Interplanetary solar radio type III bursts provide the means to remotely study and track energetic electrons propagating in the interplanetary medium. Due to the lack of direct radio source imaging, several methods have been developed to determine the source positions from space-based observations. Moreover, none of the methods consider the propagation effects of anisotropic radio-wave scattering, which would strongly distort the trajectory of radio waves, delay their arrival times, and affect their apparent characteristics (Kontar et al. 2019, 2023, Musset et al. 2021, Chen et al. 2023).
In this study, we apply the intensity fit and timing methods to investigate the source positions and directivity of an interplanetary type III burst simultaneously observed by Parker Solar Probe, Solar Orbiter, STEREO, and Wind. While the accurate identification of the true source position proves challenging through observations, ray-tracing simulations of radio-wave propagation provide an intrinsic source position. By tracking the apparent source positions after radio waves undergo scattering, it becomes compelling to employ both methods to analyze simulated time profiles and compare the deduced source position with the intrinsic and apparent positions.
Figure 1. Source positions deduced from (a)observations and (b)simulations using both the intensity fit and timing method. The intensity fit method provides direction information, and the heliocentric distance (plus signs) is derived from the density model. Adapted from Chen et al, A&A 2023
Figure 2. Heliocentric distances (upper panel) and longitudes (lower panel) of the source are deduced from observations of four spacecraft (obs) and simulations (sim) of radio wave propagation for anisotropic scattering effects. Adapted from Chen et al, A&A 2023
The source positions at each frequency obtained from observations and simulations are shown in Figure 1 and Figure 2. From observations, the radio source’s longitudes deduced from the intensity fit are comparable to the longitudes from the direction-finding measurement and the radio sources follow a rather straightforward trajectory. The radio source is found to be located further away with a decreasing frequency but did not follow a specific direction through timing method. The radial distances deduced from observations exceed the values predicted by density models, indicating alignment with radio-wave propagation affected by anisotropic scattering, which could lead to an apparent position at a heliocentric distance farther from the Sun. From radio-wave propagation simulations, the direction deduced from the intensity fit is close to that of the apparent source, which deviates from an angle from the given intrinsic source and seems to be closely aligned with the Parker spiral magnetic field. However, the heliocentric distances and longitudes determined from the timing method do not match the apparent sources, suggesting that the source positions may be underestimated.
Based on the recent paper by Xingyao Chen, Eduard P. Kontar, Nicolina Chrysaphi, Peijin Zhang, Vratislav Krupar, Sophie Musset, Milan Maksimovic, Natasha L. S. Jeffrey, Francesco Azzollini, and Antonio Vecchio, Source positions of an interplanetary type III radio burst and anisotropic radio-wave scattering, A&A 680, A1 (2023), DOI: 10.1051/0004-6361/202347185
References
Kontar, E. P., Chen, X., Chrysaphi, N., et al., 2019, ApJ, 884, 122
Musset, S., Maksimovic, M., Kontar, E., et al., 2021, A&A, 656, A34
Chen, X., Kontar, E. P., Clarkson, D. L., & Chrysaphi, N. 2023, MNRAS, 520, 3117
Kontar, E. P., Emslie, A. G., Clarkson, D. L., et al., 2023 ApJ, 956, 112
