Bz Explained: The Aurora Signal Most Forecasts Ignore
Ask a casual Aurora watcher and they'll talk about Kp. Ask someone who's out chasing the lights three nights a week and they'll talk about Bz. It's the most powerful real-time Aurora signal there is, and the one almost every mainstream forecast leaves out. Here's what it is and how to use it.
What Bz is
The solar wind carries its own magnetic field. Bz is the north–south component of that field, measured in nanotesla (nT), sampled continuously by spacecraft parked about 1.5 million kilometres upstream of Earth, currently NOAA's SOLAR-1 mission, which took over from the retired DSCOVR satellite in 2026.
Bz can be positive (northward) or negative (southward). That sign is everything.
Why southward Bz ignites the Aurora
When Bz turns negative (southward), it points opposite to Earth's own magnetic field at the point where they meet. Opposite fields reconnect, opening a door in Earth's magnetosphere that lets solar wind particles pour in toward the poles. Those particles are what light up the sky.
Why Bz beats Kp for timing
Kp is a three-hour global average, updated slowly. Bz is measured minute by minute, upstream of Earth, giving you 15 to 60 minutes of warning before the effect reaches the ground and before Kp reflects it. If you're watching Bz swing sharply south, you can be outside and ready before the display even begins.
It also explains the nights that confuse people: a Kp 4 with strongly negative Bz can produce a better show than a Kp 5 with neutral or positive Bz. The headline number said the second night was stronger; the sky said otherwise. Bz is why.
How to use it
- Treat sustained Bz below about −5 nT on a clear, dark night as a strong go signal.
- Don't panic over brief flickers: Bz is noisy, so look for it holding south for 10+ minutes.
- Combine it with your local Kp threshold and clear skies. Bz opens the door, but you still need darkness and no cloud.