Towards understanding the early kinematics of coronal mass ejections: Proba-3/ASPIICS observations of the 21 September 2025 event

The Sun is a restless star, constantly shedding a stream of charged particles known as the solar wind. Occasionally, this flow is interrupted by massive, violent eruptions called coronal mass ejections. These events blast billions of tons of magnetized plasma from the solar surface out into space, traveling at speeds that can reach hundreds of kilometers per second. When such an eruption is directed toward Earth, it can interact with our planet's magnetic field, creating spectacular auroras but also posing risks to satellites, power grids, and communication systems. For decades, scientists have understood that these eruptions begin with a slow, quiet rise of material, followed by a sudden, violent acceleration. However, a critical piece of the puzzle has remained missing: the exact moment when the slow rise turns into a rapid explosion. This transition happens in the inner region of the Sun's atmosphere, a zone that has been difficult to observe clearly because it sits in a blind spot between the instruments that see the solar surface and those that watch the distant corona.

A new study published by a team of researchers from the Royal Observatory of Belgium and international partners has finally filled this gap. Using a specialized instrument aboard the European Space Agency's Proba-3 mission, the team captured a continuous, high-definition view of a coronal mass ejection as it erupted on September 21, 2025. The event began with a quiet prominence, a loop of cool, dense gas hanging in the Sun's atmosphere, which started to rise slowly. As it climbed, the structure began to twist and destabilize. The new observations allowed the scientists to track this eruption from its very first movements, just above the solar surface, all the way out into the middle corona, without losing sight of it. By stitching together data from this new instrument with older observations from other spacecraft, the researchers were able to construct a complete timeline of the eruption's speed and acceleration, revealing exactly how the event evolved from a slow drift into a high-speed projectile.

The key to this success was an instrument called ASPIICS, which uses a unique design to block the blinding light of the Sun's surface. Unlike previous telescopes that had to rely on internal masks or could only see the corona at great distances, ASPIICS uses a separate spacecraft flying in formation to act as an external sunshade. This setup creates a dark, clear view of the solar atmosphere starting just above the surface, extending out to three times the Sun's radius. This specific region is where the physics of an eruption changes most dramatically, yet it had previously been a zone of uncertainty. The researchers focused on an eruption that occurred above the Sun's southwestern edge. They watched as a quiet prominence, which appeared as a bright, twisted structure in the images, began to lift off. For a time, the rise was slow and steady, but then the structure suddenly surged forward.

The team measured the distance the eruption traveled over time, creating a detailed map of its motion. They found that the eruption followed a distinct pattern: a slow start, followed by a sharp, impulsive acceleration. The core of the eruption, which was the original prominence material, began to rise first. As it moved, it pushed against the surrounding solar atmosphere, piling up plasma at its front to form a bright leading edge. This leading edge appeared later than the core but quickly caught up in speed. The data showed that the core reached a speed of about 400 kilometers per second, while the leading edge accelerated to roughly 450 kilometers per second within the inner corona. The acceleration was not a gentle increase; it was a rapid, non-linear surge that indicated a sudden release of magnetic energy.

To ensure these measurements were accurate and not distorted by the angle from which they were viewed, the researchers combined the new data with observations from three other spacecraft positioned at different locations in the solar system. By looking at the same event from multiple angles, they could reconstruct the eruption in three dimensions. This analysis confirmed that the eruption was expanding in a self-similar way, meaning it kept the same shape as it grew larger. The three-dimensional reconstruction revealed that the eruption was traveling at a true speed of about 515 kilometers per second when it reached a distance of four solar radii. Because the eruption happened near the edge of the Sun as seen from Earth, the angle of view was favorable, and the measurements of speed and distance were very close to the actual values, requiring only minor corrections.

The study also compared the new high-resolution data with older models of how these eruptions behave. The researchers found that the eruption's motion could be described by a smooth mathematical curve that accounts for a slow start, a burst of acceleration, and a final settling into a constant speed. This curve matched the observations perfectly, suggesting that the forces driving the eruption and the resistance from the solar wind are well understood. The leading edge and the core of the eruption accelerated almost simultaneously, reaching their peak speeds at nearly the same time. This finding challenges the idea that the leading edge and the core might behave as entirely separate entities; instead, they appear to be part of a single, cohesive structure that responds together to the magnetic forces at play.

One of the most significant outcomes of this research is the demonstration that the long-standing gap in solar observation has been closed. For years, scientists had to guess what happened between the surface of the Sun and the start of the outer corona, often relying on separate instruments that could not see the same object at the same time. The continuous tracking provided by the Proba-3 mission shows that it is possible to follow a single eruption from its birth to its maturity without interruption. The data confirms that the transition from a slow rise to a fast explosion is a rapid, violent process that happens within the inner corona. The researchers suggest that this specific event, with its clear progression from a quiet prominence to a fast-moving cloud of plasma, represents a textbook example of how these eruptions work.

The ability to measure these speeds and accelerations with such precision opens the door to better predictions of space weather. By understanding exactly how fast an eruption accelerates and in which direction it travels, scientists can improve their models of how these clouds of plasma will interact with Earth. The study does not claim to have solved every mystery of solar eruptions, but it provides a solid, measured foundation for future work. The researchers note that while this single event offers a clear picture, studying a larger number of eruptions will be necessary to determine if this behavior is universal or if different types of eruptions follow different rules. For now, the continuous, high-definition view of the 2025 event stands as a definitive record of a coronal mass ejection in its earliest, most critical moments, turning a once-blind spot into a window of clear understanding.