The European Space Agency-led Solar Orbiter mission has traced floods of energetic electrons hurled out by the Sun back to two distinct sources, marking a major advance in space weather research.
The Sun, the Solar System’s most powerful particle accelerator, can whip up electrons to nearly the speed of light and eject them into space. Known as Solar Energetic Electrons (SEEs), these particles play a key role in shaping the cosmic environment.
For decades, scientists suspected that SEEs originated from different types of solar outbursts but lacked the ability to clearly link events in space to their source on the Sun.
Now, Solar Orbiter has delivered the first direct evidence connecting electrons measured in space with their origins.
The findings show that one type of SEE is tied to intense solar flares, explosive bursts from smaller patches of the Sun’s surface, while another stems from coronal mass ejections (CMEs), massive eruptions of hot gas from the Sun’s atmosphere.
“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.
Tracing electrons back home
While the existence of two types of SEE was known, Solar Orbiter’s proximity to the Sun allowed researchers to make an unprecedented connection. By flying closer than previous spacecraft and using eight of its ten instruments, the probe observed over 300 events between November 2020 and December 2022.
“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,” Warmuth adds. “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.”
Co-author Frederic Schuller of AIP highlights the probe’s unique vantage point: “It’s the first time we’ve clearly seen this connection between energetic electrons in space and their source events taking place at the Sun.”
The study also explained a long-standing puzzle: why electrons often appear delayed after solar eruptions. According to ESA Research Fellow Laura Rodríguez-García, “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.”
Safeguarding spacecraft and astronauts
The distinction between SEE types matters for space weather forecasting. CMEs in particular are linked to swells of high-energy particles that can damage satellites, disrupt communications, and endanger astronauts.
“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.
ESA’s upcoming missions will build on this progress. The Vigil mission, set for launch in 2031, will monitor the Sun from the side to provide advance warnings of potentially hazardous events. Meanwhile, the Smile mission, due next year, will investigate how Earth’s magnetic field responds to solar storms.
“Thanks to Solar Orbiter, we’re getting to know our star better than ever,” Müller adds.
“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.”