Real-Time Solar Wind Speed & Density

These numbers come from a spacecraft, not a model. NOAA's real-time solar wind feed carries plasma and magnetic field measurements taken at the L1 Lagrange point, about 1.5 million kilometres sunward of Earth — roughly one percent of the way to the Sun — where DSCOVR sits with the older ACE spacecraft as a backup. Whatever passes the spacecraft reaches us next: about an hour at an ordinary 400 km/s, half that in a fast stream, and as little as a quarter of an hour ahead of the strongest shocks. That is the entire warning window, and it is why this reading is a measurement rather than a forecast.

Speed is the headline and the least interesting of the three. The ambient wind blows at 300 to 500 km/s. When a coronal hole rotates to face Earth it opens a high-speed stream that lifts it to 600 or 800 km/s and holds it there for a day or two. A coronal mass ejection announces itself differently, as a shock: speed, density, temperature and field strength all jump within a minute or two. That simultaneous jump is the clearest single signal in the feed that something is about to happen on the ground.

Bz is the reading that decides whether any of it matters. It is the north-south component of the interplanetary magnetic field, and when it turns negative — southward — it lies antiparallel to Earth's field at the dayside, reconnection peels the magnetosphere open, and energy pours in. A strong storm needs Bz held south, several nT or more, for hours; a brief dip does little. Bt, the total field strength, is the ceiling, since Bz can never exceed it. Fast wind with a northward Bz is a quiet night.

The sequence to watch is southward Bz first, then a rising Kp index an hour or two later as the ground magnetometers register the substorms, and the aurora along with it. Days earlier, the disturbance usually started as an eruption you can trace on the flare monitor.