L1 Point
Definition
The gravitational balance point a million miles sunward of Earth where solar wind monitoring satellites are stationed.
In plain language
Park a satellite at L1 and it hovers between Earth and Sun, sampling the solar wind before it reaches us. Everything passing L1 hits Earth about 30–60 minutes later — our early-warning buoy.
The science
The Sun–Earth L1 Lagrange point (~1.5 million km sunward) allows continuous in-situ solar wind measurement. Monitoring spacecraft there supply the real-time speed, density, and IMF data underpinning short-term aurora forecasting.
Why it matters for aurora viewing
The 30–60 minute L1 lead time is the entire basis of "go outside now" aurora nowcasting — it is where live telemetry numbers come from.
See this in action on tonight's data.