What is a CME?
A coronal mass ejection (CME) is a massive eruption of plasma (charged particles) and magnetic field from the Sun's corona. Unlike a flare (radiation), a CME is actual matter and field moving through space. CMEs can contain billions of tonnes of material and travel at speeds from about 300 to over 2000 km/s. When they are Earth-directed, they can reach our planet in roughly 1–3 days and trigger geomagnetic storms.
CMEs and Flares
CMEs often occur together with solar flares, but they are distinct: a flare is electromagnetic radiation; a CME is mass and field. Not every flare has a CME, and not every CME is associated with a strong flare. Spacecraft such as SOHO and STEREO image CMEs and help forecast arrival time and impact.
Effects on Earth
When a CME hits Earth's magnetosphere, it compresses and disturbs the magnetic field. The Kp index rises, aurora becomes more likely (including at mid-latitudes for strong events), and power grids, satellites, and radio can be affected. Cosmic Radar shows reported CMEs in the daily report so you can see when Earth-directed events are expected.
Sources and further reading
- NOAA – Coronal mass ejections – CME overview and forecasting
- SOHO LASCO – CME imagery
- NASA DONKI – Space weather events catalogue