Space Weather 101
🌊 Coronal Mass Ejections (CMEs)
View live Coronal Mass Ejections (CMEs) data →What is Coronal Mass Ejections (CMEs)?
A coronal mass ejection (CME) is a massive burst of plasma and magnetic field launched from the Sun's corona into space, often (but not always) associated with a large solar flare. Unlike the constant solar wind, a CME is a distinct, one-time event — essentially a giant cloud of solar material hurled outward, which can be Earth-directed or aimed elsewhere entirely.
Why it matters
CMEs are the primary driver of the most severe geomagnetic storms. When an Earth-directed CME arrives, it compresses and disturbs Earth's magnetosphere far more dramatically than the ordinary solar wind alone, making CME tracking central to space weather forecasting.
Typical values
CME speeds vary enormously, from slow ones around 250-300 km/s to fast, energetic ones exceeding 2,000-3,000 km/s. Typical Earth-directed transit time is 1-3 days, though the fastest recorded events have arrived in under 18 hours.
How scientists measure it
Coronagraphs — instruments that block out the Sun's bright disk to reveal the much fainter corona around it — image CMEs as they erupt and expand outward, allowing scientists to track their speed and estimate whether they're Earth-directed.
Why it affects Earth
When a CME's magnetic field connects favorably with Earth's (similar to the Bz effect), it can trigger major geomagnetic storms, vivid aurora at unusually low latitudes, and — in the most extreme cases — real disruption to power grids and satellite operations.
FAQ
How long does a CME take to reach Earth?
Typically 1-3 days, though speed varies enormously — the fastest recorded CMEs have arrived in well under 24 hours, while slower ones can take 3-5 days or more.
Does every CME cause a geomagnetic storm?
No — only Earth-directed CMEs matter for Earth, and even then, the resulting storm's strength depends heavily on the CME's magnetic field orientation (its effective Bz) when it arrives.