A Study of "Spread?F" Ionospheric Echoes at Night at Brisbane. I. Range Spreading (Experimental)

Abstract

SummaryAt frequencies well below the critical frequency, satellite echoes sometimes accompany the night-time F 2 echo, sometimes clearly separated, sometimes overlapping_ In an investigation of these range multiplets, in addition to routine P'f sounding records, continuous virtual range measurements at fixed frequency (at stations of various separations), and measurements of mean intensities, phase-path changes, and directions of arrival, have been carried out_ From a study of the results, certain simple relationships emerge, as first approximations, between the various quantities measured, namely: virtual ranges (group paths), phase-path change, zenith angle, azimuth, intensity, and the time. A parameter, having the dimensions of velocity, appearing in these relationships, has values of which 80 per cent. lie in the range 240 ± 140 km/hr, with directions of which 80 per cent. lie in the range 290 ± 60°. The mean duration of a satellite is 50 min. Satellites occur more frequently in winter than in summer and show a somewhat complex diurnal variation, which is described in detail, and which tends to recur from year to year. It is associated with the general night-time changes in equivalent height of the layer. Strongly reflecting Es shows an inverse diurnal variation. No correlation with geomagnetic disturbance indices could be found.

MU-Radar Recorded Field-Aligned Irregularities in the F2 Region and Associated Sporadic-E Disturbances.

“…The F2-layer frontal structures (bands) reported by Behnke (1979) from a northern-hemisphere mid-latitude location are moving in south-west directions. As travel speeds and structure spacings are consistent with those reported by McNicol et al (1956) and Bowman (1960aBowman ( , 1991b, it is possible that these are moving spread-F structures. However Behnke suggests "that these bands are manifestations of the so-called 'Perkins' instability" (Perkins, 1973).…”

“…It will be seen later that these times are appropriate for ES-layer disturbances propagating from the other stations. No spread-F is recorded in the first event but resolved frequency spread (McNicol et al, 1956;Bowman, 1991c) is seen to accompany the second event. The duplicate trace displaced approximately 100 km in range from the main trace is associated with some other disturbance.…”

A review of some recent work on mid-latitude spread-F occurrence as detected by ionosondes.

“…However, what is less well known is the fact that similar (although not nearly as pronounced) height rises occur with associated spread-F, at other latitude regions (MCNICoL et al, 1956;BOWMAN, 1960bBOWMAN, , 1978. Even in high-latitude regions spread-F occurrence is found to be related to higher h'F and lower foF2 values (see Fig.…”

“…However, evidence for the presence of large-scale structures at the times of spread-F occurrence has been available from the 1950s onwards, thus giving support to the concept that much larger structures are involved (ROY and VERMA,1953;UYEDA and OGATA,1954;BOWMAN, 1960aBOWMAN, , 1968aBOWMAN, , 1981KLEMPERER, 1963;KING, 1970). In addition MCNICOL et al (1956), using a set of spaced stations, detected 87 TIDs (speeds 41 to 97 ms-1) travelling generally in north-west directions and intimately associated with mid-latitude spread-F occurrence. Despite this experimental evidence, the original ideas involving a scattering mechanism became well-established in the minds of researchers, so that it is only in more recent times that large-scale structures have had reasonable support as the primary cause of the spread-F traces (see e.g., BOOKER, 1979;BOOKER et al, 1986;RASTOGI, 1989).…”

“…Furthermore, STERLING et al (1971) have reported wavelike perturbations (periodicity approximately 20 minutes) in the isoionic contours during the postsunset height rises which precede equatorial spread-F occurrence. This paper will consider not only macroscale structure changes as they apply to medium-scale TIDs which MCNICoL et al (1956) associated with mid-latitude spread-F occurrence, but also as they apply to large-scale TIDs (FRANCIS, 1975) which also at times (see e.g., BOWMAN, 1978) appear to be associated with spread-F occurrence.…”