Introduction to ionospheric heating at Tromsø—I. Experimental overview

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

HF heater-produced artificial airglow emissions at 630.0 nm were detected at the High Frequency Active Auroral Research Program (HAARP) ionospheric research facility near Gakona, Alaska (62.39° N 145.15° W), for the first time in March 1999. HF excitation was identified by virtue of two measurements: A region of emissions enhanced 50-60 R above the background of ∼100 R was observed in the approximate region illuminated by the HF heater beam; and the rise and decay of the airglow followed the HF transmitter on/off cycle with time constants of approximately 80 s and 50 s, respectively. The observations were made in close proximity to the natural aurora, which gradually moved southward over the site during the experiment and eventually overwhelmed the much weaker artificial airglow. Significant structure in the airglow region was observed, including an overall equatorward displacement relative to the nominal transmitter beam, preferential occurrence of intensity maxima along the overhead field line up to and including magnetic zenith, and periods of elongation of the airglow region along the magnetic meridian and later parallel to the encroaching auroral zone. We discuss potential sources of this structuring of the emission shape including auroral plasma transport and neutral winds. Transmitter frequencies of 3.1–3.3 MHz matched the ionospheric plasma frequency near the F region peak at high altitudes of 300–350 km. We identify the primary mechanism behind excitation of the 630.0 nm airglow as thermal electron excitation on the basis of this evidence, along with thermal balance arguments, further substantiated by the observed asymmetry in rise and decay times and a lack of detectable emissions at 557.7 nm.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000RS002399

First observations of HF heater‐produced airglow at the High Frequency Active Auroral Research Program facility: Thermal excitation and spatial structuring

Airglow enhancement observations have been considered as supporting evidence of electron acceleration in ionosphere heating experiments by high-power HF waves. Here we analyze some of the 6300-A airglow data from the Platteville, Colorado, heating experiments of 1970, employing new electron impact excitation rates for the O(1D) state and empirical, but in accord with experimental and theoretical constraints, plasma heating rates and show that these airglow enhancements should be attributed to excitation by thermal electrons. An important aspect of the present analysis is the excellent agreement of the observed and the calculated airglow enhancements over several complete transmitter on/off cycles of several minutes duration and an increasing airglow trend of 1 hour duration. The fact that the OI red line may be thermally excited and the scarcity of observations of simultaneous OI red and green line enhancements imply that electron acceleration, even to a few eV, may require very special experimental and ionospheric conditions that are not very often realized.

Reinterpretation of the 6300‐Å airglow enhancements observed in ionosphere heating experiments based on analysis of Platteville, Colorado, data

Under current solar minimum conditions at Arecibo, Puerto Rico, large (∼1000–2000 K) enhancements in electron temperature are observed in the winter, nighttime ionosphere when high-power 3-MHz radio waves reflect near the F region peak. Computational modeling has been performed to determine the cause of the unusually large temperature enhancements. The results indicate that low collisional cooling rates combined with strong thermal conduction along geomagnetic field lines play a key role in elevating F region temperatures and spreading the electron temperature enhancements far outside of the region where radio wave energy is deposited. Thus, the dramatically large temperature enhancements are attributable primarily to expected low cooling rates of the electron gas, rather than unexpectedly high heating rates.

H. C. Carlson

Papers

Thermal response of the F-region ionosphere for conditions of large HF-induced electron-temperature enhancements