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

Abstract

Abstract. This paper describes the results of a preliminary study to determine the eective heating and cooling time constants of ionospheric currents in a simulated modulated HF heating,`beam painting' con®guration. It has been found that even and odd harmonics of the fundamental ELF wave used to amplitude modulate the HF heater are sourced from dierent regions of the ionosphere which support signi®cantly dierent heating and cooling time constants. The fundamental frequency and its odd harmonics are sourced in a region of the ionosphere where the heating and cooling time constants are about equal. The even harmonics on the other hand are sourced from regions of the ionosphere characterised by ratios of cooling to heating time constant greater than ten. It is thought that the even harmonics are sourced in the lower ionosphere (around 65 km) where the currents are much smaller than at the higher altitudes around 78 km where the currents at the fundamental frequency and odd harmonics maximise.

Powerful electromagnetic waves for active environmental research in geospace

“…It is a challenging task to optimize the efficiency of ELF/VLF generation. It was found that so‐called pump beam painting during pulse‐modulated HF heating will not greatly enhance the emission at the fundamental frequency but should be effective in enhancing the even harmonics [ Barr et al , 1999]. Further, we suggest that since EM radiation depends on the rate of change of the source current, the efficiency might be increased if the total available energy is concentrated in pulses to maximize the rate of electron heating and the rate of change induced on the electrojet.…”

Comment on “The magnetic response of the ionosphere to pulsed HF heating” by K. Papadopoulos, T. Wallace, G. M. Milikh, W. Peter, and M. McCarrick

“…In fact a large part of R86 is devoted to the asymmetrical response of the magnetic field between pulse–on and pulse–off. The implications for ELF/VLF generation efficiency were also discussed in a later paper by Barr et al [1999].…”

On the generation of ELF/VLF waves for long‐distance propagation via steerable HF heating of the lower ionosphere

“…Previous studies at the Tromsø facility [ Barr and Stubbe , 1991; Barr et al , 1999], using a similar exponential model, have concluded that the ERP levels there are not sufficiently high to allow beam painting to work as proposed. Although the HAARP facility has a higher ERP at 3.25 MHz (575 MW) compared to Tromsø (300 MW), it is not certain that this ERP level is high enough, either.…”

“…The heating and cooling occurs exponentially, with characteristic times τ heat and τ cool , respectively. This curve is a first‐order approximation of the heating and cooling dynamics but one that has been employed previously [ Barr et al , 1999]. For τ cool = 165 μ s, as in the estimate given by Barr et al [1999] for the modulation frequency and a daytime ionosphere.…”