Our star, the Sun, is an inexhaustible source of energy and life, but at the same time the most powerful particle accelerator in our system. In the complex processes that take place deep in its atmosphere, the Sun constantly ejects huge amounts of energetic particles into space. Among them, the so-called solar energetic electrons (SEE), subatomic particles accelerated to speeds close to the speed of light, particularly stand out. It was long thought that these electrons were a unique phenomenon, but revolutionary discoveries by the Solar Orbiter mission, led by the European Space Agency (ESA) in collaboration with NASA, have revealed that the truth is much more complex. Scientists have succeeded in separating this stream of particles into two fundamentally different groups, successfully tracing each to its specific source on the Sun.

This discovery not only deepens our understanding of the fundamental physics of the Sun, but also has direct and crucial implications for predicting space weather – a phenomenon that can have devastating consequences for our technological civilization, from satellites in orbit to power grids on Earth. By precisely mapping the origin of these super-fast electrons, we are opening a new chapter in protecting our infrastructure and future space missions.

Unveiling the Sun's Secrets Up Close

The key to this epochal discovery lies in the unique capabilities of the Solar Orbiter spacecraft. Unlike previous missions that observed the Sun from a greater distance, Solar Orbiter's elliptical orbit brings it incredibly close to our star, at times within the orbit of Mercury. It is this proximity that allows scientists to analyze the particles in their "pristine" state, before their paths and energy are significantly altered by the long journey through interplanetary space. Observing the events from such a close distance allowed the team to determine the exact time and location of their origin on the Sun with extraordinary precision.

Alexander Warmuth from the Leibniz Institute for Astrophysics Potsdam (AIP), the lead author of the study, emphasizes the importance of this approach: "We were 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. The proximity to the Sun allowed us to measure the particles in their early, original state and thus precisely locate their source."

Two Types of Solar Electron Storms

By analyzing data collected from more than 300 individual events between November 2020 and December 2022, scientists noticed a clear division. On one side are "impulsive" events, and on the other, "gradual" ones.

Impulsive events are associated with solar flares. Solar flares are sudden and intense explosions on the Sun's surface, releasing a huge amount of energy in the form of radiation. Electrons originating from these events are ejected from the Sun in fast, short-lived bursts. We can think of them as sharp, concentrated shots of energetic particles.

In contrast, gradual events are linked to much larger and longer-lasting phenomena known as coronal mass ejections (CMEs). CMEs are giant clouds of plasma and magnetic field that detach from the Sun's atmosphere, the corona, and travel through space. Electrons associated with CMEs are released over a longer period, creating a wider and more prolonged wave of particles that washes over the Solar System. Although scientists were previously aware of the existence of these two types of events, Solar Orbiter has, for the first time, provided irrefutable evidence directly linking them to their different sources on the Sun.

Synergy of Instruments: The Key to Success

This research represents the most comprehensive study of solar energetic electrons to date, and its success lies in the coordinated use of as many as eight of the ten scientific instruments on Solar Orbiter. The mission is designed to perform two types of measurements simultaneously: remote sensing and in situ measurements.

Remote sensing instruments, such as the EUI (Extreme Ultraviolet Imager) and STIX (Spectrometer/Telescope for Imaging X-rays), constantly monitor the Sun's surface and atmosphere, capturing details of solar flares in the extreme ultraviolet and X-ray spectrum. At the same time, the Metis coronagraph blocks the blinding light of the Sun's disk to image the outer, rarer corona, allowing direct observation of the magnificent coronal mass ejections.

While these instruments "look" at the Sun, the Energetic Particle Detector (EPD) performs in situ measurements, which means the spacecraft literally passes through the clouds of electrons it is observing. The EPD analyzes their composition, energy, and direction of travel. Frederic Schuller, a co-author of the study from AIP, emphasizes: "For the first time, we have clearly seen this link between energetic electrons in space and the events on the Sun that are their source. We measured the particles in situ while other instruments were simultaneously observing what was happening on the Sun, also collecting data on the space environment between the Sun and the spacecraft."

Solving the Time-Lag Puzzle

One of the long-standing mysteries in solar physics was the apparent delay between the moment astronomers observe a solar flare or CME and the moment energetic electrons arrive at a detector in space. In some extreme cases, it seemed to take hours for the particles to "escape" from the Sun. The question was: why?

Data from Solar Orbiter now offers an answer. Laura Rodríguez-García, an ESA research fellow, explains: "It turns out that this delay is at least partially related to how electrons travel through space. There can be a delay in the release itself, but also a delay in detection." Namely, the space between the Sun and the planets is not empty. It is filled with the solar wind, a continuous stream of charged particles flowing from the Sun, carrying the Sun's magnetic field with it. On their path, electrons encounter turbulence within the solar wind, are scattered in different directions, and do not travel in a straight line. Their path is chaotic and significantly longer than a direct line. The farther the observer is from the Sun, these effects accumulate and the detection delay becomes greater.

Importance for Space Weather Forecasting and Safety on Earth

This fundamental scientific discovery has immense practical significance. Understanding and predicting space weather is crucial for the safety of our technology. Distinguishing between the two types of SEE events is critical for accurate forecasting. Events associated with coronal mass ejections (CMEs) pose a much greater threat. They carry a larger number of high-energy particles and can cause serious damage to satellites, endanger the health of astronauts by exposing them to dangerous levels of radiation, and on Earth, cause geomagnetic storms that can bring down power grids and communication systems.

Daniel Müller, ESA's Project Scientist for Solar Orbiter, points out: "Knowledge like this from Solar Orbiter will help protect other spacecraft in the future, allowing us to better understand the energetic particles from the Sun that threaten our astronauts and satellites." The ability to quickly determine, based on the first detected particles, whether they originate from a relatively harmless flare or a potentially catastrophic CME could provide valuable time to take protective measures.

The Future of Solar Observation: The Vigil and Smile Missions

The insights from Solar Orbiter are just the beginning of a new era in understanding the Sun. ESA is already planning future missions that will build on these discoveries. The Vigil mission, scheduled for launch in 2031, will apply a revolutionary approach. It will be positioned at a location from which it will, for the first time in history, be able to operationally observe the "side" of the Sun. This will allow it to detect potentially dangerous solar events, such as active regions prone to CMEs, days before they rotate towards Earth, giving us invaluable early warning.

Our understanding of Earth's response to solar storms will be further explored with the launch of ESA's Smile mission, planned for 2026. Smile will study how Earth withstands the constant solar wind and occasional impacts of powerful particles, investigating the interaction of these particles with our protective magnetic field. Together, these missions will create a comprehensive system for monitoring and protecting against the whims of our star.