Density Turbulence and the Angular Broadening of Radio Sources in the Polar and Ecliptic Planes

Abstract:

Density irregularities are responsible for the scattering of radio waves in the solar wind and astrophysical plasmas. These irregularities significantly affect the inferred physical properties of the radio sources, such as size, direction, and intensity. We present here a theory of angular broadening due to the scattering of radio waves by density irregularities that improves the existing formalism used to investigate radio wave scattering in the outer heliosphere (OH) and the very local interstellar medium (VLISM). Our extensions include adding an inner scale and both latitudinal and radial dependences for the density fluctuation spectra, as well as propagation paths for the radiation out of the ecliptic plane. Based on a pickup ion-mediated solar wind model (PUI model) by Zank et al. [2018], we estimate the turbulence and solar wind quantities for the fast polar solar wind. The predictions include the density variance, inner/dissipation scale, velocity correlation length, mean magnetic field, and proton temperature. The density turbulence amplitude is estimated in two ways. A simple scaling is used to extend the theoretical predictions of the PUI model beyond the heliospheric termination shock (HTS). The solar wind and turbulence quantities in the ecliptic plane using plasma and magnetometer data from the Voyager 2 spacecraft over the period 1977 to 2018. Based on the models and observations, we calculate the scattering angle of the radio sources in the polar and ecliptic wind. Finally, we compare the numerical results with the analytic results from Cairns [1995] and Armstrong et al. [2000].