Auroral Arc
Definition
The classic ribbon of aurora — a smooth, elongated band stretching east–west across the sky.
In plain language
Most aurora nights begin with a single quiet arc low on the poleward horizon. Arcs can hold steady for hours — or sharpen, split, and erupt into curtains when a substorm strikes.
The science
Discrete arcs map field-aligned acceleration regions where quasi-static potential drops or Alfvénic turbulence energize electrons along magnetic field lines, producing thin (sub-km to few-km) curtains extending over 1,000+ km.
Why it matters for aurora viewing
The arc is your baseline: watch its brightness and lower edge. A brightening, rippling arc is the classic minutes-before-breakup warning.
See this in action on tonight's data.