HR: 0800h
AN: SA11A-0293    [Abstracts]
TI: A Lack of Electron Density Production During Long-Pulse Ionospheric HF Heating Experiments
AU: * Moore, R C
EM: moore@ece.ufl.edu
AF: University of Florida, 319 Benton Hall, Center Drive, Gainesville, FL 32611, United States
AB: It is by now well-known that modulated high frequency (HF) heating of the lower ionosphere in the presence of the auroral electrojet current system can efficiently produce electromagnetic waves in the extremely low frequency (ELF, 3-3000 Hz) and very low frequency (VLF, 3-30 kHz) bands. One active area of research addresses the improvement of the HF-to-ELF/VLF conversion efficiency. In this paper, we experimentally evaluate the hypothesis that minutes-long HF pulses (so-called pre-heating pulses) modify the ionospheric electron density, in turn enhancing the efficiency of ELF/VLF wave generation. Recent upgrades at the High-frequency Active Auroral Research Program (HAARP) HF transmitter in Gakona, Alaska allow for dual-beam operations. During a thirty-minute period on 2 August 2007, one beam of the HAARP HF transmitter generated ELF/VLF waves (at 1215 Hz and 2430 Hz) by modulated ionospheric heating at 4.5 MHz (X-mode), stepping the peak HF power in 15 distinct log-based steps. Simultaneously, the second beam of the HAARP HF transmitter continually heated the same patch of ionosphere at 3.25 MHz (CW, X-mode) for a period of 8 minutes. This 8-minute transmission block was followed by a 7-minute period without CW heating (i.e., the first beam continued to modulate at 3.25 MHz while the second beam was OFF). The experiment was repeated twice during the 30-minute window. ELF/VLF wave observations at a ground-based receiver indicate that the electron temperature change produced by HF heating dominates the ionospheric conductivity change even for minutes-long HF heating pulses. In the case presented, ionospheric conductivity variations on the minutes-long time-scale for electron density change are not detected. It thus appears that in at least some cases, the electron density changes produced by minutes- long HF heating pulses are insignificant compared to the electron temperature changes produced (at the altitude of ELF/VLF wave generation). Possible explanations for a lack of detectable electron density change are discussed.
DE: 2403 Active experiments
DE: 2407 Auroral ionosphere (2704)
DE: 2435 Ionospheric disturbances
DE: 2467 Plasma temperature and density
SC: SPA-Aeronomy [SA]
MN: 2007 Fall Meeting