This book presents the first comprehensive account of the properties of plasma loops, the fundamental structural elements of the solar corona. Plasma loops cover a wide range of sizes, and range in temperature from tens of thousands to millions of degrees. They not only define the structure of individual active regions but connect different active regions - even across the solar equator. Loops also play an integral and decisive role in the enormous solar explosions called flares. Over recent years a wealth of space and ground-based observations of loops has been obtained in various widely-spaced regions of the electromagnetic spectrum. In this book the authors have selected the best observational material from the literature on which to base a detailed account of the properties of flare and non-flare loops. The book also explores the larger implications of the loop structures in our understanding of solar and stellar coronae. The text is enhanced by a large number of illustrations and unique and beautiful photographs obtained from the ground and from space.

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Modern measurements of the granule contrast are reviewed and compared with a curve showing the wavelength variation to be expected on the assumption of black-body emission as well as with the predictions of recent inhomogeneous models. The difference in effective temperature between granules and intergranular lanes is ~270-280 K.

High-resolution filtergrams of the quiet chromosphere, taken at seven wavelengths in Ha with the aid of a computer-controlled ~ A filter, have been used to derive the contrast of ten bright and dark mottles as functions of wavelength. The contrast profiles of bright and dark mottles are strikingly different. They disagree with Athay's (1970) 'velocity' model but, with an appropriate choice of parameters, can be brought into good agreement with Beckers' (1964) 'cloud' model. Comparison between observation and theory yields values for the source function S, optical thickness to, line broadening parameter d20, and line-of-sight velocity V for both bright and dark mottles. The values of S and to obtained for dark mottles are consistent with Beckers' (1968) spicule model.