Abstract
Characteristic times for conductivity changes in the auroral D-region caused by HF heating are deduced from the VLF/ELF signals recorded on the ground underneath the heated region. The VLF/ELF signals are caused by a heating-induced redistribution of the horizontal ionospheric currents when d.c. electric fields exist in the ionosphere. Measurements were made in the late afternoon with a dark, moderately disturbed ionosphere. The time constant for the Hall conductivity to change due to heating with 240 MW of effective radiated power is approximately 70 μs, while the time constant for cooling is approximately 120 μs. These results agree with model calculations. Delay times of the VLF/ELF pulses and their subsequent echoes result in a virtual source height of approximately 88 km and a VLF reflection height of approximately 75 km.
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