1063-780X/03/2907- $24.00 © 2003 MAIK “Nauka/Interperiodica” 0561 Plasma Physics Reports, Vol. 29, No. 7, 2003, pp. 561–565. From Fizika Plazmy, Vol. 29, No. 7, 2003, pp. 606–611. Original English Text Copyright © 2003 by Papadopoulos, Wallace, McCarrick, Milikh, Yang. 1 1. INTRODUCTION A fundamental plasma physics concept among the main pioneering by Leonid Rudakov was the concept of electron magnetohydrodynamics (EMHD) [1]. The work presented here is a classic example of EMHD application in the Earth’s ionosphere, in general, and in the electrojet, in particular. Electron Hall currents driven by ionospheric electric ﬁelds in the D-region of the high latitude zone are responsible for a most fascinating plasma property: the potential to act as a frequency transformer that converts HF power injected from a high power HF transmitter into the ionosphere into coherent lower frequency VLF/ELF/ULF waves. The conversion principle relies on modulating the electrojet currents in the ionospheric D and E regions by using amplitude-modulated HF heating. The low-frequency ﬁelds subsequently couple to the earth–ionosphere wave guide, while a fraction of their power leaks towards the magnetosphere. Despite several years of theoretical [2–10] and experimental [11–16] work, many scientiﬁc and technical issues remain unresolved. Understanding the physics underly- ing their generation is important in increasing the HF to ELF/VLF conversion efﬁciency and utilizing this tech- nique for ionospheric diagnostics. A puzzling feature of the experiments has been the variation of the conver- sion efﬁciency with frequency and the unusually large relative amplitude of the harmonics. Figure 1 shows the frequency dependence of the average ﬁeld amplitude normalized to the amplitude at 2 kHz measured near the HAARP site. These data are typical of many other mea- surements and consistent with the data reported using the EISCAT heater [12]. The most important features seen in Fig. 1 and from previous data are the following: 1 This article was submitted by the authors in English. (1) An enhanced efﬁciency relative to the neighbor- ing frequencies at 2 kHz and its harmonics. (2) If we ignore the enhanced regions, the maximum efﬁciency is in the frequency range between 2 and 4 kHz. The efﬁciency is proportional to the frequency f between 2 kHz and 500 Hz. There is a weak increase in the efﬁciency between 500 and 100 Hz. The efﬁciency is proportional to 1/f between 4 and 10 kHz. (3) Harmonics with signiﬁcant relative amplitudes up to ten or larger are present. The amplitudes of the harmonics are much higher than expected from the Fourier analysis of the HF heating waveforms. Although the various sets of data have been col- lected under different heating parameters and iono- On the Efficiency of ELF/VLF Generation Using HF Heating of the Auroral Electrojet 1 K. Papadopoulos 1, 2 , T. Wallace 1 , M. McCarrick 1 , G. M. Milikh 1, 2 , and X. Yang 1 1 Advanced Power Technologies Inc., Washington, DC, USA 2 University of Maryland, College Park, MD, USA Received October 24, 2002 Abstract—Using experimental measurements and theoretical analysis, it is shown that the HF/ELF conversion efﬁciency is controlled by the timescale for electron temperature saturation. This is a function of the ERP and frequency of the heater and the ionospheric electron density proﬁle. For the current HAARP parameters, this corresponds to frequencies between 2 and 4 kHz. Efﬁciency optimization techniques as applied to the projected upgrading of the HAARP heater to its design power of 3.6 MW are discussed. © 2003 MAIK “Nauka/Interpe- riodica”. NONLINEAR PHENOMENA 10 4 10 3 10 2 0 1.4 0.2 0.4 0.6 0.8 1.0 1.2 Field amplitude, arb. units Frequency, Hz Fig. 1. The average ﬁeld amplitude measured near the HAARP site versus the ELF/VLF frequency. The amplitude is normalized.