A Monte Carlo simulation on the scattering coefficients of solar radio wave propagation

TL;DR

This paper compares theoretical and numerical models of radio wave scattering in the solar environment, revealing the limits of quasilinear theory and proposing velocity corrections for better accuracy in 3D cases.

Contribution

It provides a detailed comparison between quasilinear diffusion coefficients and ray-tracing simulations in 2D and 3D density fluctuations, introducing a velocity correction for improved modeling.

Findings

Quasilinear theory agrees with simulations under weak scattering.

Strong scattering occurs near the plasma frequency or with large fluctuations.

Velocity correction improves diffusion coefficient accuracy in 3D scenarios.

Abstract

Radio waves undergo scattering by small-scale density fluctuations during propagation through the solar-terrestrial environment, substantially affecting the observed characteristics of solar radio bursts. This scattering process can be effectively modeled as photon diffusion in phase space. In this study, we present a comprehensive comparison between the quasilinear diffusion coefficients and those calculated by ray-tracing the photon trajectories in numerically generated, broadband, isotropic density fluctuation fields in both two-dimensional (2D) and three-dimensional (3D) configurations. The comparative analysis demonstrates that for weak scattering, the simulated diffusion coefficients agree well with the quasilinear theoretical predictions. However, when the radio frequency approaches the electron plasma frequency and/or the density fluctuation amplitude becomes significant, photons experience strong scattering. Under such conditions, the quasilinear theory tends to underestimate the scattering strength of photons induced by 2D density fluctuations while overestimating the scattering strength in 3D cases. Furthermore, we implement a group velocity correction to the theoretical diffusion coefficients, based on the effective propagation speed averaged over all test photons. The corrected coefficients provide an accurate quantification of the scattering strength for radio waves propagating through 3D density fluctuations. The physical mechanisms underlying these phenomena are elucidated in the discussion.