Data obtained on consecutive orbits of the topside sounder Alouette enable contour maps of ƒχF2, in which local time is independent of longitude, to be constructed over North America and the North Atlantic Ocean. They (along with data from individual orbits) reveal that at night pronounced minimums of ƒχF2 exist which are narrow in latitude and extend in a magnetic east‐west direction. On these maps, the minimums of ƒχF2 surround some point on the earth's surface in the region of the magnetic pole. The average position of the most southerly minimum in the vicinity of 75°W geographic longitude moves southward from about 62°N geographic latitude at 14 hours local time to a minimum latitude of about 45°N at local midnight and then northward to about 49°N at 07 hours local time. The location of this minimum varies with magnetic activity, and during magnetic disturbances it can occur 10 degrees or more south of its undisturbed location.

4bstract. Some of the traces obtained from the topside sounder ionogrnms recorded at low tl~udes are identified as being caused by propagation along magnetic field-aligned sheets of 1000zation. The electron-density distribution along the magnetic field line passing through thẽ tellite obtained from one such trace is compared with the vertical electron-density distribu-tI~n obtained from the regular topside F-Iayer traces. Close agreement of these two is obtamed, indicating not only that propagation occurs along field-aligned sheets of ionization but also that radio waves propagated along the field lines are reflected very near the verticalincidence reflection level. A study of traces resulting from propagation along field-aligned sheets of ionization reveals that the electron-density gradient perpendicular to the magnetic field of one particular sheet was approximately 4 times greater than that in the regular ionosphere. The half-thickness of this sheet was approximately 0.6 km, and the maximum electron density in the sheet was estimated to have been 1 per cent above the background ionization. Radio energy that propagates at oblique incidence can become trapped by a field-aligned sheet of ionization. Thus a combination of obliquely incident propagation followed by propagatioñ l~ng field-aligned sheets of ionization can occur. The energy propagates along the sheet until It IS reflected and then retnrns to the satellite along almost the same path. Improvements to an existing explanation of spread F at equatorial latitudes are suggested on the basis of this type of propagation. Radio propagation along a model field-aligned sheet of ionization is investigated by ray-tracing techniques. A ray travels back and forth across a field line, the distance decreasing between consecutive crossings until the ray becomes reflected. In a particular case investigated the ray was reflected 1.5 km above the vertically incident reflection level.

Ionograms recorded with ionospheric sounders aboard rockets and satellites show signals (resonances) which can persist from a fraction of a millisecond to many milliseconds after the termination of the transmitted pulse. Many of the characteristics of the resonances at the plasma frequency ƒN, the upper‐hybrid frequency ƒT, the harmonic gyrofrequencies nƒB, where n ≥ 2, and the maximum frequencies of the Bernstein modes ƒQn can be explained by propagating electrostatic waves. At frequencies near ƒN, ƒT, and nƒB, electrostatic waves of slightly different frequencies generated by the transmitted pulse propagate in the ionospheric plasma, become reflected at distances up to several hundred meters away from the satellite, and return to the satellite, producing a continuous receiver response following the transmitted pulse. The resonance observed at the gyrofrequency ƒB is not yet understood. Nonlinear properties of the receiving system and/or the plasma can result in resonances observed at the sum and difference frequencies of the principal resonances. Other resoncance phenomena are also discussed.

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.

In an ionospheric modification experiment at Arecibo in July 1976 it was discovered that the height of the enhanced plasma line due to photoelectrons does not agree with the height of the enhanced plasma line due to the HF heating wave. The properties of the Barker decoder, used in the experiment, indicate that the photoelectron enhanced plasma line occurs at the height expected by theory for a uniformly varying ionosphere, whereas the observed HF enhanced plasma line occurs a few kilometers above this height. The Langmuir waves responsible for the observed HF plasma line at Arecibo probably exist near the largest or first maximum of the Airy function, which describes the standing HF electric field. This is about 200 m below the heater wave reflection height. This observation requires that the Langmuir waves responsible for the HF‐induced plasma line be generated in ionization irregularities and subsequently propagate in the irregularities to the appropriate plasma frequency for detection by the Arecibo radar.