Channel probe observations of the auroral ionosphere during high-power auroral stimulation heating experiments

Perturbations of the bottomside auroral ionosphere caused by a high-power HF heater have been observed using a bistatically configured ionospheric channel probe. The heater transmitter power and antenna directivity resulted in a power density of the order of 1 mw/m² at E layer heights and 0.2 mw/m² at F layer heights. O mode polarization was used for heating at a frequency of 2.85 MHz. Effects were observed at both E and F layer heights. The F layer effects consisted of enhanced attenuation, modified group delay at frequencies above and below the heater frequency, and a change in the character of the received signal from that of a time stable specular return to a time variable irregular return. All of these effects were most pronounced at frequencies closest to the heater frequency and diminished as the frequency deviated from the heater frequency. The formation of a localized heated cavity in the F layer is thought to be responsible for the observed changes in F layer signal group delay. Some form of parametric decay instability is thought to be responsible for the irregular character of the return signal during heating and for the enhanced attenuation which primarily affected the O mode of the probing signal. Heating induced Doppler effects were also noted in a second experiment. In particular, the main Doppler component exhibited a sharp change with the onset of heating followed by a steady change in Doppler from an initially large positive value to a large negative value at the time of heater shutdown. After heater shutdown the Doppler frequency relaxed to a steady value which differed from its preheating value. The spread of the Doppler return was also observed to broaden during heating which is consistent with the irregular character of the return during heating. The E layer return consisted of normal E layer echoes at frequencies below 2.5 MHz and of weak slant sporadic E echoes at and above 2.5 MHz. The slant sporadic E echoes were strongly attenuated during heating. An additional E layer effect involving what appears to be a localized, nonblanketing enhancement of f0E was observed during heating. The most likely explanation for the enhanced attenuation of the slant sporadic E is E layer heating caused by energetic electron precipitation from the heated F layer. The enhanced f0E could also be attributed to high-energy electron precipitation from the F layer but direct ohmic heating of the E layer is an equally plausible explanation.