Stimulated Radiation from Strong Langmuir Turbulence in Ionospheric Modification

Abstract

New measurements of stimulated electromagnetic emissions (SEE) in low duty cycle heating experiments performed at the HIPAS Observatory are presented. Two distinct types of spectra, a weak diffuse type and a stronger type with deep frequency modulations, were observed. These results have been compared with numerical predictions from a 1D driven and damped Zakharov model and are found to be consistent with Langmuir collapse processes (strong spectra) and Langmuir turbulence of the coexistence type (weak spectra). Through this joint experimental and numerical study, evidence of strong Langmuir turbulence via SEE measurements has been demonstrated. [S0031-9007(97)03881-7] PACS numbers: 52.35.Ra, 52.35.Mw, 94.20. -y Two commonly used diagnostics or measurements in high frequency (hf) ionospheric heating experiments are incoherent scatter radar (ISR) and stimulated electromagnetic emissions (SEE). ISR observations yield wave number resolved measurements of hf excited plasma waves (so called plasma lines) while SEE measurements are inherently wave number integrated. Results from the two diagnostics, then, are complementary to one another and are equally important in the understanding of hf induced turbulence in the ionosphere. Of the two types of measurements, ISR observations have received extensive experimental and theoretical attention. Strong Langmuir turbulence effects, characterized by collapsing cavitons and nucleation, predicted from strong Langmuir turbulence models [1-3] have recently been convincingly demonstrated in ISR experiments and have been shown to dominate the early-time evolution of hf induced turbulence [4-7]. SEE observations, on the other hand, have revealed many interesting spectral features of a different nature [8-15]. These features are typically observed during continuous (CW), high duty cycle, long heater pulse experiments [16]; some of them disappear for heater frequencies near multiples of the local electron gyro frequency f ce while others only exist near these frequencies.For a series of reasons, these features are currently associated with the occurrence of heater-induced small scale magnetic field aligned density striations that develop on a longer Ohmic heating time scale (see [17], and references therein). Similar manifestation of ponderomotive effects as those obtained in ISR measurements have not been reported in SEE observations. In this regard, the two diagnostics give apparently inconsistent results.In the present paper, new SEE measurements are reported, which aim at observing manifestations of ponderomotive or strong Langmuir turbulence, comparing them with theoretical predictions, and investigating to what extent the same interpretations can be supported by both SEE and ISR diagnostics. To this end, the experimen-tal method must be so as to exclude long time thermal effects such as the formation of striations. A series of controlled experiments have been conducted at the HIPAS Observatory near Fairbanks, Alaska [18]. These experiments are characterized by operating the heate...

Controlled Ionospheric Preconditioning and Stimulated Electromagnetic Radiation

“…This appears to be demonstrated from the observation of strong overshoot of the continuum feature with H 2 after preconditioning. The highly nonstationary behavior and the associated wideband spectra agree with general features of cavitation [30]. Further experiments and modeling are needed to confirm this point.…”

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

“…We did not find any clear evidence of SEE at f > 0, corresponding to so-called ''free mode'' of Langmuir waves [31,32]. SEE with a spectral shape as described in [12] were observed only when pump pulse edges happened to leak into our Fourier analysis time window.…”

“…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].…”

Ponderomotive narrow continuum (NCp) component in stimulated electromagnetic emission spectra

“…SEE has been extensively investigated at the ionospheric modification facility in Tromsø, Norway [ Stubbe et al , 1984, 1994; Leyser et al , 1990], at several Soviet/Russian facilities [ Boiko et al , 1985; Erukhimov et al , 1987b; Leyser et al , 1993; Frolov , 1995; Frolov et al , 1999a], at the facility in Fairbanks, Alaska [ Armstrong et al , 1990; Cheung et al , 1997], and at the facility in Arecibo, Puerto Rico [ Thidé et al , 1989, 1995]. SEE in its stationary state, typically reached after tens of seconds following the HF pump wave switch‐on, shows a number of distinct spectral features, many of which were named by Stubbe et al [1984].…”