Results of a High Power Oblique Incidence HF Ionospheric Modification Experiment

An oblique ionospheric modification experiment using high power transmitter was carried out between Delano, CA and Shreveport, LA. A probe system, transmitting on a frequency close to that of the VOA heater signal was used to determine the effects of heating by observing the variation in its amplitude, angle of arrival and Doppler. A mid-point Digisonde was used for frequency management and as a HF radar for the detection of small scale heater-induced self-focussing irregularities. For this period VOA was not operating at its full power of 90 dBW so the experiment was largely considered as a fact finding exercise for future campaigns. As expected, data from both the probe system and Digisonde showed no unambiguous evidence as the heating cycle. Processed probe data for this period had significantly higher frequency spectral components than predicted and contaminate the spectra in the region near the heater cycle. These short period received signals are most likely due to naturally occurring structure in the ionosphere, such as multiple acoustic gravity waves, and the interference between the different propagation modes of the probe system. Digisonde drift data detected a large number of well defined large scale irregularities and gave a relatively smooth spectrum at higher frequencies. The large amount of high frequency components in the Fourier spectrum highlights the need in future experiments to assess ionospheric conditions with a view to placing the heater cycle period in a clear region of the spectrum.