| Summary: | This report presents F-region electron densities and electron temperatures observed during the year 1966 at the Millstone Hill Radar Observatory (42.6°N, 71.5°W) by the UHF Thomson (incoherent) scatter radar. The measurements were usually made for periods of 24 hours twice per month, and covered the altitude range 150 to 750 km approximately. The time required to collect all the measurements spanning this height interval was 30 minutes, i.e., half that of previous years. The results exhibit a greater amount of random time variation than seen heretofore, partly as a result of the better time resolution achieved and partly because each day has been analyzed individually, i.e., we have discontinued the practice of computing only the monthly mean behavior. We believe, however, that the largest part of the random variation is real and results from fluctuations in the solar EUV flux, which increase in magnitude as the sunspot number rises. Also, we expect a growing incidence of fluctuations produced by large-scale traveling ionospheric disturbances as we approach sunspot maximum. Despite this variability, the characteristics “winter” and “summer” type behavior reported for Millstone in previous years is clearly recognizable. On four days in which pronounced effects due to geomagnetic storms are evident, the layer rose to great heights in the late afternoon and achieved an abnormally high density (and lower-than-normal temperature). The reverse behavior was encountered the next morning. This sequence is similar to that first seen in June 1965, and its interpretation in terms of current ideas is summarized.
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