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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