Ground‐based optical and digital ionosonde measurements were conducted at Thule, Greenland to measure ionospheric structure and dynamics in the nighttime polar cap F layer. These observations showed the existence of large‐scale (800–1000 km) plasma patches drifting in the antisunward direction during a moderately disturbed (Kp ≥ 4) period. Simultaneous Dynamics Explorer (DE‐B) low‐altitude plasma instrument (LAPI) measurements show that these patches with peak densities of ∼106 el cm−3 are not locally produced by structured particle precipitation. The LAPI measurements show a uniform precipitation of polar rain electrons over the polar cap. The combined measurements provide a comprehensive description of patch structure and dynamics. They are produced near or equatorward of the dayside auroral zone and convect across the polar cap in the antisunward direction. Gradients within the large scale, drifting patches are subject to structuring by convective instabilities. UHF scintillation and spaced receiver measurements are used to map the resulting irregularity distribution within the patches.

All‐sky imaging photometer measurements and ionospheric soundings taken at Thule, Greenland (86° corrected geomagnetic latitude), in December 1979 and January 1982 reveal the large‐scale organization of the winter polar cap ionosphere. Three groups of forms have been identified: (1) The most predominant features are sun‐aligned, generally unstructured, subvisual, F region arcs, extending for more than 1000 km (limit of all‐sky field of view) across the polar cap. In general, these arcs drift from dawn to dusk at speeds between 100 and 250 m s−1; however, stagnation of arc drift and drift reversals have been observed. The arcs are produced by soft particle precipitation. (2) During a magnetically disturbed period the arcs disappeared, and large patches of enhanced F region ionization drifted at speeds of 250 to 700 m s−1 across the field of view in the antisunward direction. Although arcs are produced by soft particle precipitation, preliminary results from the Dynamics Explorer satellite do not indicate any localized soft electron precipitation into the patches. (3) On a few occasions both forms were observed simultaneously. Between F region sun‐aligned arcs, which were drifting from dawn to dusk, small patches of ionization were observed moving at much higher speed in the antisunward direction. Both the arcs and the patches appear as strong localized irregularities in the ionospheric soundings. The Doppler information provided by the aircraft's Digisonde 128PS was used to relate backscatter traces to individual arcs or patches and to track individual features over more than 1500 km. For a large number of observations the time variation of measured Doppler velocities suggests specular reflection from electron density enhancements associated with the optical forms, rather than scatter from field‐aligned irregularities imbedded in the arcs or patches. A simple velocity filter applied to the ionogram data permitted the generation of range‐time characteristics for selected ionization drift velocities. A tendency for high velocities to occur during magnetically active periods was found. The ionospheric soundings showed that the F region arcs are bands of enhanced ionization, imbedded in a background ionosphere with a base height (h′F) of approximately 250 km and a critical frequency of approximately 4 MHz (∼2 × 105 el cm−3). The virtual heights in the ionograms did not change during transit of the arcs through the zenith. During the active periods, when the antisunward moving patches were observed, the background ionization dropped to less than 3 MHz (105 el cm−3) while the base height (h′F) moved up to heights above 400 km. The strongly ionized patches (f0 F2 > 8 MHz), however, were observed to reach a minimum virtual range of ∼250 km during the zenith transit, leading during their passage through the zenith to rapid h′F fluctuations of the order of 200 km within minutes.

Coordinated measurements of F region plasma patches were conducted on February 3/4, 1984, from Thule and Sondrestrom, Greenland. Optical, ionosonde, amplitude scintillation, total electron content (TEC), and incoherent scatter radar measurements were combined to reveal several new aspects of the structure and transport of these localized regions of enhanced F region ionization. For the first time these patches were directly tracked flowing in the antisunward direction over distances of 3000 km from the center of the polar cap to the poleward edge of the auroral oval. Quantitative measurements of TEC show increases of 10–15 TEC units within the patches, above a background polar cap value of 5 TEC units. Amplitude scintillation measurements show the presence of ionospheric irregularities through the entire patch, with a weak indication of stronger scintillation on the trailing (or E × B unstable) edge.

An investigation of the polar cap ionosphere near the peak of the last solar cycle identified polar cap F layer arcs and ionization patches as unique features of the polar cap ionosphere, and as sources of severe scintillations observed on 250‐MHz satellite beacon signals. The continuing investigations in January and December 1983 and January 1984 have shown that arcs and patches persist as the dominant features of the winter polar cap ionosphere during periods of low sunspot numbers. Improved ionospheric soundings made at Thule, Greenland (86°CGL), showed a clear diurnal variation for the occurrence of the patch‐type ionization. Discussion of various possible mechanisms producing the observed ionization patches leads to the conclusion that the solar produced ionosphere equatorward of the dayside cusp is the source region of the ionization patches. Polar plasma convection transports this ionization across the cusp and the central polar cap. The local time dependence of the occurrence of the patches at Thule is shown to be a manifestation of the well‐known universal time control of the polar cap F region. A strong positive solar cycle dependence of the scintillations was measured during three extended campaigns and confirms earlier measurements. The diurnal variation of scintillations is almost flat at solar maximum and has a local time variation very similar to that of the patch type ionization at solar minimum. Both arcs and patches contribute to substantial scintillations around solar maximum, while only the patches are responsible for the considerably weaker scintillations during solar minimum.

Radio wave and optical experiments were conducted onboard a U.S. Air Force research aircraft in March 1977 and March 1978 at low magnetic latitudes to investigate the effects of F region electron density irregularities on transionospheric communications links. Imaging photometer, ionosonde, and satellite amplitude scintillation measurements were used to monitor the development and motion of F region 6300-/• O I airglow depletions, spread F, and scintillation producing irregularities that are all associated with low-density bubbles in the postsunset equatorial ionosphere. The 6300-/• airglow depletions are the bottomside signatur e of low plasma density within the bubbles. Examples of multiple airglow depletions and their relation to variations in the F layer virtual height (h'F) and to the occurrence of amplitude scintillations on 250-MHz satellite signals are described. Estimates of the average bottomside electron density, from simultaneous ionosonde measurements and 6300-A airglow intensities, show electron density decreases of---66% within the bubbles. These decreases are approximately the same for bubbles observed at the magnetic equator and near Ascension Island (18øS magnetic latitude). The measurements at Ascension Island show that airglow depletions extend away from the magnetic equator into the southern 6300-A intertropical arc. Variations in the maximum poleward extent of airglow depletions and of associated ionospheric irregularities that give rise to amplitude scintillations were observed. These latitudinal variations are interpreted, using field line mapping considerations, as variations in the maximum altitude of plasma bubbles over the magnetic equator. A north-south flight confirms that the overall pattern of airglow depletions and associated ionospheric irregularities extends continuously across the magnetic equator to +_ 15 o magnetic latitude.