The European Space Agency-led Solar Orbiter mission has split the flood of energetic particles flung out into space from the Sun into two groups, tracing each back to a different kind of outburst from our star.
The Sun is the most energetic particle accelerator in the Solar System. It whips up electrons to nearly the speed of light and flings them out into space, flooding the Solar System with so-called 'Solar Energetic Electrons' (SEEs).
Researchers have now used Solar Orbiter to pinpoint the source of these energetic electrons and trace what we see out in space back to what's actually happening on the Sun. In a paper to be published in Astronomy & Astrophysics on 1 September, they explain that they found two kinds of SEE with clearly distinct stories: one connected to intense solar flares (explosions from smaller patches of the Sun's surface), and one to larger eruptions of hot gas from the Sun's atmosphere (known as 'coronal mass ejections', or CMEs).
"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 lead author Alexander Warmuth of the Leibniz Institute for Astrophysics Potsdam (AIP), Germany.
A clearer connection
While scientists were aware that two types of SEE event existed, Solar Orbiter was able to measure a large number of events, and look far closer to the Sun than other missions had, to reveal how they form and leave the surface of our star.
"We were only able to identify and understand these two groups by observing hundreds of events at different distances from the Sun with multiple instruments – something that only Solar Orbiter can do," adds Alexander. "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."
Flight delays
The researchers detected the SEE events at different distances from the Sun. This let them study how the electrons behave as they travel through the Solar System, answering a lingering question about these energetic particles.
When we spot a flare or a CME, there's often an apparent lag between what we see taking place at the Sun, and the release of energetic electrons into space. In extreme cases, the particles seem to take hours to escape. Why?
"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 co-author and 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."
The space between the Sun and the planets of the Solar System isn't empty. A wind of charged particles streams out from the Sun constantly, dragging the Sun's magnetic field with it. It fills space and influences how the energetic electrons travel; rather than being able to go where they like, they are confined, scattered, and disturbed by this wind and its magnetism.
The study fulfils an important goal of Solar Orbiter: to continuously monitor our star and its surroundings to trace ejected particles back to their sources at the Sun.
"Thanks to Solar Orbiter, we're getting to know our star better than ever," says Daniel Müller, ESA Project Scientist for Solar Orbiter. "During its first five years in space, Solar Orbiter has observed a wealth of Solar Energetic Electron events. As a result, we've been able to perform detailed analyses and assemble a unique database for the worldwide community to explore."
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," adds Daniel. "The research is a really great example of the power of collaboration – it was only possible due to the combined expertise and teamwork of European scientists, instrument teams from across ESA Member States, and colleagues from the US."
Looking ahead, ESA's Vigil mission will pioneer a revolutionary approach, operationally observing the 'side' of the Sun for the first time, unlocking continuous insights into solar activity. To be launched in 2031, Vigil will detect potentially hazardous solar events before they come into view as seen from Earth, giving us advance knowledge of their speed, direction and chance of impact.
Our understanding of how our planet responds to solar storms will also be investigated further with the launch of ESA's Smile mission next year. Smile will study how Earth endures the relentless 'wind', and sporadic bursts, of fierce particles thrown our way from the Sun, exploring how the particles interact with our planet's protective magnetic field.
Solar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA.
