project leader.

This sixth volume in the ISSI Space Sciences series is the outcome of a process of carrying out the ISSI study project on source and loss processes of magnetospheric plasma. The goal has been to give an authoritative overview of all aspects of the topic in a well-organized form, for active researchers in the field and for young scientists who are starting their research in space physics. In order to represent the full diversity of experience and perspective that exists in the science community, some 50 leading scientists from all over the world were invited to participate in the project and contribute to the text.

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.

The present 15th volume of the ISSI Space Science Series is devoted to Auroral Plasma Physics. The aurora is arguably the most intriguing phenomenon in space plasma physics. Not only is it the most spectacular manifestation of the Sun-Earth connection chain, but the underlying plasma processes are expected to be ubiqui- tous in the plasma universe. Recognizing the enormous progress made over the last decade in the understanding of the physics of the auroral acceleration processes, it seemed timely to write a comprehensive and integrated book on the subject. Re- cent advances concern the clarification of the nature of the acceleration process of the electrons that are responsible for the visible aurora, the recognition of the fundamental role of the large-scale current systems in organizing the auroral mor- phology, and of the interplay between particles and electromagnetic fields. The project began in March 1999, as a natural follow-up of the project on Magnetospheric Plasma Sources and Losses that resulted in volume 6 of this se- ries, with a planning meeting by a core-group that coordinated the project. The group consisted of J. E. Borovsky, Los Alamos National Laboratory; C. W. Carl- son, University of California, Berkeley; G. Haerendel, Max-Planck-Institut fur ex- traterrestrische Physik, Garching; B. Hultqvist, Swedish Intitute ofSpace Physics, H. E. J. Koskinen, Finnish Meteorological Institute, Helsinki; W. Lotko, Kiruna; Dartmouth College, Hanover, New Hampshire; K. A. Lynch, University of New Hampshire, Durham and G. Marklund, Royal Institute ofTechnology, Stockholm. G. Paschmann, ISSI, Bern, was the project leader.