Observation of artificial spread-F and large region ionization enhancement in an HF heating experiment at HAARP

A large-scale ionospheric modification by HF heaters was explored via HAARP digisonde operated in a fast mode. The results show that the ionogram virtual heights and the height spread of the ordinary-wave sounding echoes were changed significantly by the O-mode heater; the X-mode heater imposed no noticeable effect on the ionograms. The enhanced virtual height spread exceeds 40 km, more than 15% of sounding echo's average virtual height. The heater downshifted/upshifted the virtual height in the low/high frequency region around the heater frequency by as much as 15 and 7.5 km. The modifications were developing to last more than 10 seconds after the heater was turned off. The perturbed ionosphere took more than 60 seconds to recover. The modified electron density distribution indicates that the electron density and temperature increases exceed 10% and 25% over a large altitude region (>30 Km) from below to above the HF reflection height.