The dependence of F-region electron heating on HF radio pump power: Measurements at EISCAT Tromsø

[1] Measurements of ionospheric electron temperature enhanced by the action of a powerful high-frequency radio wave on the F-region at the EISCAT facility near Tromsø, Norway are analyzed to obtain the electron heat source due to the radio wave as a function of transmitter power. The absorption of the wave in the D-region is accounted for and is found to have a significant influence on the F-region heating, especially due to variations in the D-region electron density during the experiment. It is found that the efficiency of F-region heating expressed in terms of the electron heating rate as a function of radio wave power flux is higher at higher transmitter powers. This behavior seems to be consistent with the development of geomagnetic field-aligned plasma density irregularities which are associated with the conversion of the radio wave to electrostatic upper-hybrid waves. At the highest power fluxes, the efficiency appears to be close to 100%.