How to read solar wind data
These numbers are measured upstream, at L1 between the Sun and Earth, and describe wind that has not reached us yet: the travel time above is how long it still has to go.
Bz
Bz is the north–south part of the interplanetary magnetic field (IMF) the wind carries, and the number that decides most. Read it by direction and by duration. Published storm criteria all ask for southward Bz held for hours — two hours at −5 nT or below for a moderate storm, more than three at below −10 nT for an intense one. That is why the signal above waits before it changes, and why a short swing to the south in the sparkline is not yet news.
Speed
Faster wind carries far more energy: it grows with the square of the speed. Slow wind runs at 300–500 km/s, wind from coronal holes at 500–800, and wind behind a coronal mass ejection (CME) can reach about 2,000 km/s. Speed and Bz work together as one number, the electric field Ey: 600 km/s with Bz at −5 nT gives 3 mV/m, the level of the moderate-storm criterion.
Density
Density adds pressure to the wind that hits Earth's magnetic field. It is not what opens the way in: that is southward Bz, and a northward Bz largely seals the field whatever the density.
Bt
Bt is the total strength of the magnetic field, all three directions together; Bz is its north–south part. A strong Bt matters for aurora when much of it points south — read it next to the Bz row, not on its own.
Hemispheric power
This one is not measured: it is the OVATION model's estimate of the aurora's energy, computed from the wind above. It is the quickest summary of how bright and wide the auroral oval is right now; the aurora map shows where it lies.
How far ahead this tells you
There is no Bz forecast: Bz is measured at L1, not predicted. The signal describes the next few hours, because Earth's magnetic field responds one to four hours after southward Bz. For tonight, see where the aurora is likely tonight and Kp now and the 3-day forecast; for the weeks ahead, the 27-day outlook. What any Kp means where you are is in the Kp index guide.
