MIT Haystack Observatory, Westford, Massachussets, USA
Following Gordon's prediction of incoherent scattering of radio waves by the ionosphere in 1958 and Bowles' observations of that scattering later the same year, there was a flurry of activity to theoretically describe the observed returns. From 1961 to 1962 over 20 journal articles and technical reports were published, largely completing the theory of incoherent scatter as we understand it today. This period will be discussed from a historical perspective with an emphasis on the contributions of Tor Hagfors. In brief, to quote an unpublished manuscript of Tor's, "In spite of the difference in approaches to the problem, there seems to be a common feature in all of them, viz. they all appear to be sufficiently complicated to make physical interpretation difficult. Since it appears to be of importance to have available simple physical explanations for the different properties of the scattering process, an attempt is made to rederive the expressions for the scattering from first principles and fairly readily understandable pictures."