On the nature of equatorial spreadF

Abstract

Ionospheric propagation via scattering from the F region was sought at 50 Mc/s over a transequatorial path (with midpoint near Huancayo, Peru) employing a 2580‐km transmitter‐receiver separation. Propagation via F scatter was present over this path about 10 per cent of the time, though only at night. A condition closely related to the occurrence of F scatter was the presence of equatorial spread‐F configurations on the Huancayo ionograms. The height of the propagation medium supporting this F scatter was usually identifiable with the lowest height of the associated equatorial spread F on the ionograms. On the basis of an interpretation of experimental results, equatorial spread F is shown to arise from scattering by relatively thin sheets of irregularities in electron density which occur at the bottom of the F layer or as much as 100 km lower. The thickness of these patches is estimated at the order of 50 km. The scattering irregularities comprising these patches are found to be elongated along the earth's magnetic lines of force. Electron irregularities of scale 10 meters or smaller measured in at least one dimension transverse to the magnetic field lines, and 1000 meters or longer measured along the magnetic field lines, are shown to exist as high as 450 km in the nocturnal equatorial F region during equatorial spread‐F conditions. Equatorial spread‐F echoes observed at Huancayo are demonstrated to be arriving at all elevation angles in the magnetic east‐west plane. The geographical extension of a given scattering sheet in the magnetic east‐west direction was at times as great as 1000 km. An observational procedure is suggested for distinguishing two fundamental varieties of spread‐F echoes appearing on the equatorial ionograms. A necessary condition, that the contours of mean electron density be parallel to the magnetic lines of force, is proposed as a controlling factor for the occurrence of spread F in the equatorial ionosphere.

Satellite traces, range spread-F occurrence, and gravity wave propagation at the southern anomaly crest

“…The equatorial belt between magnetic latitudes 20 • N and 20 • S is a well defined region where spread-F is a normal occurrence and has been described and studied in depth since the 1950s (Wright, 1959;Cohen and Bowles, 1961;Balsley et al, 1972;Aarons, 1978;Fejer et al, 1979;Argo et al, 1986;Mendillo et al, 1992;Whalen, 2000;McNamara et al, 2008). This is because, while the post-sunset rise (PSSR) of the equatorial F layer is well correlated climatologically with the equatorial spread-F (ESF) (Fejer et al, 1999;Hysell and Burcham, 2002), the day-to-day variability of ESF remains a long-standing enigma.…”

Equatorial bubbles observed at the north and south anomaly crests: Dependence on season, local time, and dip latitude

“…The signatures employed are the diffuse echoes that were first described in equatorial ionograms by Booker and Wells [1938]. These echoes were identified as range spread F (RSF) resulting from field-aligned irregularities by Cohen and Bowles [1961], who named them equatorial spread F. Weber et al [1978] found these echoes to be associated with bubbles as identified by their coincidence with depletions of airglow at 6300 •.…”

An equatorial bubble: Its evolution observed in relation to bottomside spread F and to the Appleton anomaly

“…The effect of the enhanced electron density is to create errors in the Global Positioning System and radar location and also to interfere with HF propagation. Both bubbles and BSSF produce a signature in vertical ionograms that was named equatorial spread F by Cohen and Bowles [1961], a term that unfortunately has come to be equated with bubbles. This work will describe this signature as range spread F (RSF) and will distinguish the bubble from BSSF by determining that the bubble extends to much higher latitude hence altitude and is confined to a narrow longitudinal extent as it drifts eastward.…”