Theoretical model for long time stimulated electromagnetic emission generation in ionospheric radio modification experiments

Abstract

Abstract. Experimental evidence points toward magnetic-field-aligned small-scale plasma density striations as responsible for the long time features of stimulated electromagnetic emissions (SEE) in ionospheric radio modification experiments. This paper emphasizes that such striations act as resonators for the generation of trapped upper hybrid oscillations. The sidebands observed in long time SEE experiments are associated with nonstationary behavior, such as parametric instabilities, of these trapped oscillations. A theoretical model for this is formulated and studied numerically. Source spectra for SEE generation, consistent with the main properties of observed downshifted maximum and broad continuum features, will be presented. The first is associated with a trapped oscillations decay instability, while the latter is associated with cavitation in the pre-formed striations.

An interferometer experiment to explore the aspect angle dependence of stimulated electromagnetic emission spectra

“…Such turbulence can consist of either Langmuir and ion-acoustic waves (Langmuir turbulence) or upper and lower hybrid waves (upper-hybrid turbulence). Previous studies, based on SEE, anomalous absorption, and electron temperature measurements, indicate that there is a minimum in the level of upper-hybrid excitation at the gyroharmonic frequencies (Stubbe et al, 1994;Robinson et al, 1996;Frolov et al, 1998Frolov et al, , 2001Mjølhus, 1998). More recently, both airglow and coherent backscatter radar signals have been shown to vanish when the pump frequency is tuned to the third gyroharmonic (Kosch et al, 2002a), supporting the role of upper-hybrid turbulence as the source of the energetic electrons.…”

“…However, it is well known that upper-hybrid turbulence, occurring at the upper-hybrid level several km below the critical height, is important for the production of plasma density striations and SEE. The SEE features, such as the downshifted maximum and broad continuum, may be signatures of cascading and cavitating upper-hybrid turbulence in the same way that the cascade and broad ISR spectra are the signatures of cascading and cavitating Langmuir turbulence (Mjølhus, 1998). Upper-hybrid turbulence is almost certainly, perhaps in conjunction with Langmuir turbulence, involved in the production of the energetic electrons which produce RF-induced airglow (Gustavsson et al, 2002;Kosch et al, 2002aKosch et al, , 2004Istomin and Leyser, 2003).…”

“…The BUM 1 has been linked to Langmuir turbulence (Frolov and Nedzvedski, 2004;Frolov et al, 2004 1 ), and the pump-beam-following motion is consistent with the high-pump-power fourth-gyroharmonic (approximately 5.423 MHz) optical observations, also linked to Langmuir turbulence. By contrast the DM is linked to upper hybrid turbulence (Mjølhus, 1998;Leyser, 2001). The angles calculated from the present data show that both the BUM 1 and the DM may be equally affected by the HF beam direction change, although the DM appears to reside at higher elevation angle than the BUM 1 .…”

<i>Letter to the Editor</i> UHF electromagnetic emission stimulated by HF pumping of the ionosphere

“…Certainly, the conversion of the plasma modes, which is responsible for the HF SEE generation (Grach, 1985;Karashtin et al, 1986;Mjolhus, 1998;Grach et al, 1998), cannot provide the radiation at the frequencies exceeding the Langmuir frequency f P e ∼ f 0 by two orders. Then, in order to attribute the signal observed to scattering of the Earth's black-body radiation, with the temperature ∼ 300 K off the pump-made plasma density irregularities, with the scales ∼ c/f ∼ 0.5 m (here, c is the speed of light), a relative irregularity intensity dN 2 ∼ 3 · 10 −5 is necessary.…”

“…The instabilities lead to the development of different HF electrostatic plasma waves and LF disturbances (Hagfors et al, 1983;Showen et al, 1987;Minkoff et al, 1974). In their turn, the HF plasma waves generate the HF stimulated electromagnetic emissions (HF SEE) (Thide et al, 1982;Leyser et al, 1992Leyser et al, , 1993Karashtin et al, 1986), occurring as a result of conversion of the plasma modes into electromagnetic waves (Grach, 1985;Mjolhus, 1998;Grach et al, 1998), and accelerate electrons until energies 5-30 eV (Carlson et al, 1982). The electron acceleration causes an airglow enhancement (Sipler et al, 1974;Haslett and Megill, 1974;Bernhardt et al, 1991) and additional ionization (Vas'kov et al, 1981;Grach et al, 1997) of the ionospheric plasma due to excitation of neutral particles by an electron impact; the LF Correspondence to: S. M. Grach (sg@nirfi.sci-nnov.ru) disturbances cause scattering (Fialer, 1974) and anomalous absorption of radio waves .…”