Substorm

Auroras don't glow steadily — they surge. The magnetosphere quietly banks energy for one to three hours, then unloads it all at once, sending the sky from a dim arc to full rippling curtains. That surge is a substorm.

Substorms follow a growth–expansion–recovery sequence: dayside reconnection loads flux into the magnetotail, tail reconnection then dipolarizes the field, injecting energetic electrons that produce auroral breakup, typically recurring several times nightly during active periods.

The substorm is the show. If the sky looks quiet, wait — the loading/unloading cycle means eruptions often come in waves all night.