HF‐VHF communications experiment using man‐made field‐aligned ionospheric scatterers

Barry, G. H.

doi:10.1029/rs009i011p01025

Cited by 22 publications

(11 citation statements)

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“…This fading is not well represented in Figure 5 because of the long temporal averages employed for the data presentation. The fading at Arecibo appears to follow a Rayleigh distribution function; this is similar to results obtained at Platteville, Colorado [Barry, 1974]. The observed fading is most likely caused by the random motion of a limited number of "scattering centers" within the HF-modified volume.…”

Section: Experimental Facilitiessupporting

confidence: 78%

Simultaneous measurements of HF‐enhanced plasma waves and artificial field‐aligned irregularities at Arecibo

Noble¹,

Djuth²

1990

J. Geophys. Res.

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Two radar systems with beams intersecting in the HF‐modified F region were used to simultaneously measure HF‐enhanced plasma lines (HFPLs) and artificial geomagnetic field‐aligned irregularities (AFAIs). These measurements were made with the high‐power HF facility located near the Arecibo Observatory, Arecibo, Puerto Rico. The Arecibo 430‐MHz radar was used for the HFPL observations, and a portable 49.92‐MHz backscatter radar was deployed on the island of Guadeloupe to monitor the AFAIs. The experiment was designed to examine the degree to which HF‐induced plasma turbulence influences the development of AFAIs. When the HF beam is stepped up in power, sustained HFPLs and AFAIs are first observed at the same HF power level, indicating that ponderomotively driven instabilities may be involved in the early time development of AFAIs. As the HF power is increased, the HFPL backscatter power begins to saturate at ∼70 MW effective radiated power (ERP). However, the backscatter from AFAIs is linearly dependent on HF power, even at the highest (120 MW ERP) HF power levels available at Arecibo. This suggests that additional processes may contribute to the development of AFAIs. For example, ponderomotively driven instabilities may give rise to weak geomagnetic field‐aligned irregularities that are subsequently driven unstable by processes excited near the upper hybrid resonance. It is also likely that AFAIs greatly impact the development of HF‐induced plasma turbulence at late times (>1 s) following HF turn‐on. Once the ionosphere is preconditioned by high‐power HF modifications, AFAIs and HFPLs can be simultaneously sustained at a much lower HF power level than that needed to originally excite them. The nature of the preconditioning process is currently not well understood. New theoretical initiatives are clearly needed to guide future experimental activity in this area.

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Section: Experimental Facilitiessupporting

confidence: 78%

Simultaneous measurements of HF‐enhanced plasma waves and artificial field‐aligned irregularities at Arecibo

Noble¹,

Djuth²

1990

J. Geophys. Res.

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“…From the point of view of communications or channel characteristics the scattering region may be characterized by a time-delay multipath associated with the physical thickness of the scattering region, a Rayleigh fading characteristic produced by the differential motion of the individual scattering irregularities, and a mean Doppler shift proportional to the radio frequency used. The possibility of utilizing AFAS as a means for communicating over long .distances has been further explored and demonstrated for the Platteville facility by Barry [ 1974].…”

Section: Discussionmentioning

confidence: 99%

Field‐aligned scattering from a heated region of the ionosphere—Observations at HF and VHF

Fialer¹

1974

Radio Science

172

106

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Radio scattering measurements have been made of a region of the ionosphere above the Institute for Telecommunication Sciences' ionospheric heating facility at Platteville, Colorado. Strong scattering was observed at frequencies in the HF and VHF bands. The scattering has been shown to originate from electron density fluctuations that are highly elongated in the direction of the geomagnetic field. Radar scattering cross sections of 70 to 80 dbsm are measured in the HF and low VHF bands. The scattering cross section decreases at a rate of 20 to 30 db per octave at the high end of the VHF band. The scattering irregularities are produced by operation of the heating facility with o‐mode polarization at frequencies that reflect within the ionosphere. The strength of the scattered energy decreases at a rate of at least 10 db per degree for observation geometries that do not match the geometry for specular reflections from long cylinders aligned with the geomagnetic field. The spatial distribution of irregularities has been estimated for both E‐ and F‐region heating. The temporal properties of the scattering have also been determined including the frequency spectrum of the scattered signals. The processes by which ionospheric heating produces these irregularities are not yet fully explained. Several potentially useful applications of radio scattering from such irregularities are discussed.

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“…0148-0227/78/068A-0078501.00 Showen [1975] shows that in the heated region at Arecibo the HFPL's can first be detected within 4 ms and can become intense within tens of milliseconds. This is insufficient time [Barry, 1974] for HF-induced irregularities to grow significantly; hence it appears that natural irregularities must be responsible for the observed HFPL's, at least in their initial stages.…”

Section: Introductionmentioning

confidence: 99%

The role of field‐aligned ionization irregularities in the generation of the HF‐induced plasma line at Arecibo

Muldrew¹

1978

J. Geophys. Res.

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By using the data from the July 1976 ionospheric modification experiments at Arecibo it was discovered that the Langmuir waves responsible for the enhanced plasma line are generated at the height in the ionosphere at which the Airy function, which describes the standing electric field of the high‐frequency (HF) modifying wave near reflection, has its main maximum. It would be impossible for the enhanced Langmuir waves which backscatter the 430‐MHz radar signal to exist at this height in a uniformly varying ionosphere. This implies that underdense ionization irregularities exist in the ionosphere that allow the Langmuir waves to be generated and to propagate to the appropriate ionization density for detection by the incoherent backscatter radar. This discovery puts a new light on the generation mechanism of the HF‐enhanced plasma line, since up until now it has been implicitly assumed in theoretical work that the parametric decay process occurs in a uniformly varying ionosphere, that is, an ionosphere without irregularities. The ionization irregularities responsible for the HF‐enhanced plasma line are probably field aligned with an electron density deviation decrease of about 4% or more and with a diameter between about 1 m and a few hundred meters. It is proposed that Langmuir waves are amplified parametrically as they propagate in the duct to where their wave normals are parallel to the radar beam. The observed frequency displacement from the radar frequency (430 MHz) of the parametric decay line at Arecibo is about fHF ‐ 3.5 kHz, where fHF is the frequency of the HF heating wave (4–10 MHz). Numerical calculations for typical daytime conditions show that the maximum coupling between the HF and Langmuir waves occurs for a frequency offset between the two waves of about 3.4 kHz for the field‐aligned irregularity or duct model, which is in good agreement with observations. The observed frequencies of the peaks in the HF‐enhanced plasma line spectrum, which are displaced from 430 MHz ± fHF by approximately odd harmonics of the ion acoustic or offset frequency, can be explained with the duct model. The ‘broad bump’ observed in some plasma line spectra probably results from cascading of intense Langmuir waves which are produced parametrically and propagate approximately along the axis of the duct. It is proposed that the background or ‘immature’ spectrum may result from a four‐wave decay process. The ‘overshoot’ phenomenon (intense plasma line signals occurring immediately after transmitter turn‐on) may be due to the effects of HF heating on the ducts and on the ambient plasma.

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HF‐VHF communications experiment using man‐made field‐aligned ionospheric scatterers

Cited by 22 publications

References 1 publication

Simultaneous measurements of HF‐enhanced plasma waves and artificial field‐aligned irregularities at Arecibo

Simultaneous measurements of HF‐enhanced plasma waves and artificial field‐aligned irregularities at Arecibo

Field‐aligned scattering from a heated region of the ionosphere—Observations at HF and VHF

The role of field‐aligned ionization irregularities in the generation of the HF‐induced plasma line at Arecibo

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