Bispectral analysis of spacecraft radio scintillation*

Abstract

Strong intensity scintillations are characterized by a positively skewed probability density function. For such non-Gaussian processes, the power spectrum is not a complete statistical description. In this paper I present the simplest of the higher-order spectra, the bispectrum, of interplanetary intensity scintillation (IPS) data taken in the transition between weak and strong scattering. The bispectrum is the Fourier transform of the third-order lagged product, ⟨x(t)x(t + τ 1)x(t + τ 2)⟩. As the spectrum gives the contribution to the variance from two Fourier components having frequencies that add to zero, the bispectrum gives the contribution to the third moment from the product of three Fourier components having frequencies that add to zero. The bispectra computed here are of the monochromatic S-band beacon onboard the Pioneer Venus spacecraft. This use of a monochromatic point source eliminates complications in the interpretation caused by filtering,of the third moment by finite bandwidth and finite ...