A Double Layer Review (Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May 1978)

A review is given of the main results on electrostatic double layers (sometimes called 'space charge layers' or 'sheaths') obtained from theory and laboratory and space experiments up to the spring of 1977. The paper begins with a definition of double layers in terms of potential drop, electric field, and charge separation. Then a review is made of the theoretical results obtained so far. This covers, among other things, necessary criteria for existence and stability, quantitative estimates of charge separation and thickness, and some probable cause of DL-formation in terms of an instability. Next, experimental results obtained in the laboratory are compared with the theoretical results. Due to recent progress in experimental technique, the interior of a double layer can now be studied in much more detail than was possible before. By means of barium jets and satellite probes, double layers have now been found at the altitudes that were previously predicted theoretically. The general potential distribution above the auroral zone, suggested by inverted V-events and electric field reversals, is corroborated.