Controlled Ionospheric Preconditioning and Stimulated Electromagnetic Radiation

Abstract

New results of stimulated electromagnetic emissions (SEE) from the HIPAS Observatory are reported. A novel hf heating sequence was used to first precondition the ionosphere, and SEE was then excited with low-amplitude test pulses. Through this approach, the nonlinear physics of SEE was studied. The correlation between small-scale field-aligned density striations and SEE generation was demonstrated, and SEE was excited at power density of 24 dB less than normally required. The results compare well with theoretical predictions of SEE generation via trapped upper hybrid oscillations decay and cavitation within striations. [S0031-9007(98)06204-8] PACS numbers: 94.20.Tt, 52.35.Mw, 52.35.RaThe ionosphere provides an "outdoor laboratory" [1] ideally suited for the study and excitation of thermal and nonlinear plasma instabilities through high frequency (hf) ionospheric modification experiments. A manifestation of thermal instabilities is the observation of magnetic field-aligned density striations detected via radar scattering [2][3][4] and in situ rocket measurements [5]. A manifestation of nonlinear plasma processes is the observation of stimulated hf radiation from the heated ionosphere, commonly called stimulated electromagnetic emissions (SEE), through frequency analysis of the returned hf signal after its transit through the ionosphere [6][7][8][9][10][11][12][13][14][15][16]. Two of the more robust SEE spectral features observed under high duty cycle hf heating are a broad, diffuse, and down-shifted (with respect to the heating frequency, f hf ) continuum, with frequency extension of tens of kilohertz or more, called the broad continuum; and a discrete, narrow, down-shifted peak, called the downshifted maximum, with an offset frequency near the lower hybrid frequency, f lh [11,12].It has been suggested that these SEE features are associated with the occurrence of heater-induced small-scale (meter-size) striations [17][18][19][20][21] which act as resonators for electrostatic oscillations at the upper hybrid frequency, f uh . In contrast to Langmuir oscillations near the reflection layer where f hf ϳ f p ͑z o ͒ [f p ͑z o ͒ is the plasma frequency at the reflection layer z o ], these oscillations have wave vectors predominantly perpendicular to the ambient magnetic field. Recently, Mjølhus [22,23] proposes a SEE theory including the presence of striations and has calculated the radiation source current spectra. Trapped upper hybrid oscillations or eigenstates, with frequency of f f hf f uh ͑z͒, are first generated via conversion of pump wave off meter-size striations. The parametric decay of these states, with f 0 f hf 2 f dm (f 0 is the decay wave frequency), results in the generation of the down-shifted maximum feature. Optimum growth of this trapped upper hybrid decay instability is predicted to occur when the frequency offset, f dm ͑ f hf 2 f 0 ͒, is near f lh . The resultant radiation source current spectrum contains discrete spectral peaks that can be identified with the down-shifted maximum feature. As the p...

Spatiotemporal evolution of radio wave pump‐induced ionospheric phenomena near the fourth electron gyroharmonic

“…Experimental observations have shown that most of the SEE spectral characteristics depend on the pump frequency ( f 0 ) in relation to the F‐region electron cyclotron frequency ( f ce ) [ Leyser et al , 1989, 1990; Stubbe et al , 1994; Honary et al , 1999; Cheung et al , 1998] and/or F‐region critical frequency [ Leyser et al , 1990; Armstrong et al , 1990]. The first classification of the SEE spectral features was performed by Stubbe et al [1984] who categorized them either as “gyrofeatures” which exist only at f 0 ≈ nf ce , where electron gyroharmonic number n = 3 or greater, such as the up‐shifted maximum (UM), or universal features, which exist at all frequencies except for pumping close to a gyroharmonic, for example, the down‐shifted maximum (DM).…”

Self‐focused radio frequency L wave pumping of localized upper hybrid oscillations in high‐latitude ionospheric plasma

“…L wave pumping of the long predicted localized UH oscillations [ Vas'kov and Gurevich , 1976; Dysthe et al , 1982; Gurevich et al , 1995; Istomin and Leyser , 1997] provides a unified interpretation for a range of experimental results, including pump beam self‐focusing in geomagnetic zenith, annular optical emission regions and equatorward displacement of the excitation region. Localized UH oscillations have previously been detected indirectly in stimulated electromagnetic emissions during controlled ionospheric preconditioning experiments [ Cheung et al , 1998].…”

Competition between Langmuir and Upper-Hybrid Turbulence in a High-Frequency-Pumped Ionosphere

“…It was discovered experimentally already in 1981 that when a narrow-band high-frequency (hf) radio beam of high intensity irradiates the ionosphere at a frequency f 0 which is close to the local ionospheric plasma frequency f p , a structured, wide-band secondary radiation escapes from the interaction region [1], a phenomenon now known as stimulated electromagnetic emission (SEE) [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. By varying the injected hf beam systematically in terms of frequency, intensity, and duty cycle and analyzing the secondary radiation, and hence the associated hf radio beam-excited ionospheric plasma turbulence and wave conversion processes, we have been able to study the competition between the effects due to ponderomotive parametric instabilities (PPI) [22 -24] and those due to thermal parametric instabilities (TPI) [25,26].…”