Enhancement of phase and intensity scintillation, as a radio line of sight scans through grazing incidence on the local L shell in the nightside diffuse-auroral ionosphere, has been well documented by means of data from the DNA Wideband satellite. In this paper we describe systematic behavior of the phase spectrum found in the enhancement region over Poker Flat, Alaska. Routine Wideband processing included spectral analysis of 20-s (•,, 60-km) segments of VHF and UHF phase records and log linear fits thereto. Tabulation of the resulting power law spectral indices, p, disclosed increased values in the scintillation enhancement region. It has been established that the increase in p is the signature of a physically real phenomenon and not merely an artifact of statistical nonstationarity arising from the narrowness of the scintillation strength enhancement. Moreover, it has been found that p is not increased in strength enhancements occurring close to the magnetic zenith. Indeed, in some cases, it is substantially decreased. A possible source of these unexpected spectral behaviors is size-dependent anisotropy, an idealization of the irregularities responsible being small-scale field-aligned rods imbedded in large-scale shell-aligned sheets. We also present examples of multiple-regime power law spectra, characterized by an increased spectral index at short structure wavelengths (less than a few hundred meters north-south) and, more frequently, by an increased index at large wavelengths (more than a few kilometers). These spectral breaks occur both separately and together and both within and outside the scintillation enhancement region.
Recent scintillation observations have disclosed the existence of sheetlike electron density irregularities aligned along L shells in the auroral zone ionosphere. In this paper we exploit the aspect sensitivity of phase scintillation to identify the dominant three‐dimensional configuration of irregularities in four latitude/time zones: (1) equatorward of the high‐latitude scintillation boundary on the nightside of the earth, (2) poleward of the nightside boundary, (3) equatorward of the boundary on the day side, and (4) poleward of the dayside boundary. We find the sheetlike irregularities to be confined to region 2, with few exceptions. The dominant configuration in the other zones, including zone 4, is that of axially symmetric rodlike irregularities aligned along the magnetic field.
Numerical results are presented which describe the statistics of scattered signals observed with an interferometer. The ratio of nonscattered to scattered flux and the wavefront autocorrelation are related to the distributions and average characteristics of real amplitude product and phase difference. It is shown how a combination of pure amplitude and phase‐dominated information can yield unique solutions for the two wavefront parameters without a priori information concerning them. The treatment is based on assumption of a randomly phased angular spectrum.
Quantitative results for 49 visibility fades observed from College, Alaska, during 1965 are presented. A majority of the fades revealed ionospheric optical depths in excess of unity at 68 MHz. Optical depth is numerically equal to mean‐square fluctuation in radio‐frequency phase across a plane at the base of the scattering region; thus, the fades often were characterized by rms phase deviations in excess of one radian at 68 MHz. The wavefront phase structure immediately after scattering was found to have scales in the range of tens and hundreds of meters. The observations usually were not consistent with the demands of a Gaussian autocorrelation function, as is commonly assumed. Rather, the perturbed wavefronts displayed evidence of quasi‐periodic structure in the observed dimensional range.
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