Multi-spacecraft Radio Observations Trace the Heliospheric Magnetic Field by D. L. Clarkson et al.
Solar flares accelerate energetic electrons that escape into interplanetary space, guided by the Parker spiral magnetic field, and are responsible for the generation of the interplanetary Type III solar radio bursts. With multiple spacecraft now in orbit around the Sun (see e.g. Musset et al 2021), we are in a unique position of observing the propagation of radio emission through the heliosphere from multiple vantage points.
Recent study by Clarkson et al (https://www.nature.com/articles/s41598-025-95270-w) demonstrate that the magnetic field not only guides the emitting electrons, but also directs radio waves via anisotropic scattering from density irregularities in the magnetised plasma of the interplanetary space.
**Figure 1.** _Overview of a type III burst observed by four spacecraft. (a) Dynamic spectra. (b) Time profiles at four frequencies with intensity scaled to 1 au. (c) Intensity peaks from panel (b) and directivity fitting. (d) Longitudes of the fitted maximum intensity. The symbols show the spacecraft positions in the heliosphere._
To study this effect across large distances in the heliosphere, we use observations of 20 Type III bursts between ~0.9-0.2 MHz from Parker Solar Probe, Solar Orbiter, STEREO-A and WIND spacecraft that were distributed around the Sun. Figure 1 shows an example event where the same burst is observed by spacecraft separated by ~180 degrees, with the brightest intensity observed by Solar Orbiter. The intensity distribution is considered through fitting the peak fluxes at each spacecraft (scaled to 1 au) for a given frequency with the equation (see e.g. Musset et al 2021),
\\[I_{sc} = I_0\exp{\left(-\frac{1-\cos{(\theta_{sc}-\theta_0)}}{\Delta\mu}\right)},\\]
where $\theta_0$ gives the angle of maximum fitted intensity, and $\Delta\mu$ describes the width of the directivity pattern (Figure 1c). In each event studied, we find an eastward longitudinal shift of the fitted peak intensity, and characterize the deviation by $\Delta\theta=\theta_0 – \theta_0(0.9\,\mathrm{MHz})$. Figure 2 shows that between 0.9-0.2 MHz, the peak has shifted by -30 degrees on average. If one assumes that the radiation propagates scatter-free with any angular deviation only produced via the emitter motion along the Parker spiral, then such a scenario would require an unrealistically slow solar wind speed of ~50 km s$^{-1}$, inconsistent with in-situ measurements.
**Figure 2.** _Deviation in longitude of the fitted peak intensity from that at 0.9 MHz. (a) Fits to observational data (grey). The red lines show the average and standard deviation. The blue and green bands show the deviation due to the scatter-free cases. (b) Simulation data using different turbulence scaling factors._
To reproduce the strong variation of 1 au flux with heliocentric longitude, we include a Heliospheric magnetic field model in the form of a Parker spiral to the simulation framework described by _Kontar et al. 2023_, with a spread in solar wind speeds between 340-420 km s$^{-1}$ and a scaling of the turbulence amplitude $(0.5-2)\times$. The resulting photons are scattered across a wide range of heliocentric longitudes, yet there is a distinct channelling along the field direction (Figure 3). We find that for typical Heliospheric conditions, the simulation results reproduce the average ~30 degree deviation between 0.9-0.2 MHz (Figure 2b) with a spread of ~10 degrees depending on the solar wind speed or turbulence conditions.
**Figure 3.** _Polar plots of the time-averaged simulated photon propagation in the heliosphere for (a) a fundamental emitter (blue star) and (b) a harmonic emitter (green star). The coloured histograms show the photon positions with the average wavevector at a given location shown by the black arrows. The inset shows the approximate directivity at a distance where the scattering rate is significantly lower._
**Conclusion**
Assuming that the magnetic field guides only the emitting electrons whilst the radiation is weakly scattered cannot explain the directivity pattern in multi-spacecraft observations without invoking a much steeper curvature of the Parker spiral. We demonstrate that the emitted radio _waves_ are also guided along the interplanetary field due to anisotropic scattering, affecting the radiation received by observers that are spatially separated around the Sun. The eastward deviation of the type III radio burst intensity with decreasing frequency (increasing distance) allows for the magnetic field to be traced to distances greater than that of the emitter path, offering a powerful diagnostic tool for space weather studies and a potentially wide-ranging diagnostic of the magnetic field structure of different astrophysical environments in which radio sources are embedded.
**Based on the recent paper** by Clarkson, D.L., et al. _Tracing the heliospheric magnetic field via anisotropic radio-wave scattering_ _,_ Sci Rep 15, 11335 (2025). DOI: 10.1038/s41598-025-95270-w
**References****:**
Clarkson, D.L., et al., Sci Rep 15, 11335 (2025)
Musset, S. et al., 2021, Astronomy & Astrophysics, 656, A34
Kontar, E. P., et al. 2023, ApJ, 956, 112
