Dynamic spectra of interplanetary scintillations

Interplanetary scintillations (IPS) are fluctuations in the apparent intensity of radio sources caused by fluctuations in the electron density of the solar wind1. Observations at a single wavelength provide information on the spatial spectrum of electron density at scales of the order of the radius of the first Fresnel zone. However, dynamic spectra covering a wide range of wavelengths show evidence of fluctuations on a much larger scale2. Unfortunately, the problem of estimation of the spatial spectrum at this larger scale is analytically intractable. Here we compare the observations2 with a numerical simulation of the diffraction of plane waves by a two-dimensional random phase screen. The frequency–time structure of the simulated IPS shows good qualitative agreement with the observations when the turbulence spectrum in the phase screen is a simple power law with the Kolmogorov exponent. No ad hoc large-scale structure is necessary to account for the observations.