Arecibo Observatory, Arecibo PR, USA
It is unusual for one individual to make so many original contributions in so many areas including theory and experiment in multiple scientific fields. This talk discusses seemingly unrelated areas, ionospheric modification science and novel radar techniques, and shows how Professor Hagfors in at least one case contributed to both in the same experiment. He first invented the chirp plasma line technique, which allows high signal-to-noise ratio measurements anywhere in the F region. This high sensitivity can be achieved with an unmodulated long pulse only at the F peak. Then he applied this technique to measurements of the enhanced plasma line using both Arecibo and EISCAT radars and HF heaters to help in characterize the the effect of a powerful HF wave on the ionospheric plasma. This technique gives both frequency and range measurements with good resolution and is matched to the characteristics of the plasma. Techniques are very much a product of the technology at the time they were introduced, and so in the rest of the talk we shall try to give some historical perspective on Professor Hagfor’s contributions. One example is a brief look a surface wave filters for decoding Barker codes. (He did not invent this, but recognized it early on and brought one to Arecibo from Norway in the early 1970s.) Another is an inverse technique, before the term was even in general use, to extract Arecibo F region ionospheric vectors velocities from a time sequence of line of sight measurements with the azimuth angle continuously changing. This technique was used until recently when it became possible to work with an entire experimental set and thus better allow for changes in the vector with time.