Excitation of ELF waves in the Schumann resonance range by modulated HF heating of the polar electrojet

Abstract

We report the successful generation and detection of ELF waves in the frequency range of 6-76 Hz by polar electrojet modification using the High-Power Auroral Stimulation (HIPAS) HF Heater Facility near Fairbanks, Alaska. Magnetic field amplitudes of-1 pT and vertical electric field amplitudes of---0.2 mV have been observed at a receiving site 35 km from the HIPAS site. The wave amplitude does not depend strongly on the ELF frequency, however, the amplitude is closely related to the level of electrojet activity inferred from magnetometer chain data and from ionosonde measurements. The 1-MW HF heater is modulated at low ELF frequencies using an "array dephasing" technique where the eight-element antenna array is alternately phased for peak vertical gain and dephased for a spread pattern at the ELF rate. This method can be employed at any modulation frequency without concern for transmitter power supply resonances. Measurements of the wave amplitude are made by conventional analog lock-in techniques and by continuously digitizing the waveform output of the wideband magnetic search coils and the electric field detector. With the digitized waveform, digital postprocessing can be used to extract the low-level coherent signal from the Schumann resonance noise background using a software lock-in routine.Paper number 90RS00041. 0048-6604/90/90RS-00041 $08.00 ionosphere and magnetosphere and into the oceans and the Earth's crust. Previous ELF measurementsat the HIPAS facility were concentrated in the range of 1-5 kHz [Ferraro and Li, this issue]. An extensive data set has been collected in this band, and a wide variety of parametric variations have been investigated.

ELF wave generation in the ionosphere using pulse modulated HF heating: initial tests of a technique for increasing ELF wave generation efficiency

“…8 are very similar, both showing a periodic gradual increase in signal strength with increasing duty cycle of the modulation waveform. Also both the fundamental and harmonic frequencies show preferential signal generation during`X' as opposed to`O' mode heating as reported in most previous studies (Barr et al, 1997;McCarrick et al, 1990;Stubbe et al, 1981).…”

Self-consistent modeling of induced magnetic field in Titan’s atmosphere accounting for the generation of Schumann resonance

“…The first point of divergence concerns the natural generation mechanism of ELF magnetic field components presumably being the energy source of the Titan's SRs (Béghin, 2014), while the same process does not seem to occur naturally on Earth. However, the first successful attempt of artificial triggering of ELF emissions in the polar electrojet by High Frequency (HF) heating experiments (Mc Carrick et al, 1990) and furthermore performed quasi-routinely in VLF range (e.g., Cohen and Golkowski, 2013) bring a significant support to the model proposed here, though no experimental evidence is known to date about a natural ELF self-modulation of terrestrial electrojets. This is probably because the necessary increase of conductivity which is thought to exist naturally in Titan's ionopause for triggering the ion-acoustic instability and subsequently yielding the ELF modulation of the current, seems not occurring on Earth.…”

“…The physical process of ion-acoustic instability is basically the same as observed with the experiments of HF heating of the ionosphere in the Earth's Polar Regions in which the increase of Pedersen conductivity is triggered artificially (e.g. Stubbe and Koppa, 1977;Mc Carrick et al, 1990;Cohen and Golkowski, 2013), contrary to the natural process involved for Titan. The specific criterion for triggering the ion-acoustic instability is when the electron-ion drift velocity V drift exceeds the adiabatic ion-acoustic speed c s , which reads…”

Ionospheric very low frequency transmitter

“…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.…”