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 temperature at D region altitudes. In this paper we present new observations at a station in Alaska positioned to provide measurements of VLF signals that have propagated beneath the ionospheric region heated by the high-power amoral stimulation (HIPAS) HF heating facility near Fairbanks, Alaska. Analysis of data from HIPAS campaigns conducted in fall 1992 and spring 1993 has shown that in roughly 60% of the cases analyzed, the amplitude of the 23.4-kHz signal from the NPM transmitter in Hawaii as observed in Fort Yukon, Alaska, exhibited a measurable change in amplitude with the same on/off modulation pattern as that of the HIPAS HF transmissions at 2.85 MHz. In almost 70% of the cases analyzed, the same signal exhibited similar measurable changes in phase. The amplitude changes ranged from -0.2 dB to +0.5 dB, and the sensitivity of the measurement was approximately +/- 0.02 dB. The phase changes ranged from -4.5 degrees to -0.3 degrees, and the sensitivity of the measurement was typically +/- 0.4 degrees. It is demonstrated that the phase and amplitude changes can be used as diagnostic tools to determine characteristics of the ambient electron density profile above the HIPAS facility.