A Study of "Spread?F" Ionospheric Echoes at Night at Brisbane. II. Interpretation of Range Spreading

Abstract

SummaryA review is given of various hypotheses which might explain the behavioU1' of "range multiplets ", in which satellite echoes accompany the F. echo at frequencies well < below the critical penetration frequency. It is shown that these cannot arise from stratification of the layer. Some few may be due to reflection from the F layer of waves previously scattered upwards by an Es irregularity. The great majority, however, appear to originate in reflections from irregularities in the F layer itself. This is confirmed by the directional observations, multiple. hop echo records, correlations between separated stations, and the usual shape of the range-t~e curves, as described in Part I.Evidence is presented indicating irregularities with fronts up to several hundred kilometres long, with curvatu,res ranging up to 10-0 km-1• The irregularities appear to take various forms, of which the simplest are (a) an inverted trough, around 100-200 km wide, (b) a gap, of roughly the same width, with reflecting surfaces which have, at some points, curvatures up to at least 10 km-l, (0) a step, with height and breadth not exceeding some 10-20 km, again with edges which are strongly curved in parts. Each irregularity probably persists for a period of the order of 1-2 hr and is usually propagated through the F region with a nearly un~~orm velocity of the order of 200 kID/hr. The balance of the evidence indicates that this is not related to any general drift of the region.

Ionospheric motions observed with high-frequency back-scatter sounders

“…The type of irregularities responsible for backscatter ripples is evidently the same type as those investigated by Munro [1948Munro [ , 1949Munro [ , 1950Munro [ , 1953, Munro and Heisler [1956 a, b], Price [1954], andMcNicol andWebster [1956] in Australia, Beynon [1948), Bramley and Ross [1951], and Bramley [1953] in England, and Pierce andMimno [1940), andToman [1955] in the United States. Most of Munro's results indicate a velocity of 300 to 600 lilll/hr.…”

Coupled electrodynamics in the nighttime midlatitude ionosphere

“…Fronts and Electrical Coupling. The term front, used to describe TIDs and spread F [e.g., McNicol and Webster, 1956;Bowman, 1968] and Es [e.g., Bowman, 1960;Goodwin, 1966], refers to an identifiable feature (e.g., isodensity contours) that is essentially linear over large geographical distances. McNicol and Webster [1956] have described spread F in terms of fronts that extended several hundred kilometers.…”

Seasonal and solar cycle variations of spread F at the equatorial anomaly crest zone.

“…Generally speaking, F layer spread echo starts to appear in the sonograms after sunset and diminishes quickly around sunrise. According to their diffusion characteristics on the vertical incidence ionogram, spread F echoes are usually classified into two types: frequency spread F and range spread F (McNIcOL et al, 1956). If the diffusion is pronounced along the section of the trace that sweeps upward and makes the critical frequency of F2 layer difficult to estimate, the spread F is classified as 'frequency spread F'.…”