Plain-language answer: A coronal mass ejection is a large eruption of magnetized plasma from the Sun into space.
Why it matters
Only some CMEs intersect Earth; arrival time, speed, density, and magnetic orientation determine how strongly the solar wind couples to the magnetosphere.
Use it correctly
Check whether the eruption is Earth-directed, follow updated arrival forecasts, then watch solar-wind and geomagnetic observations rather than assuming impact.
The common mistake
Treating every visible solar eruption as an imminent geomagnetic storm at Earth.
Decision boundary
Current regulation, program rules, manufacturer limits, exact station configuration, and live observations control. Recheck changing rules, solar conditions, and operating status before acting.
