Over the past few years it has become clear that the heating phenomenon in chromospheres and coronae very likely cannot be explained by a single process alone, but is rather due to a multitude of mechanisms. Some of these mechanisms operate globally, others only in particular physical situations or in very special magnetic field geometries. At the present time, it is not possible to decide which mechanisms are the important ones. This is due to the relatively primitive theoretical development of many mechanisms, and to the paucity of definitive observational tests which are capable of deciding between alternative mechanisms, or of testing specific predictions of particular mechanisms. This book is devoted to an exposition of the observational and theoretical work on stellar atmospheric heating which is pursued today, as propounded by the scientists conducting this research. As a consequence, the reader will not find here an exhaustive compendium of all possible such heating processes, but rather only discussions of those mechanisms which have survived the challenges of past observational tests (the interested reader will find a more extensive review, based on the published literature up to mid-1989, in Narain and Ulmschneider, Space Science Reviews, to appear 1990).
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