The solar transition region, which spans the temperature range from about 20,000 to 1,000,000 K, separates the chromosphere from the corona. All the energy that heats the corona and powers the solar wind must pass through this part of the solar atmosphere. This book summarizes recent ultraviolet and extreme ultraviolet observations of the transition region, the empirical models derived from them, and the physical models that try to explain both the observations and the empirical models. The observational focus is on quiet solar transition region observations made with Skylab and subsequent rocket and satellite experiments. The book also presents a unified discussion of the analysis of ultraviolet and extreme ultraviolet spectroscopic data, including the determination of the emission measure and density and temperature diagnostics. This will be useful to astrophysicists who are confronting high-resolution ultraviolet and extreme ultraviolet data from astrophysical plasmas for the first time.

The discovery of chemical elements in celestial bodies and the first estimates of the chemical composition of the solar atmosphere were early results of Astrophysics - the subdiscipline of Astronomy that was originally concerned with the general laws of radiation and with spectroscopy. Following the initial quantitative abundance studies by Henry Norris Russell and by Cecilia Payne-Gaposchkin, a tremendous amount of theoretical, observa tional, laboratory and computational work led to a steadily improving body of knowledge of photospheric abundances - a body of knowledge that served to guide the theory of stellar evolution. Solar abundances determined from photospheric spectra, together with the very similar abundances determined from carbonaceous chondrites (where extensive information on isotopic composition is available as well), are nowadays the reference for all cosmic composition measures. Early astrophysical studies of the solar photospheric composition made use of atmosphere models and atomic data. Consistent abundances derived from different atmospheric layers and from lines of different strength helped to confirm and estab lish both models and atomic data, and eventually led to the now accepted, so-called "absolute" abundance values - which, for practical reasons, however, are usually given relative to the number of hydrogen nuclei.

The widespread tendency in solar physics to divide the solar atmosphere into separate layers and to distinguish phenomena of solar activity from phenomena of the quiet Sun emphasizes the wide ranging diversity of physical conditions and events occurring in the solar atmosphere. This diversity spans the range from a neutral, essentially quiescent atmosphere to a highly ionized, violently convective atmosphere; from a domain in which magnetic field effects are unimportant to a domain in which the magnetic pressure exceeds the gas pressure, and from a domain in which the particle motions are Maxwellian to a domain in which an appreciable fraction of the particles is accelerated to relativistic energies. It is now widely recognized that the chromosphere and corona have a common origin in the mechanical energy flux generated in the hydrogen convection zone lying beneath the photosphere. Furthermore, magnetic field phenomena appear to be as vital to the structure of th~ quiet Sun as to the active Sun. For these reasons it appears desirable to present a unified treatment of the entire solar atmosphere, both active and quiet, in a single volume. On the other hand, such a treatise must be very long if it is to avoid being superficial, and it is very difficult for a single author to write authoritatively on such a wide range of topics.

The complex internal structure of the Sun can now be studied in detail through helioseismology and neutrino astronomy. The VI Canary Islands Winter School of Astrophysics was dedicated to examining these powerful new techniques. Based on this meeting, eight specially-written chapters by world-experts are presented in this timely volume. We are shown how the internal composition and dynamical structure of the Sun can be deduced through helioseismology; and how the central temperature can be determined from the flux of solar neutrinos. This volume provides an excellent introduction for graduate students and an up-to-date overview for researchers working on the Sun, neutrino astronomy and helio- and asteroseismology.

The polarization of light is the key to obtaining a wealth of essential information that lies encoded in the electromagnetic radiation from cosmic objects. Spectropolarimetry and imaging polarimetry provide powerful diagnostics of the physical conditions in astrophysical plasmas, which cannot be obtained via conventional spectroscopy. Whilst its application to other fields of astrophysics is still at an early stage of development, spectropolarimetry is being used with great success in solar physics. The book contains the lectures delivered at the XII Canary Islands Winter School of Astrophysics. Written by eight prestigious astrophysics researchers, it covers the physics of polarization, polarized radiation diagnostics of solar magnetic fields, stellar magnetic fields, polarization insights for active galactic nuclei, compact objects and accretion disks, astronomical masers and their polarization, infrared-submillimeter spectropolarimetry, and instrumentation for astrophysical spectropolarimetry. This timely volume will provide graduate students and researchers with an unprecedented introduction to the field of Astrophysical Spectropolarimetry.