Multi-Spacecraft Fleet Tracks Solar Electron Blast from Corona to Earth

On 28 January 2025, the Sun fired a comparatively modest but scientifically precious salvo toward the inner solar system: an extreme ultraviolet jet, a chorus of type III radio bursts, and a spray of energetic electrons that swept past three spacecraft stationed near 1 AU. A team of solar physicists led by Shiwei Feng of Shandong University and Xinhua Zhao of the National Space Science Center in Beijing has now reconstructed that event end to end, following the electrons from their birthplace in the low corona all the way to the detectors of STEREO-A, Wind, and their companions. The study, published in Solar Physics, demonstrates a workflow that stitches together remote-sensing imaging, radio triangulation, and in situ particle measurements into a single, coherent propagation story across time, space, and energy.

The significance of the work lies less in the size of the event than in the precision of the tracking. Solar energetic electrons are among the fastest messengers the Sun sends into the heliosphere, and their arrival times at spacecraft encode information about where they were released, how long and twisted their path was, and how much they scattered along the way. By combining multiple independent techniques, the team was able to cross-examine each stage of the journey rather than relying on a single diagnostic, offering exactly the kind of observational constraints that space weather forecasters need to convert a solar eruption into a reliable warning at Earth.

The starting point was the corona. Extreme ultraviolet imagers captured a jet erupting from the solar surface, the classic signature of magnetic reconnection in which stressed field lines snap into a lower-energy configuration and fling plasma outward. Simultaneously, radio receivers recorded type III bursts, the sweeping tones produced when beams of electrons escaping along open magnetic field lines excite plasma oscillations at the local electron frequency, which drift downward in frequency as the beams race into the decreasing density of the outer corona. A type II burst, generated by a shock wave, also accompanied the event, hinting that a coronal mass ejection or blast wave was involved in the disturbance.

To establish the geometry of the escape routes, the researchers applied a Potential Field Source Surface reconstruction, a standard magnetohydrostatic technique that extrapolates the measured photospheric magnetic field outward to a spherical source surface where the field is forced to be radial. The result showed that the type III burst sources in the corona and the direction of the EUV jet ejection generally lined up with open magnetic field lines, the natural highways along which electrons can leave the Sun without becoming trapped on closed loops. This consistency between the imaged ejecta, the radio sources, and the modeled field topology anchors the whole event in a believable magnetic setting.

The next step followed the electrons into interplanetary space. Using a Time Difference of Arrival technique, in which the same radio emission is detected at different times by separated spacecraft, the team triangulated the source of the interplanetary type III burst. The reconstructed source lay along the Parker spiral, the archimedean curve traced out by magnetic field lines frozen into the radially expanding solar wind as the Sun rotates. From the frequency drift of the burst, the researchers derived exciter velocities of 0.06c to 0.17c, meaning the electron beams driving the radio emission were traveling at between roughly six and seventeen percent of the speed of light, entirely consistent with the kinds of electron beams long associated with type III radio emission.

The in situ piece of the puzzle came from velocity dispersion analysis of the electrons themselves. Faster electrons arrive before slower ones, so if a batch of particles is released at a single instant and travels scatter-free along a common path, the onset times measured across different energy channels fall on a straight line whose intercept gives the release time and whose slope gives the path length. Applying this method to electron data from three spacecraft near 1 AU, the team derived solar release times of 08:03 to 08:23 UT for STEREO-A and 08:07 to 08:27 UT for Wind, windows comfortably consistent with the timing of the coronal jet and radio activity.

The inferred path lengths, however, told a more interesting story. For STEREO-A the derived path length was 1.2 plus or minus 0.9 AU, a value broadly compatible with an ideal Parker spiral connecting the Sun to the spacecraft. For Wind, by contrast, the analysis yielded 2.6 plus or minus 1.6 AU, a figure that exceeds the nominal Parker spiral length to Earth. The authors interpret this as evidence for a complex, non-ideal interplanetary magnetic field topology along the Wind connection, perhaps involving additional winding, field-line meandering in turbulent solar wind, or structures that forced the electrons to negotiate a longer and more convoluted route than the textbook spiral predicts.

Perhaps the most provocative conclusion concerns the identity of the electrons themselves. Comparing the release and arrival times of the locally detected electrons with those of the beams inferred from the interplanetary type III radio emission, the team found that the two populations appear not to coincide. The electrons that generate the radio bursts and the electrons that wash over the spacecraft detectors may be distinct groups, released at slightly different times or traveling along different field lines, even though both trace back to the same eruption. This distinction matters because many studies implicitly assume that the radio-emitting beam and the measured particle population are one and the same, an assumption this event suggests should be handled with care.

Methodologically, the study amounts to a template. By combining multi-viewpoint imaging, PFSS field modeling, radio TDOA localization, and multi-spacecraft velocity dispersion analysis, the researchers show how the same transient can be tracked continuously across the corona-to-1 AU pipeline, with each technique constraining the others. In an era when Solar Orbiter, Parker Solar Probe, STEREO-A, Wind, and a growing fleet of heliophysics missions sample the inner heliosphere from widely separated longitudes, such multi-spacecraft forensics are becoming the norm rather than the exception, and this event offers a clean worked example of how to do them well.

The practical payoff is for space weather. Energetic electrons, though they cannot penetrate shielding as effectively as high-energy protons, contribute to radiation hazards for satellites and astronauts, and their prompt arrival makes advance warning genuinely difficult. A framework that converts the first radio signatures of escaping electron beams, captured within minutes of a solar eruption, into quantitative estimates of release time, beam velocity, and expected path length gives forecasters a physical basis for predicting when and where particle enhancements will arrive. The 28 January 2025 event, modest as it was, shows that with enough viewpoints and the right combination of analysis techniques, the Sun’s fastest particles can indeed be followed door to door, from the reconnection site in the corona to the doorstep of an observer at Earth’s orbit.

Subject of Research: Multi-spacecraft tracking of solar energetic electrons from the solar corona to 1 AU during the 28 January 2025 event

Article Title: Tracking Solar Energetic Electrons from Corona to 1 AU: A Multi-Spacecraft Study of the 28 January 2025 Event

Article References: Feng, S., Zhao, X., Xiang, N., Qi, S., Wang, W., Chen, X., Chen, L., Zhou, Z., Kuznetsov, A., & Yan, Y. (2026). Tracking Solar Energetic Electrons from Corona to 1 AU: A Multi-Spacecraft Study of the 28 January 2025 Event. Solar Physics, 301(9), Article 142. https://doi.org/10.1007/s11207-026-02730-1

Image Credits: AI Generated

DOI: 10.1007/s11207-026-02730-1

Keywords: solar energetic electrons, type III radio bursts, EUV jet, Parker spiral, velocity dispersion analysis, time difference of arrival, PFSS magnetic field reconstruction, space weather, STEREO-A, Wind spacecraft, interplanetary magnetic field, solar physics