Could the Sun’s most dangerous particles be found to originate from two essentially different types of cosmic explosions? Fresh research by the joint ESA–NASA Solar Orbiter mission suggests as much, offering the clearest view yet of how electrons are formed at high energies and hurled across the solar system and why their arrival on Earth can be famously late.
From November 2020 to December 2022, Solar Orbiter witnessed more than 300 Solar Energetic Electron (SEE) bursts, with some accelerated to near the speed of light. By using both in situ measurements and distant imaging, the spacecraft tracked these electrons back to their roots on the Sun. The outcomes are a sharp dichotomy: “impulsive” events that go along with tight, short solar flares, and “gradual” events that go along with enormous coronal mass ejections (CMEs) that can propel billions of tonnes of plasma into space. “With a clear distinction between ‘impulsive’ particle events, in which these high-energy electrons accelerate from the surface of the sun in bursts through solar flares, and ‘gradual’ ones connected with longer-duration CMEs, which emit a more gradual swell of particles over larger time scales,” states Alexander Warmuth from the Leibniz Institute for Astrophysics Potsdam.
Their physics lies in very extreme solar plasma dynamics. In flare-driven activity, electrons are accelerated in the Sun’s low corona through magnetic reconnection the rapid rearrangement of magnetic field lines in current sheets that may fragment into turbulent structures. Magnetohydrodynamic simulations suggest that such turbulence can be “strong,” with δB/B > 1, producing localized electric fields that accelerate electrons in abrupt, discrete jumps. In CME-driven events, the most significant mechanism shifts to shock acceleration: as the CME propagates through solar wind and corona, it generates a shock front in which particles are accelerated by diffusive shock acceleration and, in extremely turbulent conditions, by turbulent reconnection. This combination can accelerate to an order of magnitude and raise maximum energies by two orders above shocks alone.
One of the most intriguing discoveries of the research addresses an old enigma: why SEEs occasionally materialize several hours after the eruption of their origin. ESA Research Fellow Laura Rodríguez-García explained, “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. The electrons encounter turbulence, get scattered in different directions, and so on, so we don’t spot them immediately.” The solar wind a constant stream of charged particles with the Sun’s magnetic field captures and deflects the electrons, maintaining their paths very non-linear. Scattering is especially large in the turbulent sheath regions behind CME shocks.
Understanding such causes and transportation effects is greater than a cognitive exercise. Gradual CME-related events spread more energetic particles with higher risks for spacecraft electronics, satellite functionality, and astronaut health. By sorting out the impulsive and gradual events in close to real time, forecasters would be able to refine space weather models to better predict the intensity and duration of hazardous particle storms. Missions like Solar Orbiter are establishing the empirical databases to train such predictive models.
Future missions will continue to leverage this capability. The SMILE spacecraft, to be launched in 2026, will take pictures of the interaction between the solar wind and Earth’s magnetic field with soft X-ray and ultraviolet auroral cameras, providing a global view of how particles from outside disrupt our planet’s magnetic shield. In 2031, ESA’s Vigil mission will place itself to sit watch over the flank of the Sun, detecting CMEs before they turn into the Earth’s line of sight and offering precious hours of warning.
The Solar Orbiter results also return to fundamental solar physics. They confirm that the Sun is a giant, multi-mechanism particle accelerator, whose magnetic topology, turbulence intensity, and shock geometry all control escaping particles’ energy spectra and composition. These results are a step towards deciphering both the Sun’s most powerful outbursts for space technologists and astrophysicists and shielding the ever-more crowded near-Earth space from their effect.
