Scientists Reveal How a Giant Solar Explosion Travels Through Space — And Predicts Its Arrival at Earth

A powerful eruption from the Sun can send enormous clouds of hot plasma and magnetic fields racing through space, potentially threatening satellites, communication networks and power grids on Earth. These eruptions, known as coronal mass ejections (CMEs), are among the main causes of severe space weather. However, scientists have long struggled to understand exactly how these giant solar explosions move, expand and change direction as they travel toward our planet.

Now, researchers led by Jason Byrne and his team have used an innovative three-dimensional (3D) reconstruction technique to investigate how a CME travels through space. By combining observations from NASA's Solar Terrestrial Relations Observatory (STEREO) mission with advanced computer simulations, the researchers were able to track the shape, direction and speed of a CME more accurately.

Their findings provide new insights into the forces controlling these solar eruptions and demonstrate how scientists could improve predictions of when a CME will reach Earth.

What Are Coronal Mass Ejections?

The Sun may look calm from Earth, but its atmosphere is filled with constantly changing magnetic fields and extremely hot, electrically charged particles called plasma.

Sometimes, these magnetic fields become unstable and release enormous amounts of energy. This can launch a massive cloud of plasma and magnetic fields into space. Such an event is called a coronal mass ejection.

CMEs can travel at speeds of up to approximately 2,500 kilometres per second. They can also carry enormous amounts of material and energy across the solar system.

When a CME reaches Earth, its magnetic field can interact with our planet's magnetic environment, called the magnetosphere. Depending on the strength and orientation of the incoming magnetic field, this interaction can trigger geomagnetic storms.

These storms may disrupt satellite operations, interfere with GPS navigation and radio communications, damage electrical infrastructure in extreme circumstances, and increase radiation risks for astronauts and some high-altitude flights.

Understanding how CMEs travel through space is therefore essential for improving space weather forecasting and protecting modern technology.

Why Is Tracking a Solar Explosion So Difficult?

One of the biggest challenges in studying CMEs is determining their actual three-dimensional shape and movement.

Traditional solar observations often capture images from a single viewpoint. These images show only a two-dimensional projection of an event that is happening in three-dimensional space.

As a result, scientists may struggle to distinguish between a CME genuinely changing direction and one simply appearing to move because of the viewing angle. Its actual size and expansion can also be difficult to measure accurately.

NASA launched the Solar Terrestrial Relations Observatory, known as STEREO, in 2006 to help overcome this problem.

The mission used two spacecraft orbiting the Sun, one ahead of Earth and the other behind it. Their different viewing angles allowed scientists to observe the same solar eruption from two perspectives.

Although these observations improved researchers' understanding of CME movement, reconstructing the complete shape of an expanding eruption remained difficult. Earlier techniques often relied on assumptions about its geometry or tracked individual features without fully accounting for the curvature of the CME's front.

To address these limitations, Byrne and his colleagues developed a new method called elliptical tie-pointing.

A New Technique Reveals the CME's True Shape

The elliptical tie-pointing technique uses images from both STEREO spacecraft to reconstruct the CME's front in three dimensions.

Instead of treating the eruption as a collection of isolated points or assuming that it has a simple, fixed shape, the method accounts for the curvature of its leading edge.

This additional information helps researchers identify where the observed features are located in space and reconstruct how the CME changes over time.

By applying the technique to successive images, the team followed an Earth-directed CME as it moved away from the Sun.

The researchers tracked its development from approximately two to 46 solar radii, equivalent to around 0.2 astronomical units. One solar radius is approximately 696,000 kilometres, while one astronomical unit is the average distance between Earth and the Sun.

This reconstruction allowed the team to examine changes in the CME's direction, angular width and speed with greater confidence than conventional two-dimensional observations alone.

The Solar Explosion Changed Direction and Expanded

One important finding was that the CME did not travel straight outward from its original source region.

Instead, it began at a relatively high solar latitude and gradually moved toward the Sun's equatorial region, where Earth's orbit lies.

Scientists have previously suggested several possible explanations for this behaviour. Magnetic fields associated with polar coronal holes may guide eruptions toward lower latitudes. The surrounding solar magnetic field, solar wind and the internal magnetic structure of the CME may also influence its path.

The researchers' reconstruction provided a clearer picture of this non-radial movement, although the precise contribution of each possible mechanism remains a subject for further investigation.

