The auroral emissions in the upper atmosphere of the polar regions of the Earth are evidence of the capture of energetic particles from the Sun, streaming by the Earth as the solar wind. These auroral emissions, then, are a window to outer space, and can provide us with valuable information about electrodynamic coupling processes between the solar wind and the Earth's ionosphere and upper atmosphere. Studying the physics of these phenomena extends our understanding of our plasma universe.

Ground-based remote-sensing techniques, able to monitor continuously the variations in the signatures of aurorae, in combination with in-situ satellite and rocket measurements, promise to advance dramatically our understanding of the physical processes taking place at the interface of the atmospheres of the Earth and the Sun. Decoding their complexity brings us closer to reliable prediction of communication environments, especially at high latitudes. This understanding, in turn, will help us resolve problems of communication and navigation across polar regions.

Aurorae have been the object of wonder and scientific curiosity for centuries. Only recently, however, have we been able to detect, with sensitive instrumentation, noontime aurorae, and persistent aurorae deep within the polar cap. This book is the first to provide a morphological and theoretical framework for understanding these dayside and polar cap aurorae.

The book also communicates the excitement of discovery, as it details the nature of these newly revealed auroral displays. It is a fascinating voyage of exploration, one appropriate for students of nature, wherever and whoever they may be.

These proceedings are based on the invited talks and selected research reports presented at the NATO Advanced Study Institute (ASI) on "POLAR CAP BOUNDARY PHENOMENA" held at Longyearbyen, Svalbard, June 4 - 13, 1997. The role of the polar cap and its boundary is very substantial in solar-terrestrial physics. At this NATO AS! a major change in thinking on the "cusp" precipitation region in the high-latitude days ide upper atmosphere was reflected, at least for intervals when the interplanetary magnetic field (IMF) is directed southward. It is likely that this has implications for northward IMF as well. The change comes from the now almost complete acceptance of the concept of magnetosheath particle entry along open magnetic field lines and the evolution of the precipitation into the upper atmosphere with time elapsed since magnetic reconnection which opened the field line. A key prediction of this view is that the low-latitude boundary layer (LLBL) is on open field lines.

Summary of the NATO Advanced Research Workshop on Physical Signatures of Magnetospheric Boundary Layer Processes T A POTEMRA, M I PUDOVKIN, R W SMITH, V M VASYLIUNAS and A EGELAND 451 PREFACE These proceedings are based on the invited talks and selected research reports presented at the NATO Advanced Workshop on "PHYSICAL SIGNATURES OF MAGNETOSPHERIC BOUNDARY LAYER PROCESSES", held at Sundvolden Hotel, Norway, 9.-14.May 1993. The international political and scientific communities have gradually realized that the Earth's environment is more fragile than previously believed. This has led to the establishment of international research programmes directed toward the understanding of "Global Change". The Earth's magnetosphere, "the Earth-space", is a part of our environment, and physical processes in the magnetosphere and coupling between the solar energy stream, the solar wind, and the Earth-space are important in the complete understanding of our environment. Variations in the electromagnetic and particle energy output of the Sun have a significant effect on global changes. The energy transfer mechanisms at the days ide magnetospheric boundary layers and their ionospheric signatures are perhaps even more important to solar terrestrial research than the night-side processes in this connection. The dayside boundary layers and the polar cusps are the Earth's windows to outer space. The present NATO ARW was the latest in a series of conferences focused on dayside magnetospheric phenomena. It is five years since the preceding Workshop on "Electromag netic Coupling in the Polar Clefts and Caps" was held at Lillehammer in September 1988.

In Nordic literature a remarkable discussion of the northern light appears in Kongespeilet (The King's Mirror) a thirteenth-century Norwegian chronicle. It is described in vivid detail as the following translated excerpts demonstrate: These northern lights have this peculiar nature, that the darker the night is, the brighter they seem, and they always appear at night but never by day, most frequently in the densest darkness and rarely by moonlight. In appearance they resemble a vast flame of fire viewed from a great distance. It also looks as if sharp points were shot from this flame up into the sky; these are of uneven height and in constant motion, now one, now another darting highest; and the light appears to blaze like a living flame. Three different theories for the origin of the northern light were suggested in this book. Numerous naturally occurring heavenly phenomena have been observed and enjoyed as long as the Earth has been inhabited, but hardly any of them has stirred man's imagination, curiosity and fear as much as the northern light. The northern light is certainly one of the most spectacular of nature's phenomena.

These proceedings are based upon introductory talks, research reports and discussions from the NATO Advanced Workshop on the "Morphology and Dynamics of the Polar Cusp," held at Lillehammer, Norway, 7-12 May, 1984. The upper atmosphere at high latitudes is called the "Earth's win dow to outer space." Through various electrodynamic coupling process es as well as through direct transfer of particles many geophysical effects displayed there are direct manifestations of phenomena occurring in the deep space. The high latitude ionosphere will also exert a feedback on the regions of the magnetosphere and atmosphere to which it is coupled, acting as a momentum and energy source and sink, and a source of particles. Of particular interest are the sections of the near space known as the Polar Cusp. A vast portion of the earth's magnetic field envelope is electrically connected to these regions. This geometry results in a spatial mapping of the magnetospheric pro cesses and a focusing on to the ionosphere. In the Polar Cusps the solar wind plasma has also direct access to the upper atmosphere. The polar regions are thus of extreme importance when it comes to under standing the physical processes in the near space and their effect on our environment. The Introductory Talks given at this workshop provided a common background for discussing and understanding the physics of the Polar Cusp. By this book we will make the information which thus was provid ed to the participants of the workshop accessible to a wider audience."

These proceedings are based upon in~roductory talks, re- search repor~s and discussions at the NATO Advanced Work- shop on ELECTROMAGNETIC COUPLING IN THE POLAR CLEFTS AND CAPS, held at Lillehammer, Norway, 20-24th September 1988. By this book we will make the information which was pro- vided to the participants of the workshop, accessible to a wider audience. Electromagnetic processes governing particle, momen- tum, and energy transfer from the solar wind via the magne- tosphere and into the earth's upper atmosphere are the main topics of solar-terrestrial research. Due to the peculiar magnetic field configuration in the magnetosphere, result- ing from the interaction with the shocked solar wind, the sunward-side boundary is mapped along magnetic field lines in~o a thin, arc-like band of the days ide polar ionosphere at the boundary of the polar cap; i.e. ~he ionospheric cleft region. The polar cusp is a separate, more limited region near magnetic noon, as defined by electron and proton precipitation detected from polar orbiting satel- lites. The basic physics of the different coupling modes at the dayside magnetopause is a matter of great controversy. This is an important problem to solve, also because similar boundaries exist in stellar objects throughout the Uni- verse. It is expected that ground-based remote sensing tech- niques, with their ability to continuously monitoring the temporal and spatial variations of the ionospheric signa- tures, will have a great impact on this problem, in parti- cular when combined with in situ measurements.