The aurora looks like weather — it drifts, brightens, and fades like something in the sky above you. But the event that actually lights it up doesn't start in the sky at all. It starts on the sun, roughly 150 million kilometers away, and by the time you see the result, you're watching the last step of a chain that began minutes to days earlier.
Step one: the sun never stops exhaling
The sun constantly sheds a stream of charged particles — mostly protons and electrons — called the solar wind. It's always blowing, even on a quiet day, at somewhere around 300–400 kilometers per second. Coronal holes (cooler, darker patches on the sun where the magnetic field opens outward instead of looping back) let this wind escape faster, sometimes past 600 km/s. Bigger events — coronal mass ejections, dense clouds of solar material thrown outward by a solar flare — arrive as a sudden surge on top of that background flow, typically one to three days after they leave the sun.
Step two: Earth's magnetic field catches it — mostly
Earth isn't defenseless against this. Our planet's magnetic field forms a bubble called the magnetosphere, and for most of the solar wind, it works like a shield: charged particles get deflected around it, the same way water splits around the bow of a boat.
The deciding factor is a single number Moose tracks constantly: Bz, the north–south orientation of the magnetic field the solar wind is carrying with it. When Bz points north, it reinforces Earth's own field, and the shield holds. When Bz swings south — even briefly — it partially cancels Earth's field on the sunward side. That's the opening. Magnetic reconnection lets solar wind energy pour into the magnetosphere instead of sliding past it.
Step three: the tail stretches, then snaps back
Energy that gets in doesn't show up instantly. It first stretches Earth's magnetic field into a long tail pointing away from the sun, like a windsock filling with air. That tail can only stretch so far before it becomes unstable. When it does, the field snaps back toward a more relaxed shape in a sudden, explosive release called a substorm — the event responsible for the aurora suddenly brightening, forming rippling curtains, and sometimes bursting into fast-moving color across a huge stretch of sky, all within a few minutes.
This is also why a substorm can seem to catch the official Kp index off guard: Kp is only recalculated every three hours from ground magnetometers, so a sharp southward Bz turn is often the earliest real signal that a brightening is imminent — sometimes 30 to 60 minutes before anything else confirms it.
Step four: the particles finally reach you — sort of
The released energy accelerates electrons down Earth's magnetic field lines toward the poles, where the field lines converge and dip into the atmosphere. Around 100–300 km up, those electrons slam into oxygen and nitrogen molecules, exciting their electrons to a higher energy state. When those electrons relax back down, they release the energy as light — the aurora itself.
- Green — oxygen, around 100–150 km up. The most common color, and usually the brightest.
- Red — oxygen again, but higher up (200 km+), where collisions are rarer and the atoms need longer to release their energy. Usually only visible during stronger events.
- Blue and purple — nitrogen, typically lower in the atmosphere, often at the bottom edge of a bright curtain.
So why does Moose watch three separate numbers?
Because the chain above has three genuinely different failure points. Kp tells you how much energy has already built up geomagnetically — a lagging, three-hour-resolution summary. Bz tells you whether the door is currently open for more energy to get in — a live, fast-moving signal. Solar wind speed tells you how forcefully anything arriving is likely to hit. Watching just one is like checking only the temperature and assuming you know the whole forecast.