The team also observed that the CME expanded significantly as it travelled away from the Sun.

Its angular width did not increase at a constant rate. Instead, the eruption underwent stronger angular expansion during its early development, with the expansion becoming more limited farther from the Sun.

This distinction is important because a CME's apparent growth in an image does not always represent its actual physical expansion. Some of the observed change can result from the viewing angle.

By reconstructing the CME in three dimensions, the researchers could better separate genuine expansion from these projection effects.

The results suggest that pressure differences between the erupting plasma and its surroundings, together with the changing geometry of the solar wind, play important roles in shaping the eruption.

How the Solar Wind Controls the CME's Speed

The researchers also investigated what happens when a CME travels farther into interplanetary space.

Close to the Sun, the eruption is strongly influenced by the energy released during its launch. Magnetic forces and pressure can accelerate the expanding cloud rapidly.

However, as the CME moves farther away, its interaction with the surrounding solar wind becomes increasingly important.

The solar wind is a continuous stream of charged particles flowing outward from the Sun. A CME moving through this stream experiences a drag force, similar in principle to the resistance an object encounters while moving through a fluid.

The direction of this force depends on the difference between the CME's speed and the speed of the surrounding solar wind.

A fast CME can slow down when it moves faster than the solar wind. A slower CME can accelerate when the solar wind moves faster than the eruption.

In this particular event, the researchers found that drag became important beyond approximately seven solar radii. The CME's motion was consistent with a quadratic aerodynamic drag regime, in which the drag force depends on the square of the relative speed.

As the eruption continued, its velocity moved closer to that of the surrounding solar wind.

The team also found that a slower solar wind stream ahead of the CME influenced its later motion, helping explain why its arrival could not be predicted accurately by considering only its initial speed.

This finding highlights why the speed measured near the Sun does not always tell scientists how quickly a CME will reach Earth.

Better Predictions of When a CME Will Reach Earth

One of the most significant outcomes of the research was the improved prediction of the CME's arrival time at the Lagrangian L1 point, a location approximately 1.5 million kilometres from Earth toward the Sun.

Spacecraft positioned near L1 can monitor the solar wind and magnetic conditions before they reach Earth's magnetosphere. These observations provide valuable early warnings of potentially dangerous space weather.

To estimate the CME's arrival, the researchers used their reconstructed 3D structure and motion as inputs for a three-dimensional magnetohydrodynamic simulation.

Magnetohydrodynamics, or MHD, is a method used to model the behaviour of electrically conducting fluids, such as solar plasma, in the presence of magnetic fields.

By incorporating the CME's changing shape and trajectory, the simulation could better account for its interaction with the solar wind.

The resulting model provided a more accurate arrival-time prediction for this particular event.

Such improvements could eventually help scientists issue more reliable warnings about solar eruptions that threaten satellites, power systems and other technologies.

However, one successful reconstruction does not mean that all CMEs can now be predicted with the same accuracy. Different eruptions have different magnetic structures, trajectories and surrounding solar wind conditions.

What Comes Next?

Despite this progress, many questions about CMEs remain unanswered. Scientists still need better theoretical models to explain how these eruptions begin, why they sometimes change direction and how their internal magnetic structures evolve during their journey through space.

Future observations from advanced solar missions, including the European Space Agency's Solar Orbiter and NASA's Parker Solar Probe, can help researchers investigate the physical processes responsible for these enormous eruptions.

Combining such observations with improved 3D reconstruction methods and realistic computer simulations could further strengthen space weather forecasting.

Conclusion

The research by Jason Byrne and his team demonstrates how a more accurate three-dimensional view can reveal the hidden physics behind a solar eruption. By reconstructing a CME's curved front, the researchers tracked its deflection, measured its changing expansion and showed how interaction with the solar wind controlled its later movement.

Most importantly, their work helped improve predictions of when the eruption would reach the L1 monitoring point near Earth.

As society becomes increasingly dependent on satellites, GPS navigation and electrical infrastructure, understanding how solar eruptions travel through space is more important than ever. Although scientists cannot yet predict every aspect of these events, advances in 3D imaging and computer modelling are bringing us closer to forecasting potentially dangerous space weather before it reaches our planet.

Reference: Byrne, J., Maloney, S., McAteer, R. et al. Propagation of an Earth-directed coronal mass ejection in three dimensions. Nat Commun 1, 74 (2010). https://doi.org/10.1038/ncomms1077