Anisotropic Radio-Wave Scattering and the Interpretation of Solar Radio Emission Observations

TL;DR

This paper presents a 3D stochastic model of radio-wave propagation in the anisotropic turbulent solar corona, explaining observed properties of solar radio bursts and emphasizing the role of perpendicular density fluctuations.

Contribution

It introduces a novel 3D stochastic framework for radio-wave transport in anisotropic plasma, linking propagation effects to observed solar radio burst characteristics.

Findings

Perpendicular density fluctuations with an anisotropy factor ~0.3 are needed.

Photon isotropization occurs near primary emission regions.

Large-scale refraction focuses waves, affecting emission directivity.

Abstract

The observed properties (i.e., source size, source position, time duration, decay time) of solar radio emission produced through plasma processes near the local plasma frequency, and hence the interpretation of solar radio bursts, are strongly influenced by propagation effects in the inhomogeneous turbulent solar corona. In this work, a 3D stochastic description of the propagation process is presented, based on the Fokker-Planck and Langevin equations of radio-wave transport in a medium containing anisotropic electron density fluctuations. Using a numerical treatment based on this model, we investigate the characteristic source sizes and burst decay times for Type III solar radio bursts. Comparison of the simulations with the observations of solar radio bursts shows that predominantly perpendicular density fluctuations in the solar corona are required, with an anisotropy factor $\sim 0.3$ for sources observed at around 30~MHz. The simulations also demonstrate that the photons are isotropized near the region of primary emission, but the waves are then focused by large-scale refraction, leading to plasma radio emission directivity that is characterized by a half-width-half-maximum of about 40~degrees near 30~MHz. The results are applicable to various solar radio bursts produced via plasma emission.