10 Minutes
The Sun as a Natural Particle Accelerator
The Sun is not only the source of light and heat for the Solar System; it also functions as a natural particle accelerator, launching streams of charged particles that travel through interplanetary space. Using data from the European Space Agency’s Solar Orbiter mission, researchers have now traced the fastest of these particles—high-energy electrons—back to two separate solar origins. This discovery resolves a long-standing ambiguity about Solar Energetic Electrons (SEEs) and sharpens our ability to predict hazardous space weather that can affect satellites, astronauts and ground infrastructure.
ESA’s Solar Orbiter revealed how the Sun launches two distinct streams of high-energy electrons, solving a key mystery and advancing space weather protection. Credit: ESA & NASA/Solar Orbiter/STIX & EPD
Mission and instruments: How Solar Orbiter made the link
Solar Orbiter operates nearer to the Sun than most previous missions and carries a carefully integrated suite of remote-sensing and in situ instruments. Between November 2020 and December 2022 the spacecraft recorded more than 300 energetic-electron events, combining direct particle measurements with simultaneous imaging and spectroscopic views of the solar atmosphere.
Solar Orbiter observed more than 300 bursts of ‘Solar Energetic Electrons’ between November 2020 and December 2022. For the first time, we clearly see the connection between the energetic electrons in space and their sources on the Sun. The energetic electrons are launched by two distinct sources: solar flares (blue dots) and coronal mass ejections (red dots). Solar flares release quick bursts of energetic electrons, whereas coronal mass ejections release broader swells of energetic electrons more gradually. Credit: ESA & NASA/Solar Orbiter/STIX & EPD
The dataset used in the study relied on eight of Solar Orbiter’s ten science instruments. Key contributions included:
- Energetic Particle Detector (EPD): in situ measurement of electron fluxes and energies as Solar Orbiter flew through the streams.
- Spectrometer/Telescope for Imaging X-rays (STIX): captured X-ray emissions produced when energetic electrons impact the solar atmosphere.
- Extreme Ultraviolet Imager (EUI) and Metis coronagraph: provided context on the solar origin—solar flares, jets and coronal mass ejections (CMEs).
By pairing in situ particle detection with remote observations of the Sun, scientists could determine both when and where electrons were accelerated and how they escaped into interplanetary space. As lead author Alexander Warmuth (Leibniz Institute for Astrophysics Potsdam) notes, “By going so close to our star, we could measure the particles in a ‘pristine’ early state and thus accurately determine the time and place they started at the Sun.”
Two distinct electron populations identified
The central result is a clear separation between two types of Solar Energetic Electron events:
- Impulsive events linked to solar flares: These produce short, intense bursts of high-energy electrons that escape rapidly along magnetic field lines. Flares occur in smaller, highly active regions of the solar surface and often drive narrow jets and localized X-ray emission.
- Gradual events associated with coronal mass ejections (CMEs): CMEs are immense expulsions of plasma and magnetic field from the Sun’s corona. They can accelerate and inject electrons more slowly and over a broader range of longitudes, producing prolonged swells of energetic particles.
“We see a clear split between ‘impulsive’ particle events, where these energetic electrons speed off the Sun’s surface in bursts via solar flares, and ‘gradual’ ones associated with more extended CMEs, which release a broader swell of particles over longer periods of time,” says Alexander Warmuth.
Solar Orbiter observed this solar flare on November 11, 2022, with its Extreme Ultraviolet Imager (EUI) and Spectrometer/Telescope for Imaging X-rays (STIX) instruments. The EUI footage (yellow) shows million-degree gas in the Sun’s atmosphere. A narrow ejection of gas from the flare called a solar jet, can be clearly seen heading towards the bottom right of the inset. Solar flares release electrons both outwards into space and inwards towards the Sun’s surface. When they hit the Sun’s surface, they generate X-rays. This X-ray emission, recorded by STIX, is overlaid in blue. Credit: ESA & NASA/Solar Orbiter/EUI & STIX
How particles travel: detection delays and transport effects
One persistent mystery addressed by the study is the variable delay between a solar eruption observed remotely (a flare or CME) and the detection of energetic electrons near a spacecraft. In some cases electrons appear to arrive hours after the initiating solar event. The Solar Orbiters dataset shows that part of this delay is not an intrinsic delay at the Sun but the result of transport effects in the solar wind.
Scattering and turbulence in the heliosphere
Interplanetary space is filled with the solar wind—a supersonic flow of plasma carrying the Sun’s magnetic field. This magnetised plasma is turbulent: irregularities and waves scatter charged particles. As electrons propagate outward, they encounter magnetic turbulence that can change their direction and speed, producing detection lags and smearing out the original burst signal.
