We present here signals received near the Troms¿ ionospheric heater facility when the heater was modulated with a 505Hz square wave. We found that whereas the harmonics of the square wave (≥1515 Hz) showed the accepted behaviour of reduced wave excitation on switching from 'X' to 'O' mode HF heating, the fundamental frequency showed enhanced excitation of more than 10dB with 'O' mode heating. The phase of the 505 Hz signals changed by typically 90¿ when the HF wave polarization was switched from 'X' to 'O' mode, but the phase of the harmonics was continuous through HF mode switching. We consider that these results indicate that during 'O' mode heating the fundamental current was sourced in the upper D/E region, where the Pedersen current dominated, whereas the harmonic currents were sourced around 80 km where the Hall current maximised. The amplitude of the signal at the fundamental frequency was very unstable during 'O' mode heating. This together with the fact that STARE data showed clear echos over Troms¿ during 'O' mode heating, which disappeared on switching to 'X' mode, leads us to conclude that parametric instabilities, modulated by the HF heater, were responsible for this enhanced ELF excitation.¿ 1997 American Geophysical Union