Aspect angle dependence of top- and bottom-side UHF radar backscatter from ionospheric HF pumping at Tromsø

Ashrafi, M.1, Kosch, M.2, Starks, M.3, Senior, A.2, Rietveld, M.4 and Yeoman, T.5

1 School of Physics and Astronomy, University of Southampton, UK
2 Communication Systems, Lancaster University, UK
3 Air Force Research Laboratory, MA, USA
4 EISCAT Scientific Association, Tromsø, Norway
5 Physics Department, University of Leicester, UK

The EISCAT high-frequency transmitter facility near Tromsø has been used to inject an O-mode wave into the ionosphere with the pump beam dipped 9-degrees south of zenith and 106 MW effective radiative power at 4.544 MHz. Due to the beam width (-6 dB), zenith angles of 0-16 degrees were illuminated, with the transmitters modulated 1-s on 29-s off. A low duty cycle at a pump frequency far removed from any electron gyroharmonic encourages Langmuir turbulence and suppresses the formation of striations (upper-hybrid resonance). The latter is confirmed by CUTLASS radar observations at 11 MHz. The UHF radar scanned in 0.5 or 1-degree steps in angular subsets of 0-20 degrees zenith angle south. Ion-line enhancements, corresponding to Langmuir turbulence, were observed in the UHF backscattered power at the O-mode bottom-side reflection height (~220 km) and the Z-mode top-side ionosphere (~310 km). The meridian scanning UHF backscatter show bottom-side enhancements at all angles, except in a narrow window ~8-10 degrees south. Within the so-called radio window top-side echoes appear. These observations confirm theory by Mjølhus and Flå [1984]. When changing the pump frequency, the top-side UHF backscatter is only observed when the ionospheric peak frequency is ~15% higher than the modifying pump frequency, as predicted by theory. The results are consistent with HF ray trace modeling.