Solar Orbiter traces space-weather particles back to solar flares and CMEs

The star at the center of the solar system does more than provide light and warmth. It also acts as the most powerful particle accelerator in the neighborhood, whipping electrons to near-light speed and hurling them across space. These energetic particles, known as solar energetic electrons, can flood the solar system and sometimes head straight back into the Sun’s own atmosphere, releasing bursts of X-rays and gamma rays.

For decades, scientists knew that these electrons didn’t all behave the same way. Some shot out in quick bursts linked to solar flares, while others spread out more slowly, tied to giant eruptions of plasma called coronal mass ejections, or CMEs. What remained unclear was exactly how these electrons were launched and why their arrival at spacecraft often lagged minutes or even hours after the flare that triggered them.

Now, with help from the European Space Agency’s Solar Orbiter, researchers have been able to capture hundreds of these events and sort them into two clear categories, offering new insight into how the Sun accelerates particles.

You can think of the Sun’s particle emissions as falling into two camps. Impulsive events are tied to small, intense solar flares. They release energetic electrons in sharp bursts, often linked with sudden flashes of X-rays and radio signals. Gradual events, in contrast, are connected to large CMEs that fling hot gas and magnetic fields out into space. These produce a wider spray of particles, released more slowly over time.

Alexander Warmuth of the Leibniz Institute for Astrophysics Potsdam explains it this way: “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.”

The distinction matters because the two groups carry different risks. Flare-related electrons are important for understanding the physics of the Sun itself, but CME-related electrons tend to pack more energy and pose a greater threat to astronauts and satellites.

To build a clearer picture, an international team created the CoSEE-Cat catalogue, short for Catalogue of Solar Energetic Electron Events. The first release covers 303 events observed between November 2020 and December 2022. Unlike past missions, Solar Orbiter combines both telescopes and particle detectors, letting scientists watch flares erupt on the Sun while measuring the electrons that arrive in space.

Eight instruments contributed to this effort. The Energetic Particle Detector measured electrons across a wide energy range, from thousands to millions of electron volts. The STIX telescope caught X-rays from flares, while the Extreme Ultraviolet Imager and other cameras tracked eruptions across the Sun. Radio receivers listened for type III radio bursts, the telltale whistles of electrons escaping into space. Coronagraphs and imagers recorded CMEs expanding outward, and instruments measuring the solar wind and magnetic field revealed the environment the electrons had to cross.

This unique combination allowed the team to trace each event back to its origin. More than 88 percent of the electron events were linked to X-ray flares, most of them weak but some reaching the stronger M and X classes. Nearly all showed visible flare or eruption activity, and most were tied to radio bursts. About a quarter were also connected to CMEs, especially the gradual events.

One of the biggest mysteries was why the arrival of electrons at spacecraft often lags behind the flare that seems to launch them. In some cases, the delay is only minutes. In others, it stretches to hours.

Part of the answer lies in how the electrons travel. The Sun constantly blows out a solar wind of charged particles, dragging its magnetic field into space. Electrons don’t move in straight lines but are guided, scattered, and slowed by this turbulent environment. Co-author Laura Rodríguez-García of the European Space Agency explains, “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.”

By comparing data taken at different distances, Solar Orbiter showed that delays often grow with distance from the Sun, especially for impulsive events. This points to scattering and transport effects rather than long delays in the initial release. Gradual events, however, may genuinely reflect slower acceleration by CME-driven shock waves.

Not all particle events look the same chemically. Impulsive events often carry unusual mixes of ions, such as extra helium-3 or higher iron-to-oxygen ratios. These traits mark them as products of localized flare activity. Gradual events, on the other hand, show more ordinary compositions, consistent with shock waves sweeping up material across larger regions.

Measurements confirmed that nearly three-quarters of the events catalogued were impulsive, while about one-fifth were gradual. A handful showed traits of both.

Another clue came from anisotropy, or how directional the electrons were. Many events showed strong directionality, suggesting they streamed straight along magnetic field lines with little scattering. Others were more spread out, hinting at complex transport through disturbed solar wind.

The catalogue also revealed how these events vary with solar activity. As solar cycle 25 ramped up, the number of energetic electron events increased. Yet month-to-month variation was large, reflecting how important magnetic connection is between the spacecraft and the flare site.

Impulsive events tended to rise quickly, with typical rise times of about seven minutes, while gradual events often took 20 minutes or longer. Peak intensities varied widely, overlapping between the two groups but with impulsive events showing higher directionality.

Mapping the source regions showed that impulsive events usually came from well-connected areas in the Sun’s western hemisphere, matching where magnetic field lines spiral out toward Earth. Gradual events originated from a broader spread of locations.

Thanks to Solar Orbiter, scientists are now piecing together a more complete story of how electrons are launched from the Sun. Impulsive events stem from compact flare sites, rise quickly, and travel with little scattering. Gradual events come from large CME-driven regions, spread out more slowly, and often show long delays.

Daniel Müller, ESA’s project scientist for Solar Orbiter, sums up the progress: “Thanks to Solar Orbiter, we’re getting to know our star better than ever. 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.”

This work goes beyond curiosity about the Sun. Energetic particles from solar eruptions can damage satellites, interfere with communications, and pose risks to astronauts. Distinguishing between impulsive and gradual electron events helps forecast which outbursts are most dangerous. Gradual events tied to CMEs, in particular, tend to hold more high-energy particles and are more likely to threaten spacecraft.

Better understanding also supports future missions. ESA’s upcoming Vigil spacecraft, planned for launch in 2031, will monitor the “side” of the Sun that soon rotates into view from Earth. This could give advance warning of eruptions before they affect satellites or power grids.

Meanwhile, ESA’s Smile mission will study how Earth’s magnetic shield responds to these solar storms. Together with Solar Orbiter’s ongoing data collection, these efforts could improve space weather forecasts and help protect technology and people in space.

Note: The article above provided above by The Brighter Side of News.

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