Coronal Mass Ejection (CME)
Definition
A huge eruption of magnetized plasma blasted from the Sun into space — the main driver of strong auroras.
In plain language
Sometimes the Sun hurls a billion-ton cloud of material outward. If that cloud is aimed at Earth, it slams into our magnetic field one to three days later and can ignite auroras far from the poles.
The science
CMEs are large-scale expulsions of coronal plasma and magnetic flux, often following flares or filament eruptions. Earth-directed ("halo") CMEs drive geomagnetic storms whose intensity depends on arrival speed and the embedded field's southward (negative Bz) component.
Why it matters for aurora viewing
CME arrivals produce the best mid-latitude aurora nights of the solar cycle. When one launches, you get 1–3 days' notice to plan.
See this in action on tonight's data.