Spectral behavior of phase scintillation in the nighttime auroral region

Authors

  • E. J. Fremouw,

  • J. A. Secan,

  • J. M. Lansinger


Abstract

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.

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