Space Encyclopedia
☄️ Coronal Mass Ejections (CMEs)
Overview
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.
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.
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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.
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