Structure and dynamics of the winter polar cap F region

Abstract

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.

Polar patches observed by ESR and their possible origin in the cusp region

“…A comprehensive review of the current understanding of the patch phenomenon has been given by Crowley (1996). Patches occur predominantly under conditions of IMF Bz southward and drift across the polar cap at speeds of typically 300± 1000 ms )1 (Buchau et al, 1983). They have been observed in summer and winter (Buchau and Reinisch, 1991) and have been seen to form in geomagnetically conjugate regions (Rodger et al, 1994a).…”

Simulation of off‐great circle HF propagation effects due to the presence of patches and arcs of enhanced electron density within the polar cap ionosphere

“…On any particular occasion, the actual flow patterns may differ significantly from those in the illustration. When the IMF is directed northwards (Bz > 0), the principal large-scale electron density structures within the polar cap ionosphere are Sun-aligned arcs (plasma striations extending for thousands of kilometers in the transpolar noon-midnight direction, but much narrower with scales around 100 km in the dawn-dusk direction [Buchau et al, 1983]). A series of arcs drifting steadily across the polar cap from dawn to dusk would lead to the expectation that increasing bearing swings would be observed during the time sector 1800 to 0600 LT, with the largest swings expected in the midnight sector.…”

“…During periods of southward directed Interplanetary Magnetic Field (IMF) (Bz < 0) and the associated high levels of geomagnetic activity, patches of plasma 100-1000 km across with electron density enhancements of up to a factor of 10 above the background densities have been observed in the high-latitude F region ionosphere. These drift antisunwards across the central polar cap at velocities of a few kilometers per second in the high latitude convection current flows Buchau et al, 1983]. When geomagnetic activity is low and the IMF is directed northward (Bz > 0) (approximately 50% of the time), Sun-Earth aligned arcs of plasma with electron density enhancements of a factor of 2-3 above the background can occur which drift across the polar cap at velocities of a few hundred meters per second, generally in a dusk-ward direction [Buchau et al, 1983].…”

“…These drift antisunwards across the central polar cap at velocities of a few kilometers per second in the high latitude convection current flows Buchau et al, 1983]. When geomagnetic activity is low and the IMF is directed northward (Bz > 0) (approximately 50% of the time), Sun-Earth aligned arcs of plasma with electron density enhancements of a factor of 2-3 above the background can occur which drift across the polar cap at velocities of a few hundred meters per second, generally in a dusk-ward direction [Buchau et al, 1983]. The plasma striations are elongated for thousands of kilometers in the transpolar noon-midnight direction but are much narrower (around 100 km) in the dawndusk direction.…”

Reconfiguration of polar-cap plasma in the magnetic midnight sector

“…Of relevance to the current study are density enhancements on horizontal scales of ∼100 s to 1000 s km in the F-region, known as polar patches. These patches were studied first using ionospheric sounders and airglow emissions (Buchau et al, 1983;Weber et al, 1984), but they have since been investigated by a variety of experimental techniques, including radio scintillation (Buchau et al, 1985), incoherent scatter radar (Pedersen et al, 1998), HF radar (Ogawa et al, 1998) and radio tomography (Walker et al, 1999). However, comparisons of features observed by different techniques are not Correspondence to: S. E. Pryse (sep@aber.ac.uk) always straight-forward, with different techniques using different criteria to define a patch.…”