Low ‐ Frequency Waves in HF Heating of the Ionosphere

Ionospheric heating experiments have enabled an e xploration of the ionosphere as a large‐scale natural l aboratory for the study of many plasma processes. These experiments inject high‐frequency (HF) radio waves using high‐power transmitters and an array of ground‐ and space‐based diagnostics. The heating of the plasma has led to many new phenomena such as modulation of the ionospheric current system and generation of low‐ f requency electromagnetic radiation [Papadopoulos et al.,1989; 2011a, b; Stubbe, 1996], stimulated emissions [Leyser, 2001], plasma waves and turbulence [Guzdar et al. 2000], small‐scale irregularities or striations [Mishin et al., 2004], and pump‐induced optical processes [Bernhardt et al., 1989; Pedersen et al., 2009]. Among the ionospheric heating facilities worldwide, the High Frequency Active Auroral Research Program (HAARP) located at Gakona, Alaska, has the highest power HF transmitter and a c omprehensive set of diagnostics. The transmitter array of 180 crossed dipoles that are arranged in a 12 by 15 r ectangular grid are phased together to produce up to 3.6 MW of radiated power in a band from 2.8 to 10 MHz. The HF heating experiments conducted using HAARP facility has generated low‐frequency waves [Papadopoulos et al., 2011a, b] in all the three ranges, namely ultra low f requency (ULF, <10 Hz), extremely low frequency (ELF, 0.3–3 kHz), and very low frequency (VLF, 3–30 kHz). The low‐frequency waves generated in the ionosphere during heating experiments with modulated HF waves (1–10 MHz) originate from multiple physical mecha­ nisms that operate at different altitudes and conditions. A mechanism that is already recognized is the modula­ tion of the D/E region conductivity by modulated HF heating, and this requires the presence of an electrojet current, namely the auroral electrojet. The associated modification of the electrojet current creates an effec­ tive antenna radiating at the m odulation frequency [Stubbe et al., 1981; Papadopoulos et al., 1989; Stubbe, 1996]. This mechanism of low‐frequency wave genera­ tion by a modulated heating of the auroral electrojet, at ~80 km altitude in the D/E region, is referred to as the polar electrojet (PEJ) antenna. In another mechanism, modulated HF waves heat the plasma in the F region, producing a local hot spot and thus a region of strong gradient in the plasma pressure. This induces a diamag­ netic current on the time scale of the modulation fre­ quency, which excites the hydromagnetic waves. In this mechanism, there is no quasi‐steady or background cur­ rent, and the wave excitation is c ontrolled, for example, by the plasma conditions, HF modulation frequency, and size of the heated region. This is the mechanism that has been s tudied in simulations of the high‐latitude ionosphere [Papadopoulos et al., 2011a; Eliasson et al., 2012] for conditions typically corresponding to the HAARP f acility. Experiments at HAARP have verified this mechanism and provided details of the key features [Papadopoulos et al., 2011b]. Another mechanism for generating low‐frequency waves in the ELF range was motivated by observations from the DEMETER satel­ lite during experiments at HAARP with no modulation of the HF power. These waves have been identified as Low‐Frequency Waves in HF Heating of the Ionosphere