Solar Wind
The solar wind is a continuous stream of charged particles blowing off the Sun in every direction. Its speed and density decide how hard it pushes on Earth's magnetic field — and, when the field's tilt is right, how bright the aurora burns.
See the live solar wind →What the solar wind actually is
The Sun's outer atmosphere — the corona — is so hot, over a million degrees, that its own gravity can't hold it down. It boils off into space as a thin plasma of protons and electrons, dragging a piece of the Sun's magnetic field along with it. That outflow is the solar wind, and it never stops: even on a quiet day the Sun is bleeding roughly a million tonnes of material into space every second. By the time it reaches Earth, about 150 million kilometres away, it has thinned out to only a handful of particles per cubic centimetre — a better vacuum than anything we make in a lab, yet it carries enough energy to light up entire polar skies.
Two numbers describe the wind's punch: its speed and its density. A slow, thin wind slides past Earth almost unnoticed. A fast, dense wind slams into the magnetosphere, compresses it, and pumps energy into the system that eventually spills out as the auroral oval around each pole.
Speed and density: the two dials
Solar wind speed typically runs between about 300 and 800 km/s, though extreme gusts from a strong eruption can top 1,000 km/s. The background wind sits near the low end; the excitement comes when speed climbs. Faster wind delivers more energy per second and, just as importantly, tends to carry a more turbulent magnetic field — which raises the odds of the crucial southward tilt that unlocks the aurora.
Density — the number of particles packed into each cubic centimetre — usually sits in the low single digits but can spike sharply, especially at the leading edge of an eruption where the wind piles up. High density means more mass hitting the magnetosphere, a stronger shove, and a brighter, more active display. The two dials work together: fast and dense is the combination that lights up the sky, but only if the magnetic field cooperates. That third ingredient is the Bz component, and it's the real gatekeeper.
Where the wind comes from
Not all solar wind is the same. It arrives in two main flavours, and knowing which one is heading our way tells you a lot about what the night might hold.
Coronal holes
Open regions in the corona where the magnetic field lets plasma escape freely, launching steady high-speed streams. They recur roughly every 27 days as the Sun rotates. Space weather →
CMEs
Coronal mass ejections are violent, one-off blasts of billions of tonnes of plasma. When one is aimed at Earth it arrives as a dense gust that can trigger the strongest storms. Geomagnetic storms →
Background wind
Between events, a slower, calmer stream flows out of the quiet Sun. It keeps the magnetosphere ticking over but rarely lights the sky on its own. Aurora forecast →
Aurora Power
We fold speed, density, Bz and the total field into one 0–9 score so you don't have to watch every dial yourself. See it live →
How it fuels the aurora
Fast, dense wind is only half the story. The magnetosphere is a good shield, and it deflects most of what hits it. The door opens when the magnetic field embedded in the solar wind tilts southward — a negative Bz. A southward field links up with Earth's own northward field on the dayside, letting solar wind energy flow in instead of bouncing off. Now the wind's speed and density really count: with the door open, faster and denser wind loads far more energy into the system, which discharges as substorms and drives the KP index up into storm territory (G1 through G5). When Bz swings north, the door effectively slams shut and even a raging wind fades to a quiet glow.
This is why AuroraSignal watches the wind and the field together. We read the stream from spacecraft parked at the L1 point, about 1.5 million kilometres upstream of Earth — a gravitational sweet spot between us and the Sun. Because the wind reaches those satellites before it reaches us, their measurements give roughly 30 to 60 minutes of warning: enough time to grab a coat and find dark sky before the show arrives. Check whether the pieces are lining up on the tonight page or watch it unfold on the live tracker.
Solar wind FAQ
How fast does the solar wind travel?
Usually between roughly 300 and 800 km/s. The slow background wind hovers near the lower end, while high-speed streams from coronal holes and gusts from a coronal mass ejection can push past 800 km/s — and the shock front of a powerful eruption can briefly exceed 1,000 km/s. Even at these speeds it takes the wind two to four days to cross from the Sun to Earth.
Does faster solar wind always mean aurora?
No — speed and density set the potential, but the magnetic tilt decides whether it's realised. If the Bz component points north, even a fast, dense wind is largely deflected and the sky stays quiet. When Bz turns and holds southward, that same wind can load the magnetosphere and drive a strong storm. Speed matters most once the door is open. More on Bz →
Why measure the solar wind at the L1 point?
L1 is a spot about 1.5 million km sunward of Earth where the Sun's and Earth's gravity balance, so a spacecraft can hover there and sample the wind before it reaches us. Reading it upstream buys roughly 30–60 minutes of lead time on speed, density and the magnetic field — the basis for every short-term aurora forecast. Space weather →
Watch the solar wind in real time
Live speed, density, Bz and a single 0–9 Aurora Power score — pulled from spacecraft upstream of Earth and updated every minute.
Open the live tracker →