What Is A Coronal Mass Ejection?
QUICK ANSWER
A coronal mass ejection (CME) is a massive eruption of plasma and magnetic field from the Sun's outer atmosphere (the corona). CMEs can launch billions of tons of solar material into space at speeds over a million miles per hour. When directed at Earth, they can trigger geomagnetic storms.
Coronal mass ejections are some of the most dramatic events the Sun produces. They're enormous bubbles of plasma and magnetic field that erupt from the Sun's corona and travel through space at extreme speeds. When directed at Earth, CMEs can trigger geomagnetic storms, beautiful auroras, and sometimes serious disruptions to satellites and power grids. CMEs are the main drivers of major space weather events.
What is a CME?
A huge eruption of solar plasma. According to NASA, coronal mass ejections are immense clouds of solar material blasted into space by the Sun at over a million miles per hour, often following a solar flare. CMEs can carry billions of tons of solar material and intense magnetic fields. They're triggered by magnetic reconnection events in the Sun's corona, where twisted magnetic field lines suddenly snap and release enormous energy. Each CME is a distinct event, though several can occur in quick succession during active periods.
How fast do they travel?
From relatively slow to extraordinarily fast. The fastest CMEs travel at millions of miles per hour. When directed at Earth, fast CMEs can reach our planet in as little as 15 hours, while slower CMEs can take several days. Most arrive 1 to 3 days after eruption. The arrival speed and density determine how strong the resulting geomagnetic storm will be. CME speed is measured by coronagraphs on spacecraft like SOHO and STEREO, which can detect them within minutes of eruption.
How are CMEs different from solar flares?
They're related but distinct phenomena. Solar flares are sudden bursts of electromagnetic radiation from active regions on the Sun, lasting minutes to hours. CMEs are physical ejections of plasma and magnetic field that take days to travel through space. Flares often (but not always) accompany CMEs. Flares affect Earth's atmosphere through radiation arriving at light speed (8 minutes from Sun), causing radio blackouts. CMEs affect Earth through plasma arriving days later, causing geomagnetic storms. Both can occur together during major solar events.
When are CMEs most common?
Near solar maximum. The Sun goes through an 11-year cycle of activity with alternating periods of high and low activity. CMEs are far more common near solar maximum, when active regions and sunspots dominate the Sun's surface. The current solar maximum (2024-2025) has produced an unusually large number of CMEs, including the May 2024 series that produced the strongest geomagnetic storm in over 20 years. During solar minimum, CMEs are rare but still occur occasionally.
A coronal mass ejection (CME) is a massive eruption of plasma and magnetic field from the Sun's corona. CMEs can launch billions of tons of solar material into space at over a million miles per hour. When directed at Earth, they trigger geomagnetic storms, auroras, and sometimes disruptions to satellites and power grids. CMEs are far more common near solar maximum and are distinct from but often accompanied by solar flares.
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