On the altitude of the ELF/VLF source region generated during “beat‐wave” HF heating experiments

Abstract

Modulated high frequency (HF, 3–10 MHz) heating of the ionosphere in the presence of the auroral electrojet currents is an effective method for generating extremely low frequency (ELF, 3–3000 Hz) and very low frequency (VLF, 3–30 kHz) radio waves. The amplitudes of ELF/VLF waves generated in this manner depend sensitively on the auroral electrojet current strength, which varies with time. In an effort to improve the reliability of ELF/VLF wave generation by ionospheric heating, recent experiments at the High‐frequency Active Auroral Research Program (HAARP) facility in Gakona, Alaska, have focused on methods that are independent of the strength of the auroral electrojet currents. One such potential method is so‐called “beat‐wave” ELF/VLF generation. Recent experimental observations have been presented to suggest that in the absence of a significantD‐region ionosphere (∼60–100 km altitude), an ELF/VLF source region can be created within theF‐region ionosphere (∼150–250 km altitude). In this paper, we use a time‐of‐arrival analysis technique to provide direct experimental evidence that the beat‐wave source region is located in theD‐region ionosphere, and possibly the lowerE‐region ionosphere (∼100–120 km altitude), even when ionospheric diagnostics indicate a very weakD‐layer. These results have a tremendous impact on the interpretation of recent experimental observations.