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

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%.