A theoretical model for the broad upshifted maximum in the stimulated electromagnetic emission spectrum

Abstract

A second‐order four‐wave interaction process including two pump photons, an upper hybrid plasmon, and an electron Bernstein plasmon is studied. The pump is the second harmonic of the HF heater in the plasma. It is found that, when the heater wave frequency f0 is above a harmonic of the electron cyclotron frequency fc, frequency‐upshifted upper hybrid waves and frequency‐downshifted electron Bernstein waves can be excited above the upper hybrid resonance layer via the considered process. The process occurs in a local region where the heater wave frequency is about the mean of the upper hybrid wave frequency and the electron Bernstein wave frequency. Moreover, in this interaction process, a low‐frequency electrostatic oscillation in the frequency range of the lower hybrid wave is generated through nonlinear coupling of the HF heater wave with the excited high‐frequency electrostatic waves. However, this wave does not satisfy the linear dispersion relation of the lower hybrid wave and is thus a driven wave. The excited frequency‐upshifted upper hybrid waves can then scatter off field‐aligned density irregularities to generate O ‐ mode emissions with frequencies around 2f0 − nfc This is consistent with observations of the broad upshifted maximum (BUM) feature in the stimulated electromagnetic emission (SEE) spectrum. The concomitantly excited frequency‐downshifted electron Bernstein waves are found to have much smaller amplitudes; hence their scattering products are also relatively weak. This explains why only BUM lines are detected. Furthermore, the driven low‐frequency fluctuations can also be the scatterers to convert the upper hybrid waves into emissions with frequencies aroundy fBUM + fLH and fBUM − fLH, where fLH is the lower hybrid wave frequency. This is suggested to be the generation mechanism of the second BUM feature which appears when the shoulder of the BUM feature is not very high.

Effect of ionospheric self‐conditioning and preconditioning on the broad upshifted maximum component of stimulated electromagnetic emission

“…Time constants for the decay of the conditioning effect are more widely spread ranging between 12 and 15.5 s based on two-pump interaction experiments and 28 s derived from the decay of single-pump residual conditioning effects. A breakdown of these results is shown in Table 1. FAI (striations) are of interest because they are involved in the conversion of electrostatic waves into electromagnetic waves [Huang and Kuo, 1994] The comparability of the time constants for the BUM decay and buildup reported in this paper with those of the FAI measured by Hysell et al [1996] suggests that striations could be the conditioning agent responsible for the observed BUM transients. However, other measurements involving simultaneous monitoring of the DM and the BUM (Figures 5, 11, 16, and 19), and other one-pump, two-pump, and power-stepping experiments [Frolov et al, 1996[Frolov et al, , 1997b[Frolov et al, , 1997d Other two-pump experiments, using narrowband receivers at selected BUM frequencies, resulted in quite different interaction results from those discussed in this paper [Frolov et al, 1996, Figure 8; Frolov et al, 1997b, Figure 5].…”

“…Interactions capable of generating BUM waves that satisfy the ob-referred to as four-wave interactions [Leyser et al, 1989[Leyser et al, , 1990[Leyser et al, , 1993; Bud'ko and Vas'kov, 1992; Goodman et al, 1993, Tripathi andLiu, 1993;Huang, 1994;Huang and Kuo, 1994]. These interactions involve the decay of a two photon pump into an upshifted UH wave at a frequency fuI-i = 2fp -nfce, a downshifted EB wave at nfce, and a lower hybrid wave.…”

Gyroharmonic properties of the middle-scale artificial ionospheric turbulence during heating of the ionospheric F 2 layer by a high-power O-mode radio wave

“…It is important to emphasize that some of the available data concerning the asymmetry of the characteristics of artificial ionospheric turbulence with respect to the electron gyroharmonic frequency show strong dependence of the observed phenomena on the gyroharmonic number n, and sometimes the effect is of different sign for different n. Unfortunately, the absence of systematic studies of different characteristics of plasma turbulence as functions of ∆f for different harmonic numbers does not permit one to formulate empirical models of different components of turbulence and to determine the reason for the minimum intensity of artificial F -spread (and, therefore, the minimum intensity of artificial middle-scale inhomogeneities of the plasma density) for ∆f ≈ −20 kHz. It can be assumed that such sort of effects should be determined by the gyroharmonic properties of the upper-hybrid and Bernstein plasma oscillations during interaction of a high-power O-mode radio wave with plasma [32,33]. Clarifying the reasons for the observed phenomena requires additional data on their properties for different harmonics of the electron gyrogrequency as functions of the magnitude and sign of detuning from nf ce .…”

Experimental Studies of the Effects Observed During the Nonlinear Interaction of Two High-Power Radio Waves in a Magnetoplasma

“…It is believed that the BUM-2 component is formed in the region of the upper-hybrid resonance and that its generation mechanism is determined by the four-wave interaction involving two quanta of the electromagnetic wave, one quantum of the upper-hybrid wave, and one quantum of the Bernstein wave. Transformation of the upper-hybrid wave to the electromagnetic wave yields the BUM-2 emission [30]. It should also be noted that, as is shown in the experiments with additional heating [29], the BUM generation can possibly be influenced by suprathermal electrons accelerated in regions with strong plasma turbulence which is excited near the level of the resonance interaction between the high-power radio waves and the plasma.…”