Using amplitude modulation derived from repetitive pulses of varying duty cycle with the Troms¿ heater facility, we determine the ratio of the heating to cooling time constants of ELF currents generated in the D-region. This radio is found to decrease from 1.03¿0.07 at 510 Hz to 0.28¿0.03 at 6010 kHz. These results, which were obtained using an effective radiated power of 240 MW, suggest that future heater facilities designed to optimise ELF radiation efficiency using the technique of 'beam painting' will have to operate at considerably higher power levels than the Troms¿ facility if any practical advantages are to be gained.