Polar Cap
Definition
The region poleward of the auroral oval, threaded by magnetic field lines open to the solar wind.
In plain language
Inside the auroral ring lies a quieter zone where field lines connect directly to interplanetary space. Auroras here are subtler โ faint sun-aligned arcs rather than the oval's bright curtains.
The science
The polar cap maps open flux connected to the IMF; its area grows during flux loading and shrinks after substorms. Polar cap arcs and patches occur mainly under northward IMF conditions.
Why it matters for aurora viewing
If you travel extremely far north (Svalbard in winter), you may sit poleward of the show โ sometimes the best aurora is south of you.
See this in action on tonight's data.