Faraday rotation and signal dispersion: The geometrical optics approximation,an exact solution,and first order smoothing theory

We critically discuss the three approximations which have been employed to estimate the influence of interstellar fluctuations in both electron density and magnetic field on Faraday rotation measure and signal dispersion measure in the radio band. We demonstrate that: (i) the geometrical optics approximation employed by Ginzburg and Eruhimov (1971) relies on the unproven assertion thatall ray paths are essentially the same as the geometrical distance between source and observer; (ii) the exact solution is physically meaningless; (iii) the purported proof of Ginzburg and Eruhimov that earlier work by Lerche is in error, is itself in error and also self-contradictory; (iv) the first order smoothing theory employed earlier by Lerche gives the exact correction to the phase of the ordered field provided only that theirreducible part of a three-point correlation function is negligible.Receipt delayed by postal strike in Great Britain.