Ionospheric research 50 years ago, today and tomorrow

Fifty years ago the processes responsible for the formation of the ionosphe ric layers were well established, Appleton, Bartels and Chapman having been in the vanguard of the exploration phase of ionospheric research. Routine observations were being carried out in some locations, but in other areas, such as Antarctica, the first measurements were only being undertaken. Thus , a global perspective of the ionosphere was still emerging. In the interve ning years, there have been many very significant findings. Energy from the atmosphere both above and below the ionosphere has been shown to have a ma jor influence its structure and dynamics. Winds up to 1 km s(-1) blow at F- region altitudes and the solar wind imposes a cross-polar cap potential > 1 00 kV on the ionosphere when the interplanetary magnetic field has a strong southward component. The ionosphere itself is not simply a passive window on atmospheric and plasma processes. It is the major source of plasma for t he magnetosphere, and its lower boundary is critical in the reflection of p lanetary waves back into the stratosphere and troposphere. Evolving technol ogy has made a vast improvement in our ability to observe and model the ion osphere; same key developments are summarised. The paper also provides a fe w illustrations of the topics studied over the last 50 yr, namely F-region storms, plasma irregularities and long-term changes. Although the climatolo gy of the ionosphere is well understood and can be modelled successfully, p redicting the ionospheric weather accurately is not yet possible. The paper concludes by identifying some key topics where further understanding will be essential if reliable modelling and forecasting are to be achieved. (C) 2000 Elsevier Science Ltd. All rights reserved.