Solar Orbiter traces superfast electrons back to Sun

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. They find 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 swell of particles over longer periods of time and over broader angular ranges,” says lead author Alexander Warmuth of the Leibniz Institute for Astrophysics Potsdam (AIP), Germany.

While scientists were aware that two types of SEE events existed, Solar Orbiter was able to comprehensively 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 were able to measure the particles in a pristine state and could thus accurately determine the time and place where they started at the Sun.

The study is the most comprehensive of SEE events to date, and produces a catalogue that will only grow through Solar Orbiter’s lifetime. It used eight of Solar Orbiter’s ten instruments to observe more than 300 events between November 2020 and December 2022. This research showcases 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, coordinated by scientists from AIP.

“It’s the first time we’ve clearly seen this connection between particles in space and their source events taking place at the Sun,” adds co-author Frederic Schuller, also of AIP. “We measured the energetic electrons in situ – that is, Solar Orbiter actually flew through the electron streams – while simultaneously using more of the spacecraft’s instruments to observe what was happening at the Sun. We also gathered information about the space environment between the Sun and spacecraft.”

The researchers detected 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 CME, there’s often an apparent lag between what we see taking place at the Sun, and the release of SEEs into space. In extreme cases, the particles seem to take hours to escape. Why?

“It turns out that this is related to how the electrons travel through space – it’s not a lag in release, but 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 particles travel; rather than being able to go where they like, the SEEs 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 particle events. As a result, we’ve been able to perform detailed analyses and assemble a unique database for the worldwide community to explore.”

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 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 SEE is hugely relevant for our forecasting.

Solar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA.