Our sun can seem deceptively peaceful from Earth, but only because we have the luxury of living 150 million kilometers away. Up close, it’s a nuclear-fuelled carnival ride of terror, launching countless tiny particles at speed far into interplanetary space.
“The Sun is the most energetic particle accelerator in the Solar System,” writes a team of researchers behind a study on the energetic particles that stream forth in solar flares and coronal mass ejections (CMEs).
According to the study’s lead author Alexander Warmuth, each of those events delivers streams of particles with very distinct features hinting at a different birthplace and backstory.
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“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,” says Warmuth, a heliophysicist at the Leibniz Institute for Astrophysics Potsdam (AIP) in Germany.
The latter, more gradual events “release a broader swell of particles over longer periods of time and over broader angular ranges,” Warmuth adds.
Using data from the European Space Agency-led Solar Orbiter, which gets as close as 42 million kilometers to the Sun, Warmuth and his team measured those particles on location, focusing on a type known as solar energetic electrons (SEEs).
A dichotomy of SEEs was already well established, but Solar Orbiter provided a wealth of data from an unprecedented proximity that revealed new details about where exactly each type of SEE came from.
“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 only Solar Orbiter can do,” Warmuth says.
“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,” he adds.
The study is based on observations of more than 300 SEE events between 2020 and 2022, representing the most exhaustive such analysis so far.
“It’s the first time we’ve clearly seen this connection between particles in space and their source events taking place at the Sun,” says 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.”
The probe’s eccentric orbit offered data on events at different distances from the Sun, yielding new insight about how these electrons behave on their travels. That includes a potential explanation for confusing lags between visual signs of solar flares and radio bursts, and the subsequent release of SEEs into space.
“It turns out 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 heliophysicist Laura Rodríguez-García.
“The electrons encounter turbulence, get scattered in different directions, and so on, so we don’t spot them immediately,” she adds. “These effects build up as you move further from the Sun.”
The probe was meant to produce insights like these, the authors note, and it should continue to illuminate solar secrets for years to come.
“Thanks to Solar Orbiter, we’re getting to know our star better than ever,” says Daniel Müller, ESA project scientist for Solar Orbiter.
That kind of familiarity is valuable for many reasons, including its potential for helping us protect spacecraft and their crews.
“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,” he says.
The study was published in Astronomy & Astrophysics.