Hemispheric Power (HPI)
Definition
An estimate, in gigawatts, of the total auroral energy raining into one hemisphere.
In plain language
HPI answers "how bright is the whole oval right now?" Quiet times run 5โ20 GW; strong storms push past 100 GW โ hundreds of power plants' worth of energy lighting the sky.
The science
HPI is derived from satellite particle measurements and auroral models, estimating integrated precipitating energy flux per hemisphere. It updates frequently and tracks auroral luminosity better than 3-hour Kp.
Why it matters for aurora viewing
A fast-refreshing brightness gauge: rising HPI is a real-time cue that the oval is intensifying, often before official indices catch up.
See this in action on tonight's data.