SUN · FIELD GUIDE
What Is the Solar Wind — and Why Bz Matters
Aurora chasers obsess over one number — 'Bz is going south.' Here's what the solar wind is and why that one value decides the night.
The Sun doesn't just shine — it blows. A steady gale of charged particles streams off its scorching outer atmosphere, fills the entire solar system, and rushes past Earth day and night. That's the solar wind, and reading it is the key to aurora forecasting.
A wind made of plasma
The wind is plasma — mostly protons and electrons — escaping the Sun's million-degree corona faster than the Sun's gravity can hold it back. Two numbers describe it:
- Speed — usually 300–400 km/s, climbing to 700–800+ km/s in a high-speed stream.
- Density — how many particles per cubic centimetre.
A fast, dense wind packs more energy. But here's the twist that catches people out: speed and density alone don't guarantee aurora.
The number that decides the night: Bz
Tangled into the solar wind is a piece of the Sun's own magnetic field, called the interplanetary magnetic field (IMF). The component that matters is its north–south tilt, written Bz and measured in nanotesla (nT).
- Bz positive (northward) — the wind's field points the same way as Earth's at the front of the magnetosphere. They repel; little energy gets in. Quiet skies.
- Bz negative (southward) — the two fields point opposite ways and snap together like two bar magnets, opening a door that lets the wind's energy pour into Earth's upper atmosphere. That energy excites the air, and it glows: aurora.
This is why seasoned aurora watchers care more about a sustained southward Bz than about the headline Kp index. Kp is a backward-looking, three-hour average; Bz is the live switch.
Why the forecast is only an hour ahead
These readings come from NASA's DSCOVR spacecraft, parked at the L1 point about 1.5 million kilometres toward the Sun. It samples the wind 15 to 60 minutes before that same wind reaches Earth — a priceless but short head start. That's the honest limit of aurora forecasting: beyond an hour or so, nobody can be sure what the Sun will send next.
Putting it together
Watch the live solar wind: if the speed is up and Bz has turned firmly negative and is holding there, the odds are good. Then check whether that's enough to reach your latitude on the aurora forecast. The Sun supplies the energy; Bz opens the door; your latitude decides whether the show reaches you.
Frequently asked questions
What is the solar wind?
The solar wind is a continuous stream of charged particles — mostly protons and electrons — flowing outward from the Sun's outer atmosphere in all directions. It fills the solar system, blows past Earth at hundreds of kilometres per second, and carries a piece of the Sun's magnetic field with it.
What is Bz, and why does it matter?
Bz is the north–south component of the magnetic field embedded in the solar wind, measured in nanotesla (nT). When Bz turns negative (southward), it links up with Earth's own magnetic field and pours energy into the upper atmosphere — which is what lights up the aurora. A sustained, strongly southward Bz is the single best predictor of a good aurora night.
How fast is the solar wind?
Typically 300–400 km/s, but high-speed streams can reach 700–800 km/s or more. Faster wind carries more energy, but on its own a fast wind won't produce aurora unless Bz is also southward.
Where are these numbers measured?
By NASA's DSCOVR spacecraft at the L1 Lagrange point, about 1.5 million kilometres sunward of Earth. It samples the wind 15–60 minutes before it reaches us, which is why a confident aurora forecast is only about an hour ahead.
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