How Coronal Mass Ejections Work
A coronal mass ejection (CME) is the largest kind of solar eruption. When a tangled magnetic loop in the Sun's corona snaps and reconnects, it launches a bubble of magnetized plasma weighing up to a billion tons into space at hundreds to thousands of km/s.
The Journey to Earth
A typical CME takes 1โ3 days to reach Earth. The fastest observed CMEs, like the July 2012 event, cross the distance in under 20 hours. Slower CMEs can take 4โ5 days. The Drag-Based Model uses ambient solar wind speed to predict transit time and impact.
What Happens on Arrival
The CME's leading shock compresses Earth's magnetosphere, causing a sudden sharp jump in ground magnetometers called an SSC (storm sudden commencement). If the CME's internal magnetic field is southward, sustained reconnection drives a geomagnetic storm and aurora for many hours.
Halo CMEs
When a CME is launched directly toward Earth, coronagraph images show it as an expanding ring around the Sun โ a 'halo CME'. These are the events forecasters watch most carefully because they'll hit head-on rather than glance past.
Frequently Asked Questions
Do all CMEs cause storms?
No. Only Earth-directed CMEs with a southward internal magnetic field produce significant storms. Glancing blows and northward-field CMEs often pass with little effect.
How is a CME different from a flare?
A flare is a burst of light; a CME is a physical cloud of matter. They often occur together but not always.