LOCALIZATION OF A TYPE III RADIO BURST OBSERVED BY THE STEREO SPACECRAFT

Ray tracing calculations show that (1) emissions from a localized source escape as direct and reflected waves along different paths, (2) the reflected waves experience higher attenuation and group delay because they travel longer path lengths in regions of reduced refractive index, and (3) widely separated spacecraft A and B can detect the direct as well as reflected emissions escaping along different directions. It is proposed that the source of a radio burst observed by twin spacecraft A and B can be localized if at a given frequency the emission at one of them is identified as the direct emission and is identified at the other as the reflected emission by comparing the observed time delays Delta T, as well as intensity ratios I-B/I-A with the corresponding values of the direct and reflected emissions obtained for a given coronal model. A type III event observed by the STEREO spacecraft A and B shows that its characteristics are consistent with direct and reflected emissions by being less intense and delayed at A in comparison to that at B. By applying the proposed technique to this event, the location of its source is found to lie between the turning point of the ray and the harmonic layer corresponding to f(pe) = f/2, where f and f(pe) are the frequency of the emission and the electron plasma frequency, respectively. The comparisons of the widths of the fundamental and harmonic emission cones with the angular separation of spacecraft A and B indicate that the mode of the observed emission is probably the harmonic.