Senior, A. and Kosch, M. J.
Lancaster University, Lancaster, UK
The F-region "cutoff" of the natural (photoelectron-enhanced) plasma line was observed with the EISCAT UHF radar during O-mode HF heating with the EISCAT HF facility near Tromsų, Norway. The F-region critical frequency was slightly above the HF pump frequency of 4.544 MHz during the daytime in January 2006. Large electron temperature enhancements (~2000 K) were observed. The natural plasma line cutoff frequency was observed to increase by ~200 kHz coincident with the increases in electron temperature, but the cutoff spectrum also became weaker and less well-defined while the pump wave was switched on. The shift of the cutoff appears to be consistent with the shift in Langmuir frequency estimated from the electron density and temperature derived from the analysis of the UHF ion line spectrum. This suggests that the large temperature enhancements observed by the UHF radar during heating can be correctly interpreted in terms of a Maxwellian plasma and non-Maxwellian effects due to electrons accelerated by plasma turbulence play only a minor role. The weakening and spreading of the plasma line during heating is probably due to the formation of density irregularities in the plasma.