Concluding remarks, session 1: Instabilities and irregularities

Abstract

The topic of instabilities and irregularities is a comparatively recent field, in which a great deal of insight has been developed through interdisciplinary contacts between ionospheric and plasma physicists. Historically, the landmark year for the opening of the field was 1963, which saw the independent development by Farley [1963] and Bunemann [1963] of the theory of the two‐stream instability (now generally accepted as the cause of the Type I electro jet irregularities), and the development by Simon [1963] and Hoh [1963], in a laboratory‐plasma context, of what is now variously known as the gradient‐drift, crossfield, or E×B instability (which is becoming accepted as the source of Type II irregularities). In all fairness, however, it is worth remembering that Martyn [1959] had earlier published a qualitative description of the gradient‐drift instability and its possible application to ionospheric spread‐F.Since 1963, a great deal of effort has gone into studying the application of these instabilities to the E region, particularly in the equatorial electro jet, but also more recently in the auroral electro jet, where conditions are much more complicated. As shown by several of the papers presented at this conference, we have come a long way since the early days, when only linear theory was available and all one could do was to predict thresholds for the instability. Unfortunately, as is usual when knowledge increases, the picture has also become somewhat fuzzier and less distinct than it was: even the distinction between the instabilities is not as clear as it was, since essentially the same equations are used to describe both, and the irregularities they create feed back on each other. It is also now clear that much of the time we are in fact seeing secondary irregularities, at least at the frequencies commonly used in radar observations. These irregularities are generated as nonlinear consequences of the growth of the primary irregularities, which may for the most part be visible only at much lower radio frequencies, such as those used by ionosondes.