Solar Wind Bz: Why "South Is Good" for Aurora

If Kp is the tide, Bz is the surf. Bz measures the north–south orientation of the interplanetary magnetic field (IMF) carried by the solar wind — and its sign decides whether the solar wind can actually dump energy into Earth's magnetosphere.

The physics in one paragraph

Earth's magnetic field points north at the equator. When the solar wind's field points south (negative Bz), the two fields oppose each other and connect — a process called magnetic reconnection. That connection opens a conduit for solar wind energy to pour into the magnetosphere, ultimately powering the aurora. When Bz points north, the fields align, reconnection shuts down, and even fast solar wind mostly slides past.

How to read the number

Space-weather feeds report Bz in nanotesla (nT). As a rule of thumb: Bz near zero is neutral; sustained Bz of −5 nT or stronger is good news; −10 nT or stronger for an hour or more is the classic setup for an aurora burst, often culminating in a substorm an hour or two later. Brief southward dips are common and usually fizzle — sustained negative Bz is what matters.

Why burst hunters watch Bz instead of Kp

Kp updates every 3 hours; Bz updates in near-real-time from the DSCOVR and ACE satellites parked a million miles sunward of Earth. Solar wind takes roughly 20–40 minutes to travel from those satellites to us, which means the Bz reading is effectively a 30-minute aurora forecast — the most actionable lead time that exists. Serious chasers watch Bz flip south and head out before Kp moves at all.

How we use it

Our engine pairs the NOAA Kp forecast with the current Kp observation, and our methodology page documents exactly how each factor is weighted. Bz is the factor we recommend watching manually when the answer is MAYBE: a strong southward swing can turn a marginal night around within an hour.

Using Bz in practice

Bz is the "should I go now?" number. When your city's verdict is MAYBE and you can see the sky is clear, watch the Bz trend: a sustained swing to −5 nT or stronger is your green light for the next hour or two, because the solar wind measured at the satellite arrives roughly half an hour later and the magnetosphere responds within the hour. Practical pairing: Bz tells you when the burst is coming; your city page tells you whether your sky can show it. The two together beat either alone.

Common Bz mistakes

Reacting to every dip: brief southward flickers happen dozens of times a day and mostly fizzle — sustained negative Bz of an hour or more is the real signal. Ignoring the delay: people see a −15 nT reading and run outside immediately; the payload arrives 30–90 minutes later, so use the delay to drive somewhere dark. Reading Bz without a sky check: a perfect Bz setup under solid cloud is still a NO — that's what the hard rules on our city pages encode.