Observations of the HF-enhanced plasma line with a 46.8MHz radar and reinterpretation of previous observations with the 430 MHZ radar

Abstract

During 5.1 MHz HF transmissions below the F-region penetration frequency by a nearby ionospheric modification facility, radar echoes coming from F-region heights at a frequency of 46.8 + 5.1 = 51.9 MHz were received while a VHF radar using the 305 m spherical reflector at Arecibo was transmitting pulses coherently on 46.8 MHz. The bandwidth of the echoes was less than 10 Hz. The so-called decay line due to the parametric decay instability was expected at a frequency lower by about 300 Hz but it was not observed. Density variations caused by the ponderomotive force resulting from the standing wave pattern at 5. 1 MHz were tentatively invoked to explain the observations. Poorly understood aspects of previous analogous observations with the 430 MHz radar can be similarly explained.