radio-induced aurora showed that the enhancement caused by the HF radio waves also remained localized near the field-aligned position. Coherent HF radar backscatter also appeared strongest when the pump beam was pointed field-aligned. These results are similar to some Langmuir turbulence phenomena which also show a strong preference for excitation by HF rays launched in the field-aligned direction. The correlation of the position of largest temperature enhancement with the position of the radio-induced aurora suggests that a common mechanism, upper-hybrid wave turbulence, is responsible for both effects. Why the strongest heating effects occur for HF rays directed along the magnetic field is still unclear, but self-focusing on field-aligned striations is a candidate mechanism, and possibly ionospheric tilts may be important. INDEX TERMS: 2403 Ionosphere: Active experiments; 6929 Radio Science: Ionospheric physics (2409); 7839 Space Plasma Physics: Nonlinear phenomena; KEYWORDS: HF-heating, ionospheric modification, electron heating, EISCAT, striations, aspect angle

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Ionospheric electron heating, optical emissions, and striations induced by powerful HF radio waves at high latitudes: Aspect angle dependence

Optical measurements made at the Arecibo Observatory during the 1987 heating campaign showed large temporal and spatial variations in 630.0-nm airglow enhancements during times of continuous power transmissions of high-power radio waves. Photometric data displayed fluctuations of 60 R or more in the red-line (630.0 nm) emission from atomic oxygen. These fluctuations were associated with heater-induced cavities which drifted and evolved in the modified ionosphere. Data from the Arecibo incoherent scatter radar were used in conjunction with airglow images to provide a physical interpretation of the modification process. Electrons were accelerated by large amplitude Langmuir waves excited by parametric decay instabilities occurring near the wave reflection points inside the density cavities. Inelastic collisions with oxygen atoms produced excited states which yielded enhanced 630.0-nm and 557.7-nm emissions. A numerical model has been used to relate the enhanced airglow intensities to the energy spectrum of the accelerated electrons. The measured airglow could have been produced by an isotropic source at 340 km altitude that accelerated 0.01% of the ambient electrons into a suprathermal Maxwellian distribution with a temperature of 2.05 eV. Experimental and theoretical studies suggest that airglow clouds were directly coupled to plasma density cavities because (1) these cavities trapped themore » HF radio beam and (2) electrons accelerated out into regions of reduced plasma concentration were less effectively thermalized and, consequently, were more effective for collisional excitation of neutral species.« less

Airglow enhancements associated with plasma cavities formed during Ionospheric Heating Experiments

High-power electromagnetic waves beamed into the ionosphere from ground-based transmitters illuminate the night sky with enhanced airglow. The recent development of a new intensified, charge coupled-device imager made it possible to record optical emissions during ionospheric heating. Clouds of enhanced airglow are associated with large-scale plasma density cavities that are generated by the heater beam. Trapping and focusing of electromagnetic waves in these cavities produces accelerated electrons that collisionally excite oxygen atoms, which emit light at visible wavelengths. Convection of plasma across magnetic field lines is the primary source for horizontal motion of the cavities and the airglow enhancements. During ionospheric heating experiments, quasi-cyclic formation, convection, dissipation and reappearance of the cavites comprise a major source of long-term variability in plasma densities during ionospheric heating experiments.

https://science.sciencemag.org/content/sci/242/4881/1022.full.pdf

Artificial airglow excited by high-power radio waves.

HF-induced airglow at 630 nm was observed by the Digital All-sky Imager, located near Skibotn in Norway, at F-region altitudes above the EISCAT HF facility near Tromso on 21 February 1999. The transmitter was operated in a 4-min on, 4-min off sequence at 4.04 MHz O-mode with the beam pointing vertically. The airglow reached a peak intensity of about 100 R above background and appeared equatorward of the HF beam’s projection on the reflection altitude, which was obtained from ionograms. Generally, the region of maximum airglow was displaced towards the magnetic field line (zenith angle = 12.8° S) passing through the HF facility. This is a unique feature of these observations. From mid-latitude studies, such airglow is thought to be excited either by electrons energised to several eV by plasma turbulence, or by thermal electron temperature enhancement. Such localisation towards the magnetic field is unexpected for both mechanisms of airglow generation and suggests this feature may be important at high latitudes.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000GL003754

High‐latitude HF‐induced airglow displaced equatorwards of the pump beam

[1] It is well known that strong electron heating by a powerful HF-facility can lead to the formation of electron and ion density perturbations that stretch along the magnetic field line. Those density perturbations can serve as ducts for ELF waves, both of natural and artificial origin. This paper presents the first experimental evidence of plasma modifications associated with ion outflows due to HF heating. The experiments were conducted using the HAARP heater during times that the DEMETER satellite flying at an altitude of approximately 670 km was close to the magnetic zenith of HAARP. The DEMETER satellite has provided in situ measurements of the ion temperature and composition. The experimental results are compared with the numerical model of inter-hemispheric artificial ducts, and it is found that they are in qualitative agreement.

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Formation of artificial ionospheric ducts

High-power hf electromagnetic waves have been used to generate large electron-density cavities in the night-time F-region ionospheric plasma. Previous modification experiments have described induced density perturbations of only a few percent. Here we report the first observations of density depletions exceeding 50%, extending hundreds of kilometers along the geomagnetic field. These cavities are thermally driven, with electron temperatures in the depletions increased by factors of 3 to 4. The depletion dynamics exhibits many of the characteristics associated with thermal cavitons.

J.P. Sheerin

Papers

Observations of ionospheric cavities generated by high-power radio waves.

Spatial and temporal evolution of 630.0 nm airglow enhancement during ionospheric heating experiments