Multispacecraft analysis and modeling of type III radio burst exciter deceleration in inhomogeneous heliospheric plasma F. Azzolini et al. – Community of European Solar Radio Astronomers

Electron beams accelerated in solar flares and escaping from the Sun along open magnetic field lines can trigger intense radio emissions known as type III solar radio bursts. However, the propagation dynamics of these electrons are still poorly understood and observationally limited (Dulk et al 1987, Krupar et al. 2015). Historically, measurements from a single spacecraft have suggested deceleration, but suffered from large uncertainties due to the unknown position of the source. By taking advantage of four spacecraft located at different heliospheric longitudes, a multi-perspective view can be obtained that allows them to locate the sources of the explosions.

Using simultaneous observations from four spacecraft, Parker Solar Probe (PSP), STEREO-A, Solar Orbiter and Wind, Azzollini and Kontar, 2025 determine the positions, velocities, and accelerations of the Type III burst exciters, correcting for the angular separations of the source and spacecraft (see Figure 1 and Figure 2).

Figure 1: Maximum type III gust fluxes measured by four different spacecraft (left) and spacecraft positions (right) in HEE coordinates during July 11, 2020. (2:30 TU) event. The direction of maximum directivity is found. wearing the maximum streams of STEREO-A, PSP, Wind and SolO at 979 kHz.

Figure 2: Dynamical (left) and time-frequency (right) spectra on July 11, 2020 by the PSP, STEREO-A, and SolO spacecraft (top to bottom). For each spacecraft, peak flux frequencies (and fit) are plotted on the right for times: frequencies selected by the green dashed box, which contains peak flux points (green “X” symbols), along with their fitted curve (green dashed line), while fitted emitter positions as a function of time and normalized residuals of the fit are shown on the right. The blue and red lines correspond, respectively, to the Fundamental and Harmonic components.

Analysis of a series of type III bursts reveals that exciter velocities decrease with heliocentric distance approximately as ($u(r)( \propto r^{-0.37 \pm 0.14}$ ), while acceleration decreases as ( $a(r) \propto r^{-1.71 \pm 0.20} $).

To interpret these results, a simple gas dynamic model was used (see simulations by Kontar, 2001) which describes the electron beam as a plateau distribution that interacts resonantly with Langmuir waves in a plasma whose density decreases radially. The model predicts a velocity scale and an acceleration scale, in remarkable agreement with observations. This agreement supports the hypothesis that plasma density gradients govern beam deceleration by modifying the Langmuir wave spectrum and inducing momentum loss through wave refraction and damping.

Interestingly, observations also show differences in velocity and acceleration of the same type III using dynamical spectra observed by different spacecraft. We suggest that the difference could be related to the additional time delay caused by radio wave scattering between the spacecraft and the source.

Conclusion

This study advances our understanding of solar type III radio bursts by combining observational data from multiple spacecraft with theoretical models. The findings demonstrate the critical role of plasma density inhomogeneity in shaping the dynamics of electron beams and their associated radio emissions. By addressing the limitations of single spacecraft observations and providing a solid theoretical framework, the study lays the foundation for future research into the complex interactions between solar energetic particles and heliospheric plasma.

Additional information: The nugget is based on the recently published paper by F. Azzollini and EP Kontar, 2025, The Astrophysical Journal, 989 118, DOI: 10.3847/1538-4357/adee22

References

Dulk, G.A., Goldman, M.V., Steinberg, J.L., and Hoang, S. 1987, A&A, 173, 366

Kontar, E.P. 2001, Solar Physics, 202, 131

Krupar, V., et al. 2015, A&A, 580, A137