On spectra of ionospheric conductivity variations during a heating experiment

Appearance of the harmonics in the spectrum of artificial emissions generated during heating experiments has been reported earlier. Usually, it relates with square modulation of HF powerful wave and distortion of this time dependence due to characteristic time of electron heating/cooling. In the EISCAT‐Heating experiment in November 1993, a quasi‐sinusoidal modulation regime was used. Observations show the clear harmonic structure in artificial emissions in this case too. In the present paper we discuss a generation of harmonics of a modulation frequency due to nonlinear dependence of value of the ionosphere conductivity disturbance from effective radiated power (ERP). A numerical model is used for calculations of the spectrum of the integral conductivity disturbance variation as a function of electron density profile and ERP. For the modulation frequency in the range of 1–100 Hz, the characteristic time of electron heating/cooling was neglected, and intensity ratios of the second and third harmonics to the fundamental frequency reach 0.4 and 0.15, respectively. For the modulation frequency 1375 Hz, time integration of the electron energy balance equation was used. The different characteristic time constants of the electron heating/cooling at different altitudes make the behavior of the intensity ratio and phase shift of the harmonics of the integral conductivity disturbance variations more complicated. The values of the harmonic amplitude ratio in this case decrease to 0.25 and 0.1.