Backscatter echoes from a region modified by RF heating above Tromsø were observed at Kiruna on 3 and 7 MHz, at Lycksele on 14 and 17 MHz, and at Uppsala on 15.6 MHz. When the heater output is suddenly varied from off to on, the backscatter echoes appear and grow to an equilibrium value in some seconds and decay at turnoff but then in some tens of seconds. These rise and decay time constants are found to be frequency dependent, being longer at lower radar frequencies. The strongest echoes seem to be obtained when the F layer peak plasma frequency (f0 F2) is close to the heater pump frequency. Echoes are absent when the heater is operating in X mode. Variation of heater output from 100% to 12.5% of full power yields a corresponding variation in the received echo amplitude without noticeable threshold effects. Some Doppler shift measurements are also presented indicating that the modified region, as well as the background, for the days of observation has a radial motion toward north in the range 0–20 m/s as seen from Lycksele. Estimated wave propagation limitation for the 3 and 7 MHz backscatter set by the observed electron density profile indicates that on the days considered the radar reflecting volume may extend vertically from about 60 km below to 20 km above the estimated optimally heated level. Observations made are consistent if we assume a modified volume with a diameter of less than about 75 km and a vertical extent of 40–80 km composed of field-aligned striations which grow and decay in accordance with the heater output power level.
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