Type III solar radio bursts, which appear as characteristic frequency-drifting features on radio dynamic spectra, are generated by energetic beams of electrons originating in solar active regions and propagating through the solar corona and interplanetary medium. The early low frequency spacecraft observations of interplanetary type III bursts prompted efforts to locate and track the observed radio source regions through the interplanetary medium. Without imaging capabilities on interplanetary spacecraft, a number of different methods were devised and used to obtain direct or indirect information on the locations of the radio sources in the interplanetary medium. The most direct among these were direction-finding techniques based on subtle differences and variations in the spacecraft antenna responses. The culmination of this effort led to a radio visualization of the global spiral structure of the interplanetary magnetic field topology (Fainberg et al. 1972) predicted by the Parker model of the solar wind (Parker, 1958). However, from a statistical analysis of a large number of type III radio bursts, Steinberg et al.1984 concluded that the direction-finding measurements from the ISEE-3 spacecraft suggested the existence of anomalous propagation effects associated with the type III burst radiation, which could severely limit the ability of direction-finding techniques to accurately locate the intrinsic radio sources. A number of models were subsequently developed to provide quantitative insights and understanding of how the effects of interplanetary scattering of the type III radiation could alter the intrinsic source locations (e.g. Thejappa et al. 2007; Kontar et al. 2019, Chen et al. 2023).
Figure 1. (Left) Dynamic spectral plot of a type III radio burst on November 17, 2010 observed by the Wind spacecraft. The observed in-situ Langmuir Waves (LWs) signal the arrival of the electron exciter beam at Wind at 8.52 UT. The white curve is the simultaneous fit to the observed frequency drift and to the derived exciter beam kinematics. (Right) The distance-time behavior deduced for the corresponding underlying exciter beam showing the passage of the exciter beam through the STEREO A-Wind triangulated source locations at the times of the observed radiation at Wind and the arrival time of the corresponding local radiation and in-situ LWs at the Wind spacecraft at 1 AU. (details in Reiner 2026)
A recent article (Reiner 2026) revisits the potential problems associated with locating the type III radio sources in the interplanetary medium using direction-finding and triangulation techniques. The basic idea was to use the underlying exciter beam kinematics, deduced from simultaneous fit to both the remote and in-situ radio and plasma wave observations, as an independent diagnostic of the intrinsic radio source locations. These unique analyses were applied to two type III bursts, observed 10 hours apart, identified as originating from the same solar active region and for which STEREO A and Wind triangulation measurements were able to track the observed source centroids of the type III radiation, generated as the underlying exciter beam propagated along the Parker spirals through the interplanetary medium from the flare site. A choice of kinematics of the underlying exciter beam was identified that provided a near perfect fit to the observed frequency drift (shown in Figure 1), whose slope at every point, over the entire distance from the Sun to 1 AU, is proportional to the instantaneous exciter speed. The fit reproduces the arrival times of the exciter beam at the triangulated source locations in the interplanetary medium, which were found to be relatively far from the Sun, and the observed signal of the arrival of the electron beam exciters at the Wind spacecraft at 1 AU, as clearly identified by the observed local radiation and in-situ Langmuir waves, without the need to introduce anomalous propagation effects.
Conclusions
The results of these analyses, deduced from the underlying exciter beam kinematics, suggest that direction-finding and triangulation measurements made from spacecraft, properly aligned relative to the type III radio source regions, can provide information on the locations of the intrinsic radio source regions. There is no evidence that the intrinsic radio sources are significantly displaced from the triangulated positions due to propagation effects, assuming either fundamental or harmonic radiation. The source locations, deduced from the observed radiation characteristics for the two type III bursts analyzed, suggest either that the radiation is generated in high-density regions of the interplanetary medium (3xSaito1977), if the leading edge radiation is at the fundamental of the plasma frequency, or that the radiation of the type III is produced entirely at the harmonic of the plasma frequency, and that the exciter beam must decelerate significantly as it propagates through the interplanetary medium.
Based on the recent paper by M. J. Reiner “Interplanetary Type III radio Emission Locations determined from the Underlying Electron Exciter Beam Kinematics”, The Astrophysical Journal, 1001,39, (2026) https://doi.org/10.3847/1538–4357/ae4ecf.
References
Chen, Xingyao, et al. 2023, Astronomy and Astrophysics, 680, 43
Fainberg, J., Evans, L. G., & Stone, R. G. 1972, Science, 178, 740
Kontar, Eduard P., et al. 2019, The Astrophysical Journal, 884, 122
Parker, E. N. 1958, The Astrophysical Journal, 128, 664
Steinberg, J. L., Hoang, S., Lecacheux, A., Auber, M. G., & Dulk, G. A. 1984, A&A, 140, 39
Thejappa, G., MacDowall, R. J. & Kaiser, M. L. 2007, The Astrophysical Journal, 671, 894