On the radiophysics and geophysics of ionogram spread F

Abstract

We consider the interpretation of "spread F" ionograms, particularly those echoes that determine most of the radio bandwidth of the phenomenon. We compare expectations based on total internal reflection with theoretical descriptions based on underdense scattering. We conclude that "Rayleigh, .... Bragg," and "diffuse multiplerefractive" radio scattering theories are not consistent with observed properties of these echoes. The total-reflection interpretation is shown to be consistent with rocket and satellite data. Evidence for multiple refractive scatter is found, but it neither greatly nor subtly extends the observed radio bandwidth. Ionograms therefore indicate comprehensively the range of plasma densities within view of the ionosonde, even in conditions of spread F. Using digital ionosonde observations, much can be learned of the spatial structures of plasma density responsible for spread F at auroral and equatorial latitudes. As an example, we suggest how contemporary theory and simulations of the equatorial "bubble" phenomenon may be reconciled with the distinctive equatorial spread F pattern.

Short‐term delays (hours) of ionospheric spread F occurrence at a range of latitudes, following geomagnetic activity

“…As discussed in section 1, the recent observations relating to spread F ionogram traces for equatorial regions [Wright et al, 1996;Sales et al, 1996] are consistent with the hypothesis proposed for midlatitude regions [Bowman, 1990;Alimov et al, 1994] and also for equatorial regions [Bowman, 1995] that the spread results from off-vertical specular reflections. Further investigations into other similarities between these regions would thus seem appropriate, particularly as the AM equatorial spread F occurrence has the same sunspot-cycle and annual variations as found for midlatitudes [Bowman, 1993a].…”

“…More recent reports have essentially confirmed the existence in equatorial regions of wavelike isoionic contours [Flaherty et al, 1996;Sales et al, 1996, Weber et al, 1996. Recent analyses also give support to the hypothesis that most of the ESF traces on ionograms are produced by off-vertical specular reflections [Wright et al, 1996;Sales et al, 1996].…”

A tutorial on the basic principles of microwave reflectometry applied to fluctuation measurements in fusion plasmas

“…These early measurements also indicated the presence of irregularities in the ionosphere indicated by amplitude fading of the reflected waves. [2][3][4] Out of these observations evolved a branch of statistical optics concerned with the physics of amplitude scintillations (fluctuations) for waves passing through irregular media. [5][6][7][8][9] One of the motivations for making use of waves reflected from plasma is to take advantage of the sensitivity of the probe beam to fluctuations near the turning point or zero in the refractive index.…”

<i>Letter to the Editor:</i> SCIPION, a new flexible ionospheric sounder in Senegal

“…On the upper part of figure 2, the vertical ionogram which is shown presents both frequency and range spreading. The interpretation of these features is still a matter of discussion (Wright et al, 1996, Lauer et al, 1996. In our example, echoes resulting from total reflection are still visible with a characteristic curvature of the range versus frequency pattern in the vicinity of the critical frequency at 8.4 MHz.…”