Multi-spacecraft Radio Observations Trace the Heliospheric Magnetic Field 

(Solar Orbiter Nugget #60 by Daniel L. Clarkson¹, Eduard P. Kontar¹, Nicolina Chrysaphi^1,2, A. Gordon Emslie³, Natasha L. S. Jeffrey⁴, Vratislav Krupar^5,6 & Antonio Vecchio^7,8)

1. Introduction

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, we are in a unique position of observing the propagation of radio emission through the heliosphere from multiple vantage points.

2. Results

In this study, we 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 spacecrafts: PSP, SolO, Stereo-A, and Wind. (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 address 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. 

To reproduce the observations, we performed simulations to follow radio-waves. The simulations show that the radio-waves are guided by the heliospheric magnetic field via anisotropic scattering (Figure 2).


Figure 2. 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.

3. Conclusions

The paper concludes 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. Therefore, 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.

This nugget is based on the recent paper Clarkson, D.L., et al. Tracing the heliospheric magnetic field via anisotropic radio-wave scattering, Science Reports, 15, 11335 (2025). ). DOI: 10.1038/s41598-025-95270-w

Affiliations

(1) School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK
(2) Ecole Polytechnique, InstitutePolytechnique de Paris, CNRS, Laboratoire de Physique des Plasmas (LPP), Sorbonne Université, 4 Place Jussieu, 75005 Paris, France.
(3) Department of Physics and Astronomy, Western Kentucky University, Bowling Green, KY 42101, USA.
(4) Department of Mathematics, Physics and Electrical Engineering, Northumbria University, Newcastle upon Tyne NE1 8ST, UK.
(5) Goddard Planetary Heliophysics Institute, University of Maryland, Baltimore County,Baltimore, MD 21250, USA.
(6) Heliospheric Physics Laboratory, Heliophysics Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.
(7) Radboud Radio Lab - Department of Astrophysics, Radboud University, Nijmegen, The Netherlands.
(8)LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France

Acknowledgements
This work is supported by UKRI/STFC grants ST/T000422/1 and ST/Y001834/1. N.C. acknowledges funding support from the Initiative Physique des Infinis (IPI), a research training program of the Idex SUPER at Sorbonne Université. A.G.E. was supported by NASA’s Heliophysics Supporting Research Program through grant 80NSSC24K0244, and by NASA award number 80NSSC23M0074, the NASA Kentucky EPSCoR Program, and the Kentucky Cabinet for Economic Development. V.K. was supported by the STEREO/WAVES and Wind/WAVES projects and by the NASA grant 19-HSR-19_2-0143. The authors thank the PSP/RFS, SolO/RPW, STEREO/WAVES, and Wind/WAVES teams for making the data available. Solar Orbiter49 is a mission of international cooperation between ESA and NASA, operated by ESA. The FIELDS experiment on the Parker Solar Probe spacecraft50 was designed and developed under NASA contract NNN06AA01C. This research has made use of the Astrophysics Data System, funded by NASA under Cooperative Agreement 80NSSC21M00561.