Scientific Overview & Physics
A coronal mass ejection (CME) is a massive eruption of plasma and magnetic field from the Sun's corona, hurled into space at speeds that can exceed 3,000 km/s for the fastest events. When an Earth-directed CME arrives, typically 1 to 4 days after eruption, it can compress the magnetosphere and — especially if it carries a southward-oriented magnetic field — trigger the strongest geomagnetic storms, including the historic Carrington Event of 1859.
How It Works
A coronal mass ejection is a huge cloud of magnetised plasma, often a billion tonnes of material, thrown out when magnetic energy stored in the Sun's atmosphere is suddenly released. CMEs often, but not always, accompany solar flares, and they can also launch when a filament of cooler gas erupts. The cloud expands as it travels. Fast ones cross the 150 million kilometres to Earth in 15 to 18 hours; typical ones take two to four days. Satellites such as SOHO and the STEREO and GOES coronagraphs block the Sun's disc so the faint cloud is visible, and scientists use those images to estimate its speed, width and direction. The key question for Earth is whether the cloud is aimed at us. A CME shown as a full or partial halo around the Sun is heading toward or away from Earth; one that appears to the side will miss. If it does arrive, what matters most is the strength and direction of the magnetic field inside the cloud: a strongly southward field produces a major geomagnetic storm.
What You Can See or Experience
At the Sun, coronagraph images show a bright expanding bubble. At Earth, an arriving CME is signalled by a jump in solar wind speed, density and magnetic field, followed by a long stretch where the field rotates smoothly. If the field turns south, the aurora can spread far beyond the polar regions, sometimes to mid-latitudes. Our dashboard shows the arrival as a spike on the solar wind and Bz panels.
Connection to Earth and Everyday Life
Large CMEs are the main cause of severe space weather. They can disturb power grids, degrade GPS and radio, force satellites to reorient, and raise radiation exposure for polar flights. The 1859 Carrington event and the May 2024 storm both came from CMEs. Forecasts of arrival time are usually good to within about 10 hours, but the strength on arrival remains hard to predict until the cloud passes the L1 monitoring spacecraft, which gives under an hour of warning.
Live Real-Time Telemetry & Tools
Key facts
- •Typical travel time to Earth: roughly 15 hours to a few days, depending on speed
- •Fastest recorded CMEs have exceeded 3,000 km/s
- •Often, but not always, associated with a solar flare at the same active region
- •Tracked in real time by coronagraphs like those on SOHO and STEREO
- •The severity of the resulting storm depends heavily on the CME's internal magnetic field orientation
Why it matters
CMEs are the primary driver of the most severe geomagnetic storms — the events capable of pushing aurora to unusually low latitudes, disrupting power grids, and degrading satellite operations. Forecasting a CME's arrival time and orientation is one of the central challenges in space weather prediction.
Explore related on this site
Frequently asked questions
Is a CME the same thing as a solar flare?
No, though they're often related. A solar flare is a burst of electromagnetic radiation that reaches Earth in about 8 minutes; a CME is a separate eruption of physical plasma and magnetic field that takes much longer — typically a day or more — to arrive, if it's Earth-directed at all.
How is a CME's arrival time predicted?
Forecasters track a CME's speed and trajectory from coronagraph imagery, then model its transit through interplanetary space to estimate arrival time — typically with an uncertainty window of several hours to about a day.
Is every CME dangerous?
No. Most CMEs miss Earth, and many that hit are weak. Only fast, well-aimed CMEs with a strongly southward magnetic field cause strong geomagnetic storms.
How is a CME different from a solar flare?
A flare is a burst of light and radiation that reaches Earth in about eight minutes. A CME is a cloud of plasma and magnetic field that takes one to four days. They often occur together but are different events.
How much warning do we get?
Days of watch time once a CME is seen leaving the Sun, then a more precise 20 to 60 minute warning when it passes the spacecraft at the L1 point just before reaching Earth.
Conclusion
A CME is the slow, heavy cousin of a solar flare. The flare reaches Earth in eight minutes as light; the CME arrives days later as matter. Watch the coronagraph images for the launch, the forecast for arrival time, and Bz for the moment it matters.