Four numbers, one that matters most
Every real-time aurora forecast eventually comes down to four measurements of the solar wind: Bz, Bt, speed and density. They look intimidating scrolling past on a NOAA plot, but the practical reading is simple once you know which one to check first -- Bz, by a wide margin. For the full walkthrough of each value and how professional aurora chasers actually read these charts, see reading a solar wind plot.
Bz: the direction that decides everything
Bz is the north-south component of the interplanetary magnetic field carried in the solar wind, measured in a coordinate system oriented to Earth's own field. When Bz pointssouthward -- shown here as a negative number -- it can connect efficiently with Earth's northward-pointing field near the equator through a process called magnetic reconnection, pouring energy into the magnetosphere. That coupling is, more than any other single factor, what actually drives geomagnetic storms and the aurora that comes with them. A northward (positive) Bz does close to the opposite: it tends to shield the magnetosphere, which is why a fast, dense stream can arrive and produce almost nothing if Bz stays positive throughout.
Bt, speed and density: the multipliers
Bt is simply the total strength of the interplanetary magnetic field, independent of direction. It sets an upper bound on how negative Bz could plausibly swing -- a rising Bt ahead of a Bz shift is often an early tell that something significant, frequently the leading edge of a coronal mass ejection, has arrived even before its direction is clear. Speed (typically 300-400 km/s at quiet background levels, up to 700-800 km/s or more in a fast coronal-hole stream, and beyond 1,000 km/s in an unusually fast CME) determines how forcefully the wind compresses Earth's magnetosphere and how much warning time is left once it is detected. Density adds to the dynamic pressure alongside speed. Both matter, but mostly as multipliers on whatever Bz is already doing -- they rarely drive a storm on their own with a persistently northward field.
Reading the trend, not just the instant
The sparklines above show the last 24 hours for each series, because a single instantaneous reading can be misleading -- solar wind data is noisy on short timescales, and what matters for aurora forecasting is usually a sustained trend rather than a one-minute spike. A Bz reading that dips to -8 nT for two minutes and bounces back is a very different signal from one that settles at -8 nT and holds for an hour. The trend arrows next to Bz and speed compare the current value against the median from six hours earlier, giving a quick sense of direction without having to eyeball the whole chart.
None of these four numbers alone answer "will I see the aurora tonight" -- that also depends on your geomagnetic latitude, whether it is dark, and the moon phase at your specific location. Get a full personal verdict that folds all of it together, or check the live Kp index for the more commonly cited single-number summary that this solar wind data ultimately feeds into.
Frequently asked questions
Which number should I check first?
Bz. A strongly negative (southward) Bz is the single best quick indicator that conditions favour aurora, regardless of what speed and density are doing. If Bz is positive, high speed and density rarely translate into much visible activity.
How negative does Bz need to be to matter?
There is no hard cutoff, but sustained readings below about -10 nT for an hour or more are generally considered a meaningful driver, and sustained readings beyond -20 nT are associated with strong to severe geomagnetic storms. A brief single-minute dip matters much less than a sustained trend.
Why do the charts sometimes show a gap?
The spacecraft measuring this data occasionally lose contact or pass through noisy data periods. A gap is a real limitation of the underlying data, not a bug in this page -- the sparklines break across missing samples rather than drawing a misleading straight line through them.
Where does this data come from and how much warning does it give?
From spacecraft stationed near the L1 Lagrange point, about 1.5 million kilometres from Earth toward the Sun, giving roughly 30 to 90 minutes of advance warning before the same solar wind reaches Earth -- the practical ceiling on how far ahead a genuinely reliable aurora forecast can look.