Airglow enhancements associated with plasma cavities formed during Ionospheric Heating Experiments

Abstract

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 the 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.

Phenomena induced by powerful HF pumping towards magnetic zenith with a frequency near the F-region critical frequency and the third electron gyro harmonic frequency

“…HF-excited airglow enhancements were first reported at 630.0 nm (Sipler and Biondi, 1972) where they are most easily seen, but they can be and are found in HF experiments, due to both thermal and accelerated electrons (Carlson, 2002 suprathermal electrons. These suprathermal electrons produce enhanced optical emissions at 557.7 (Bernhardt et al, 1989;Pedersen et al, 2003), 777.4 (Carlson, 2002Djuth et al, 2005), 844.6 (Gustavsson et al, 2005), 427.8 (Holma et al, 2006), 660.0 (Djuth et al, 1999) which can complement thermal excitation of 630.0 nm optical emissions (Carlson, 2002), and are also seen directly in enhanced incoherent scatter radar plasma lines (Carlson et al, 1982).…”

“…Gurevich et al (2001Gurevich et al ( , 2002 proposed a magnetic zenith mechanism for making a large fraction of the ERP available in the plasma production region, and means for the electron acceleration. Bernhardt et al (1989), Djuth et al (1999) Pedersen et al (2003, Gustavsson et al (2005), Holma et al (2006), Ashrafi et al (2007) have also worked on electron acceleration mechanisms and experimental evidence for their presence. Our work here shows the signatures of the magnetic zenith mechanism in terms of both rapid formation of striations and strong Langmuir turbulence when the plasma-density enhancements were produced.…”

Large airglow enhancements produced via wave‐plasma interactions in sporadic E

“…Additionally, detailed simulations of the airglow excitation process and the source electron distribution function required to produce the observed emissions are currently underway. The methodology that is being adopted for the simulations is similar to that described by Bernhardt et al [ 1989].…”

Low‐Frequency Waves in HF Heating of the Ionosphere

“…The nonlinear interaction among the wave modes with the pump wave also leads to secondary electromagnetic radiation in the VLF, ELF, and ULF bands at much lower frequencies [Barr, 1998]. A pump wave excited process at the highest frequencies leads to emissions at optical f requencies, observed in HF pumpenhanced airglow [Bernhardt et al, 1989].…”

“…These experiments inject high-frequency (HF) radio waves using high-power transmitters and an array of ground-and space-based diagnostics. The heating of the plasma has led to many new phenomena such as modulation of the ionospheric current system and generation of lowf requency electromagnetic radiation [Papadopoulos et al, 1989;2011a, b;Stubbe, 1996], stimulated emissions [Leyser, 2001], plasma waves and turbulence [Guzdar et al, 2000], small-scale irregularities or striations [Mishin et al, 2004], and pump-induced optical processes [Bernhardt et al, 1989;Pedersen et al, 2009]. Among the ionospheric heating facilities worldwide, the High Frequency Active Auroral Research Program (HAARP) located at Gakona, Alaska, has the highest power HF transmitter and a c omprehensive set of diagnostics.…”