Stimulated electromagnetic emissions and anomalous HF wave absorption near electron gyroharmonics

Abstract

Simultaneous observations are presented of stimulated electromagnetic emission (SEE) spectra and the anomalous absorption of a diagnostic HF wave, caused by powerful radio waves generated by the Troms0 heating facility. The pump frequency was varied in small steps around the third, fourth, and fifth harmonic of the electron gyrofrequency. Systematic variations with pump frequency were observed in the anomalous absorption data and the entirety of SEE properties. The anomalous absorption exhibits a pronounced minimum at a certain pump frequency near (or at) the gyroharmonic in question. Of the SEE properties the strength and discreteness of the downshifted maximum and the width of the downshifted continuum minimize together with the anomalous absorption. Other SEE properties (namely the broad upshifted maximum, the downshifted peak, and the broad symmetrical structure) exist only in well-defined narrow pump frequency ranges near the absorption minimum. Significant quantitative and qualitative differences are found between the SEE properties around the third gyroharmonic on the one hand, and the fourth and fifth gyroharmonics on the other hand. frequency. There is no other constraint on the pump frequency tee. It has been found, however, that the character of the SEE spectra changes strongly when re0 approaches an integer multiple of the electron gyrofrequency, re e • nil e [Leyser et al., 1990]. Whereas some of the SEE features become particularly strong or weak when tOo is changed in small steps (--•2,r x 10 kHz) around nile, others exist only when the condition tee -• nil e is met. This experimental finding most naturally points to the importance of electron Bernstein waves for the SEE phenomenon. High-frequency electrostatic waves propagating perpendicular to the external magnetic field are thought to be generated by linear mode conversion of the electromagnetic pump wave at small-scale field-aligned density irregularities, so-called striations [MjOlhus, 1990, and references •herein]. The same striations are also thought to be responsible for the anomalous absorption imposed upon an O-mode wave prop-Copyfight 1994 by the American Geophysical Union. Paper number 94JA00023. 0148-0227/94/94JA-00023505.00 agating through an ionospheric volume affected by the pump wave [Graham and Fejer, 1976]. Naturally, the pump itself is also subject to this anomalous absorption, giving rise to a self-absorption effect. The phenomenon of anomalous HF wave absorption (or wideband attenuation, as it was called previously) has been discovered in ionospheric modification experiments performed near Boulder, Colorado [e.g., Utlaut and Vialette, 1974]. This phenomenon has also been the research subject in a series of experiments conducted at TromsO in which several new features were found [Stubbe et al., 1982; Kapka et al., 1982; Jones et al., 1983, 1984]. In these earlier experiments, systematic pump frequency changes in small steps around nil e were not made, and simultaneous SEE experiments not performed.

Ion gyro‐harmonic structuring in the stimulated radiation spectrum and optical emissions during electron gyro‐harmonic heating

“…3f ce Ä f 0 < 3f ce + 100kHz [Stubbe et al, 1994;Hussein et al, 1998]. The offset frequency of the DP decreases from 1636 Hz to 1350 Hz as f 0 is increased from 4.25 MHz to 4.27 MHz which is consistent with the previous studies that f DP decreases as pump frequency increases toward 3f ce [Huang and Kuo, 1995].…”

Numerical simulation studies on the broad upshifted maximum of ionospheric stimulated electromagnetic emission

“…The BUM is one of the most prominent upshifted features in the SEE spectrum and has been the subject of intensive investigation in past ionospheric modification experiments [Leyser et al, 1989[Leyser et al, , 1993Stubbe et al, 1994;Frolov et al, 1996Frolov et al, , 1997Frolov et al, , 1998Wagner et al, 1999]. The BUM is a broad upshifted sideband extending more than 100 kHz above the pump frequency and develops only when the pump frequency w0 is in the vicinity of harmonics of the electron cyclotron frequency, nf•½e.…”

Spectral features of stimulated electromagnetic emission, measured in the 4.3–9.5 MHz pump wave frequency range

“…We can also note here the broad symmetrical structure (BSS) which is observed up to date only when fo -3f•, and the downshifted peak (DP), which is a well developed feature for n: 3; only weak signs of a DP have been seen for n = 4 and 5, and none for n: 6 Stubbe et al, 1994].…”

“…shifted maximum (BUM), occurring in the SEE spectra in a frequency range of about A f + _ 15-150 kHz when the PW frequency is close to an electron cyclotron harmonic frequency, n fee (in our experiments, fee • 1.35 MHz, and n = 4-7 is the harmonic mode number) [Leyser et al, 1990[Leyser et al, , 1993Stubbe et al, 1994;Frolov et al, 1996Frolov et al, , 1998], and the broad upshifted structure (BUS), which is a wideband emission generated in a frequency range of about Af + • 15-100 kHz when f0 is somewhat higher than nf• [Frolov et al, 2000].…”

“…According to Stubbe et al, [1994], all SEE components may be divided into "gyrofeatures", which exist only when fo -n f•e, and "universal features", which exist for all pump frequencies, but may strongly change their properties if fo -nfce. The major universal features are the spectral continua (NC, continuum, and BC), the DM, and the UM, whereas the major gyrofeatures are the BUM, DP, …”