Observation and theory of whistler wave generation by high‐power HF waves

Abstract

[1] Summer midnight and afternoon VLF wave generation comparison experiments were conducted on 25 July and 27 July 2011, respectively, using two CW HF X-mode waves with 11 VLF frequency differences from 2 to 21.5 kHz. The background magnetic variations were at comparable levels. During the afternoon experiment, the D region absorption was significant and increasing. The number of the ionogram echoes decreased during the afternoon experiment. VLF signals were detected from 2 to 7.6 kHz in both experiments, showing an inverse frequency dependence of intensity, although signal intensity (except at 5.5 kHz) detected during the midnight experiment was stronger than the corresponding afternoon intensity before observing a decrease of the number of ionogram echoes. However, VLF signals from 11.5 to 21.5 (except at 19.6 kHz) were also generated in the afternoon experiment concurrent with a decrease of the O-mode ionosonde echoes from 2 to 4 MHz. The concurrence of a decrease of the afternoon ionogram echoes, the unexpected generation of VLF waves at higher frequencies, and the increasing D region absorption throughout the experiment may be explained by the generation of large-scale density irregularities, which scatter the ionosonde signals as well as couple with the modulated electrojet to generate whistler waves. A theoretical formulation of the coupling mechanism for the whistler wave generation is presented.

Features of amplitude and Doppler frequency variation of ELF/VLF waves generated by “beat‐wave” HF heating at high latitudes

“…Cohen et al [] used the model approach to show that the BW source region can be created in the D region ionosphere, not the F region. More evidence and discussions on ELF/VLF wave generation in the F region are presented recently by Kuo et al [] and Rooker et al [], reporting diagnosis of BW‐generated ELF/VLF waves using ionosonde and/or radar at HAARP. During ionospheric heating experiment a powerful HF wave can excite large‐scale sheet‐like ionospheric density irregularities (or artificial ionospheric ducts) [ Lee et al , ; Cohen et al , ; Kuo et al , ; Pradipta et al , ; Rooker et al , ].…”

“…More evidence and discussions on ELF/VLF wave generation in the F region are presented recently by Kuo et al [] and Rooker et al [], reporting diagnosis of BW‐generated ELF/VLF waves using ionosonde and/or radar at HAARP. During ionospheric heating experiment a powerful HF wave can excite large‐scale sheet‐like ionospheric density irregularities (or artificial ionospheric ducts) [ Lee et al , ; Cohen et al , ; Kuo et al , ; Pradipta et al , ; Rooker et al , ]. These HF wave‐created ionospheric ducts can affect the BW ELF/VLF generation process through the coupling to the temporally modulated electrojet to produce a whistler mode current and the associated whistler (ELF/VLF) wave at the beat frequency [ Kuo et al , ; Rooker et al , ].…”

“…These studies started since the 1970s in the USSR with the help of midlatitude (Zimenki, 56.3°N, 44°E) and polar (Monchegorsk, 68°N, 33°E) heating facilities [Getmantsev et al, 1974;Kapustin et al, 1977]. Subsequent experiments were continued at high-power HF heating facilities, such as Arecibo in Puerto Rico (18°N, 67°W) [Ferraro et al, 1982;Lee et al, 1998;Pradipta et al, 2013], European Incoherent Scatter (EISCAT) in Norway (69.6°N, 19.2°E) [Stubbe et al, 1981;Barr andStubbe, 1984, 1997;Rietveld et al, 1987;Oikarinen et al, 1997;Barr, 1998], "Sura" in Russia (56°N, 44°E) [Kotik, 1994;Blagoveshchenskaya and Troshichev, 1996;Kotik and Ermakova, 1998;Belikovich et al, 2007], high-power auroral simulation (65°N, 147°W), and High Frequency Active Auroral Research Program (HAARP) (62.4°N, 145.2°W) located in USA [McCarrick et al, 1990;Jin et al, 2009Jin et al, , 2011Kuo et al, 2011Kuo et al, , 2012Kuo et al, , 2013Cohen et al, 2010Cohen et al, , 2012Moore et al, 2012;Cohen and Golkowski, 2013;Rooker et al, 2013]. According to generally accepted physical mechanisms, periodic HF heating modulates electron temperature in the D region ionosphere (~60-100 km) which leads to modulated conductivity and thus the current density.…”

Ionospheric very low frequency transmitter

“…a sum of three main causes from the elastic collisions with the neutral particles and from the rotational and vibration excitations of the neutral particles, is determined to be 21,29 dðT e Þ e ðT e Þ ffi ð2m=M n Þ en0 ½v 5=6 þ 8:38v À1=2 þ l I ðvÞ; (14) where a normalized temperature v ¼ T e /T e0 is introduced, en0 ¼ en (T e ¼ T e0 ) and a reference equilibrium electron temperature T e0 ¼ 1500 K is assumed; and l I ðvÞ ¼ v À5=2 ½14:73e 6:98ð1À1=vÞ þ 0:1e 13 In the numerical analysis, dimensionless variables and parameters are introduced:…”

“…The induced alternating current (AC) in the electrojet is then the antenna current for VLF radiation at the modulation frequency and its harmonics. Generation of VLF waves in the high latitude ionosphere via electrojet modulation has been studied experimentally [1][2][3][4][5][6][7][8][9][10][11][12][13] and theoretically. [12][13][14][15][16][17][18][19][20][21][22] The heating and cooling rates as well as the available heating and cooling times in one modulation period affect the efficacy of conductivity modulation by an intensity modulated HF heater.…”