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In recent years methods of producing controlled modifications of the ionosphere by use of high‐intensity radio‐frequency waves have been proposed; some have been tried with inconclusive results. The present paper describes detection of significant (∼30%) changes in the electron temperature in the F2 region produced by absorption of radio‐frequency energy propagated at or near the local ionospheric plasma frequency. The radio‐frequency source is the 1‐Mw cw transmitter and ∼16° beamwidth antenna array described in the accompanying paper by Utlaut [1970]. As shown by the calculations of Meltz and LeLevier [1970], electron temperature increases of ∼35% can be expected within a few tens of seconds after the transmitter, tuned to within 1% of f0F2, is turned on.

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Observations of the free atomic sodium layer near 90 km have been made as part of performance tests on a tunable dye‐laser radar. Altitude profiles of the layer obtained during parts of four nights in the fall of 1971 are consistent with those obtained by other groups but show two interesting additional features. The first is a sharp decrease in density that terminates the layer on the bottom side at a variable altitude near the mesopause. This decrease appears to become sharp only some time after twilight, suggesting that the sodium consumption mechanism undergoes a change as the nighttime chemistry is established. The second feature is a fourfold increase in sodium‐layer content during a 4‐hour period surrounding the transit of the radiant of the Geminids meteor shower on the night of December 13–14, 1971, when the shower was at its peak.

A Fabry-Perot spectrometer has been used at Arecibo, Puerto Rico, for measurement of the Doppler line width of [0 I] 6300-A emission in the night airglow. The temperature of the nighttime F region was obtained simultaneously from this technique and from analysis of data from incoherent scatter radar. In a