“It turns out that this is at least partly related to how the electrons travel through space — it could be a lag in release, but also a lag in detection,” says ESA Research Fellow Laura Rodríguez-García. “The electrons encounter turbulence, get scattered in different directions, and so on, so we don’t spot them immediately. These effects build up as you move further from the Sun.”
Because Solar Orbiter made measurements at closer distances, it could capture the electrons in a relatively 'pristine' state before cumulative scattering effects dominate. Comparing observations at different radial distances allowed the team to separate source properties (flare vs CME) from transport effects in the heliosphere.
Scientific significance and implications for space weather
Distinguishing between flare-driven and CME-driven electron populations has immediate value for space weather forecasting. CMEs generally pose a greater hazard because they contain larger numbers of high-energy particles and often drive shocks that further accelerate charged particles. By identifying which type of event produces the observed energetic electrons, forecasters can better estimate the severity and duration of particle radiation episodes.
Keeping Earth Safe Crucially, the finding is important for our understanding of space weather, where accurate forecasting is essential to keep our spacecraft operational and safe. One of the two kinds of SEE events is more important for space weather: that connected to CMEs, which tend to hold more high-energy particles and so threaten far more damage. Because of this, being able to distinguish between the two types of energetic electrons is hugely relevant for our forecasting.
“Knowledge such as this from Solar Orbiter will help protect other spacecraft in the future, by letting us better understand the energetic particles from the Sun that threaten our astronauts and satellites,” says Daniel Müller, ESA Project Scientist for Solar Orbiter. The dataset produced by the study forms a growing database that scientists around the world will use to refine models of particle acceleration and propagation.
Related technologies, operational missions and next steps
This research underscores how coordinated instrumentation—combining in situ particle detectors with high-resolution solar imaging and spectroscopy—enables breakthroughs in understanding solar particle acceleration. The lessons inform both scientific research and operational space weather infrastructure.
Solar Orbiter observed this coronal mass ejection (CME) on November 19, 2022. A CME is a vast eruption of billions of tonnes of plasma and accompanying magnetic fields from the Sun’s outer atmosphere. The Metis instrument images the Sun’s outer atmosphere by artificially covering its bright disc, similar to what happens during a total solar eclipse. In this movie, the Sun’s size and position are depicted by the white circle. Credit: ESA & NASA/Solar Orbiter/Metis
Future missions: Vigil and SMILE
Two upcoming ESA missions will extend our observational capabilities:
- Vigil (launch ~2031): Will operate at a vantage point observing the Sun’s eastern and western limbs (the “side” of the Sun relative to Earth) to provide earlier detection of CMEs that can rotate into an Earth-directed path. Vigil’s operational continuous side-view will improve lead times for forecasting CME trajectories, speed and impact probability.
- SMILE (Solar wind Magnetosphere Ionosphere Link Explorer, launch upcoming): Will study the dynamic interaction between solar wind particles and Earth’s magnetosphere, clarifying how energetic electron streams and other solar outputs perturb our planet’s magnetic shield.
These missions, together with Solar Orbiter and other observatories, will form a multi-perspective network that improves both physics understanding and operational forecasting.
Expert Insight
Dr. Maya Singh, senior heliophysicist at a major university (fictional expert), comments: “What Solar Orbiter has done is separate the fingerprints of two accelerator processes on the Sun. Observing electrons close to their origin reduces ambiguity introduced by transport effects in the solar wind. Practically, this means better models for radiation environments around spacecraft and clearer criteria for triggering protective spacecraft and astronaut protocols.”
Dr. Singh adds, “The combination of remote and in situ measurements in the same mission is a paradigm for future heliophysics missions. As we add side-viewing missions like Vigil and targeted magnetosphere missions like SMILE, our predictive capacity for space weather will improve substantially.”
Conclusion
Solar Orbiter’s observations have resolved a key question about the Sun’s fastest charged particles by showing that energetic electrons come in two principal varieties: impulsive bursts from solar flares and gradual swells tied to coronal mass ejections. By measuring more than 300 SEE events with complementary instruments and at closer distances than previous probes, the mission separated source characteristics from transport effects produced by the turbulent solar wind. The result strengthens space weather forecasting, informs protective strategies for satellites and astronauts, and sets a baseline for new missions—Vigil and SMILE—that will further expand our observational coverage of the Sun–Earth system. The growing Solar Orbiter database will continue to serve the global heliophysics community as we refine models of particle acceleration, propagation and planetary impact.
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