Pulsed radio soundings of the topside of the ionosphere in the presence of spreadF

Ionosphere Explorer I Satellite: First Observations from the Fixed-Frequency Topside Sounder

“…The second, labeled "combination mode ducted echoes," consists of rather diffuse echoes which converge with the normal echoes. These two configurations can be explained by the same mechanisms proposed to explain similar echoes observed by the second topside sounder rocket and by Alouette I (3,9). Direct-ducted echoes occur when the satellite actually passes through the ducting irregularity.…”

Topside sounding of the earth's ionosphere

“…[43] A series of experiments was conducted by two rockets flown from the range at Wallops Island in the U.S.A., carrying topside sounders operating at single frequencies near 4 and 6 MHz as shown in Table I. [34,35] These very successful experiments demonstrated the feasibility of topside sounding during both quiet and moderately disturbed ionospheric conditions. They also obtained the first evidence of plasma [42]).…”

“…An early rocket test of conditions in the topside of the ionosphere yielded very strong ducted echoes [35]. Their strength was due to the very low spatial attenuation losses for ducted propagation.…”

SpreadFover the polar cap on a quiet day

“…Although the causes of spread-F echoes are not well understood, they are generally believed to occur when the F region cannot be considered uniformly stratified but when it contains embedded irregularities such as 'clouds,' 'blobs,' or 'columns' of electrons of higher concentration tha• is found in the surrounding regions. The irregularities seem to exist not only below but aJso above the maximum of the electron concentration of the F region, as rocket and satellite flights have indicated [Seddon, 1960;Knecht and Russell, 1962]. Furthermore, field-aligned irregularities in the F region are believed to be the main source of the scatter phenomenon.…”