Solar Wind Speed
Definition
How fast the solar wind is moving past Earth, usually 250–500 miles per second.
In plain language
Speed is the punch behind the wind. Slow wind around 300 km/s rarely excites much; fast streams above 600 km/s rattle the magnetosphere and keep high-latitude skies active even without a storm.
The science
Measured upstream at L1, speed ranges ~300 km/s (slow wind) to 800+ km/s (coronal hole streams, CMEs). Coupling functions scale with speed multiplied by southward field, so velocity amplifies the effect of any negative Bz.
Why it matters for aurora viewing
A jump in speed on live telemetry often marks a stream or CME arrival — the moment to start watching Bz closely.
See this in action on tonight's data.