Observations of ULF pulsation electric fields in the D region using the high-power auroral simulation heater facility

During the two week interval from October 12 to 24, 1987, an experiment was conducted using the high-power auroral simulation (HIPAS) ionospheric heating facility near Fairbanks, Alaska. During the experiment the beam was modulated at ELF and ULF frequencies, and the data were recorded at the Geophysical Institute of the University of Alaska approximately 50 km from the HIPAS site. On several occasions naturally occurring ULF waves were present in the overhead ionosphere and were detected in the ELF polarimeter data and in the ground magnetometer data. The amplitudes of the ELF signals show high correlation with the magnitude of the naturally occurring ULF pulsations. Our analysis indicates that the ELF returns were produced by currents generated in the D region, near 75 km, where the bulk of the heater energy was deposited. The ULF pulsation signatures detected by the ground magnetometers reflect Hall currents flowing principally in the E region above 100 km. We believe that the correlation between ELF and ULF signals is produced as the naturally occurring ULF electric field penetrates to D region altitudes where it can drive currents in the heated ionosphere. It is the modulation of those currents by the heater beam which produces the ELF signals detected on the ground. ELF signal levels indicate pulsation fields of approximately 5 mV/m in the D region.

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