What is Bz and Why It’s Better Than Kp for Predicting the Northern Lights?
Bz is the vertical magnetic component of the solar wind’s Interplanetary Magnetic Field (IMF), and it’s the key to predicting Northern Lights activity. Think of it as an “on/off switch” for the aurora:
- Negative Bz (south-pointing) turns the aurora “on.”
- Positive Bz (north-pointing) turns it “off.”
When Bz is negative, Earth’s magnetic field connects with the solar wind, allowing charged particles to enter the atmosphere and create stunning auroras. The lower the Bz value (e.g., -5 to -20), the brighter and more active the Northern Lights become.
Why Bz Beats the Kp-Index
Unlike the Kp-index, which provides generalised forecasts, Bz offers real-time data. When the Bz turns south, it takes about 15–60 minutes for the aurora to appear as the solar particles reach Earth’s atmosphere.
This makes tracking Bz the most reliable way to predict the Northern Lights.
In Iceland, even a slightly negative Bz (-1 to -5) can produce auroras, while deeper drops (-10 or lower) lead to unforgettable displays. For accurate Northern Lights forecasts, always keep an eye on Bz as it will flip with a short notice!
