Observations of stimulated electromagnetic emissions in ionospheric heating experiments

Abstract

Induced sideband modes, due to stimulated electromagnetic emissions in the ionosphere, have been observed around the frequency of a high‐intensity HF radio wave transmitted from the Heating ionospheric modification facility near Tromsø, Norway. The new observational technique used amounts to analyzing the received HF spectrum directly without utilizing any diagnostic radar. It appears that we have in this way been able to identify the parameteric decay instability and, possibly, the stimulated Brillouin backscattering of the strong HF pump wave. A series of narrow spectral peaks induced in the upper sideband by Heating transmission in extraordinary mode may indicate the excitation of ion Bernstein modes. At times, very strong spectral features, upshifted from the 5.4‐MHz pump by typically 20–45 kHz, are observed. These features are still lacking a convincing theoretical explanation. By monitoring a few fixed frequencies in both the lower and the upper sidebands of the ionospherically reflected modifying wave, we have found that the intensity of the stimulated emissions exhibits a conspicuous long‐term as well as short‐term variability.

Stimulated electromagnetic emission polarization under different polarizations of pump waves

“…It was shown that the intensity of the DM component is greater under O-mode pumping. This result is consistent with previous results by Thidé et al, 1983. In addition, under X-mode pumping, the intensity and number of observed SEE features is less then under O-mode pumping.…”

“…The utilization of the different pump wave polarizations leads only to a change in the efficiency of the generation of the SEE that was found during the early phase of SEE investigations (Thidé et al, 1983). The predominant O polarization of DM was preserved in both cases of the pump wave polarizations used.…”

“…Even in cases when no SEE diagnostics were used, the results bear on the physics of the ongoing processes in the present experiments (e.g. Thidé et al, 1983;Robinson et al, 1997). In this paper we present the results from a heating campaign utilizing the SPEAR (Space Plasma Exploration by Active Radar, Robinson et al, 2006) heating facility (located near Longyearbyen, Svalbard), which transmits circular polarization, and a radio interferometer capable of recording the polarization of the received radiation (located in Barentsburg, Svalbard).…”

Geophysical phenomena during an ionospheric modification experiment at Tromsø, Norway

“…If the pump frequency f H is not extremely close to an electron gyroharmonic, there are strong SEE signals downshifted from f H with a strongly pronounced maximum downshifted by the lower hybrid frequency f LH . Such downshifted maximum, f DM f H A f LH , in the SEE spectra is well known (Thide , 1983). It is common knowledge (Leyser et al, 1989) that the lower hybrid frequency is approximately 8 kHz in the ionosphere above Tromsù.…”

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

“…The behavior of the BUM relative to the presence or absence of striations is more ambiguous. The BUM appears continuously over a range of pump frequencies starting at -10 kHz below the gyroharmonic frequency [Frolov et al, 1996] and continuing to a maximum frequency (fpmx) --100 kHz above the gyroharmonic frequency [Thid• et al, 1983;Stubbe et al, 1984Stubbe et al, , 1994Leyser et al, 1989Leyser et al, , 1990Leyser et al, , 1993Frolov et al, 1996]. Over this range of pump frequencies, striations are present everywhere except for a narrow range of frequencies surrounding the gyroharmonic frequency.…”