What drives a geomagnetic storm
Coronal mass ejections, fast wind from coronal holes and substorms, and how long each takes to reach us.
A geomagnetic storm is a period when the solar wind pours unusual amounts of energy into Earth's magnetic field. Almost all big aurora nights have one of two causes.
Coronal mass ejections (CMEs)
A CME is a cloud of billions of tonnes of plasma, with its own magnetic field, thrown out from the Sun, often alongside a solar flare from a complex sunspot group. If it's aimed at Earth, it usually arrives 1 to 4 days later; the fastest have taken less than a day.
What happens when it arrives depends mostly on the direction of its magnetic field. If it points south for hours, the result can be a severe storm and aurora far from the pole. If it points north, even a fast CME may do little. That direction can't be measured until the CME passes the spacecraft upstream of Earth, so CME forecasts always carry uncertainty. See Limits of forecasting.
On the Sun tab, CMEs show in the coronagraph views as bright clouds leaving the Sun. A halo CME, spreading all round the disc, is heading straight towards Earth or directly away from it.
Coronal holes and fast wind
A coronal hole is a region where the Sun's magnetic field opens out into space. It appears dark in extreme-ultraviolet images, and fast solar wind (600 km/s or more) streams out of it. When a hole faces Earth, the fast wind usually arrives 2 to 4 days later. These storms are usually milder than CME storms, Kp 4 to 5 or so, but they're steadier and last longer.
Because the Sun turns about once every 27 days as seen from Earth, a long-lived coronal hole comes round again a month later. That's why active nights often repeat about 27 days apart, and why NOAA's 27-day outlook is useful for planning.
Substorms
A substorm is not a storm in itself but a sudden release of energy stored in the magnetosphere's tail. The aurora brightens fast, spreads across the sky and starts to dance, often for 10 to 30 minutes. Substorms can happen several times in one night, and even on otherwise unremarkable nights. A sharp rise in Hemispheric power often marks one; Chaser can alert you to it.
Flares
A solar flare is a burst of light and X-rays that reaches Earth in about 8 minutes. Flares don't cause aurora themselves, but large ones often come with a CME that can bring aurora 1 to 4 days later.