Space Weather
IMF Bz — why direction matters more than strength
Almost nobody explains this well: Bz isn't just another number to watch — its direction is arguably the single best predictor of whether tonight's aurora activity picks up.
Live IMF Bz
Source: NOAA DSCOVR/ACE
What is IMF Bz?
The interplanetary magnetic field (IMF) is the Sun's magnetic field, carried outward through space by the solar wind. Bz is the north-south component of that field, measured in nanotesla (nT) by spacecraft positioned upstream of Earth. Bz can point either way: positive means northward, and negative means southward.
Opposes Earth's own northward-pointing magnetic field near the equator. This allows magnetic reconnection — solar wind energy connects directly with Earth's magnetosphere and pours in, driving geomagnetic storms. This is the condition aurora chasers watch for most closely.
Reinforces Earth's own magnetic field instead of opposing it. The magnetosphere stays largely closed to incoming solar wind energy, so even a fast solar wind stream is unlikely to trigger strong aurora activity while Bz stays northward.
Recent history
The last 24 hours of observed IMF Bz. Dips below zero mark southward stretches — the windows worth watching for aurora.
FAQ
How negative does Bz need to be for strong aurora?
There's no hard cutoff, but as a general guide: Bz around -5 nT sustained for a while often coincides with minor (G1) activity, -10 nT or beyond with moderate-to-strong storms, and readings of -20 nT or lower are associated with the most severe geomagnetic storms on record.
Does Bz matter more than solar wind speed?
They work together. Fast solar wind with a northward Bz usually produces little geomagnetic activity, while even moderate-speed wind with a strongly southward Bz can drive a solid storm. Southward Bz is generally considered the more decisive factor of the two.
Can Bz flip direction quickly?
Yes — Bz can swing between northward and southward within minutes to hours, especially inside a coronal mass ejection's magnetic structure. That's why this page's live reading is worth checking frequently during active space weather.