Monte Carlo Simulation of Directivity of Interplanetary Radio Bursts

Abstract

We have developed a Monte Carlo simulation code to study the effects of refraction due to spatial variation of the solar wind density and scattering due to random density fluctuations on directivities, time profiles, and sizes and positions of the apparent sources of the interplanetary type II and type III radio bursts excited at the fundamental (F) and second harmonic (H) of the electron plasma frequency, fpe. We have focused on the 120 kHz fundamental and harmonic emissions with sources at the heliocentric distances of 0.2097 AU (~115 kHz plasma level), and 0.3875 AU (~60 kHz plasma level), respectively, and computed the distributions of trajectories of traced rays in a refracting, as well as a refracting and scattering, medium. These distributions show that (1) the scattering by random density fluctuations extends the visibilities of F and H components from ~18° to ~90°, and from ~80° to ~150°, respectively; (2) the time profiles constructed using the dispersion of the arrival times of the scattered rays at any given frequency may contain two peaks corresponding to F and H emissions, or a single peak consisting of both F and H components (fundamental followed by the harmonic), or a single H peak depending on the speed of the electron beam and the observer's location; and (3) the scattering broadens the sizes of the fundamental and harmonic point sources to ~25° and ~37°, and by elevating their apparent radial distances to f/3 and f/2 levels, respectively. We also present simultaneous observations of a type II and a couple of type III radio bursts by Ulysses and Wind spacecraft separated by more than 100° and show that the widely visible radio bursts correspond either to a scattered fundamental, to a refracted or scattered harmonic, or to a mixture of scattered fundamental and harmonic emissions.