BISPECTRAL ANALYSIS OF SPACECRAFT RADIO SCINTILLATION

Strong intensity scintillations are characterized by a positively skew ed probability density function. For such non-Gaussian processes, the power spectrum is not a complete statistical description. In this pape r I present the simplest of the higher-order spectra, the bispectrum, of interplanetary intensity scintillation (IPS) data taken in the tran sition between weak and strong scattering. The bispectrum is the Fouri er transform of the third-order lagged product, <x(t)x(t + tau1)x(t tau2)>. As the spectrum gives the contribution to the variance from tw o Fourier components having frequencies that add to zero, the bispectr um gives the contribution to the third moment from the product of thre e Fourier components having frequencies that add to zero. The bispectr a computed here are of the monochromatic S-band beacon onboard the Pio neer Venus spacecraft. This use of a monochromatic point source elimin ates complications in the interpretation caused by filtering. of the t hird moment by finite bandwidth and finite angular source size. The sp ectrum, probability density function (PDF), and bispectrum are related and thus the measurement of all these quantities can be used to check for internal consistency. In this paper this internal consistency che ck is done by analysing these three statistics of IPS intensity data f or moderate strength scintillation.