VLF SIGNATURES OF IONOSPHERIC HEATING BY HIPAS

The amplitude and phase of subionospherically propagating VLF signals are known to be sensitive indicators of the electron density and tempe rature at D region altitudes. In this paper we present new observation s at a station in Alaska positioned to provide measurements of VLF sig nals that have propagated beneath the ionospheric region heated by the high-power amoral stimulation (HIPAS) HF heating facility near Fairba nks, Alaska. Analysis of data from HIPAS campaigns conducted in fall 1 992 and spring 1993 has shown that in roughly 60% of the cases analyze d, the amplitude of the 23.4-kHz signal from the NPM transmitter in Ha waii as observed in Fort Yukon, Alaska, exhibited a measurable change in amplitude with the same on/off modulation pattern as that of the HI PAS HF transmissions at 2.85 MHz. In almost 70% of the cases analyzed, the same signal exhibited similar measurable changes in phase. The am plitude changes ranged from -0.2 dB to +0.5 dB, and the sensitivity of the measurement was approximately +/- 0.02 dB. The phase changes rang ed from -4.5 degrees to -0.3 degrees, and the sensitivity of the measu rement was typically +/- 0.4 degrees. It is demonstrated that the phas e and amplitude changes can be used as diagnostic tools to determine c haracteristics of the ambient electron density profile above the HIPAS facility.