Abstract
Radio waves undergo angular scattering when they propagate through a plasma with fluctuating density. We show how the angular scattering coefficient can be calculated as a function of the frequency spectrum of the local density fluctuations. In the Earth's magnetosheath, the ISEE 1-2 propagation experiment measured the spectral power of the density fluctuations for periods in the range 300 to 1 s, which produce most of the scattering. The resultant local angular scattering coefficient can then be calculated for the first time with realistic density fluctuation spectra, which are neither Gaussian nor power laws. We present results on the variation of the local angular scattering coefficient during two crossings of the dayside magnetosheath, from the quasi-perpendicular bow shock to the magnetopause. For a radio wave at twice the local electron plasma frequency, the scattering coefficient in the major part of the magnetosheath is b(2fp) ≃ 0.5 â 4 Ã 10â9 rad2/m. The scattering coefficient is about ten times stronger in a thin sheet (0.1 to1RE) just downstream of the shock ramp, and close to the magnetopause.
Highlights
Radio waves are strongly scattered by density fluctuations when they propagate at frequencies from one to four times the local electron plasma frequency fp
In situ measurements in the solar wind at 1 AU have shown that the relative density fluctuations rN =N are more intense downstream of interplanetary shocks (Huddleston et al, 1995; see Woo, 1988); rN is the standard deviation of the density N, for fluctuations with periods in the range 10 min to 1 h
With ISEE 1-2, we cannot precisely measure density fluctuations on frequencies larger than mmax ' 1 Hz, the smallest scale length we can reach is li ' V = 2mmax 50 km: the basic assumption for scattering calculations is fulfilled because li is larger than k ' 10 km, the wavelength corresponding to the lowest frequency (30 kHz) recorded on board ISEE 3
Summary
Radio waves are strongly scattered by density fluctuations when they propagate at frequencies from one to four times the local electron plasma frequency fp. Monte-Carlo ray-tracing method, Steinberg and Hoang (1993) calculated the refracted and scattered image of AKR sources, using two different values of the angular scattering coefficient b 2fp 1:6 2 10 9 rad2 =m and b 2fp 4 2 10 9 rad2 =m at twice the local fp , in the flanks of the magnetosheath. They deduced the angular radius of the equivalent source (circular and uniformly bright). Of the refractive index is related to the spectral power of the density fluctuations by PN m
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