Space Weather 101
🌐 Bt (Total Magnetic Field)
View live Bt (Total Magnetic Field) data →What is Bt (Total Magnetic Field)?
Bt is the total strength of the interplanetary magnetic field, combining all three of its directional components (Bx, By, and Bz) into one overall magnitude, measured in nanotesla (nT). Where Bz tells you which way the field is pointing, Bt tells you how strong it is overall, calculated as the square root of the sum of each component squared.
Why it matters
Bt sets the ceiling on how strong a geomagnetic effect is even possible. A perfectly southward Bz can't drive much of a storm if the overall field is weak — the strongest storms combine a high Bt with a strongly negative Bz, meaning nearly all of that strong field is oriented favorably.
Typical values
Typical quiet-time Bt sits around 3-6 nT. Values above 10 nT are elevated, and during major storms Bt has been observed climbing past 20-30 nT.
How scientists measure it
Measured by the same L1 spacecraft magnetometers used for Bz, simply combining all three measured components into one magnitude value.
Why it affects Earth
Bt effectively caps the maximum possible geomagnetic effect for a given solar wind stream — forecasters look at Bt alongside Bz's direction to judge just how powerful a potential storm could be.
FAQ
Can Bz be bigger than Bt?
No — Bt is the total combined magnitude, so it's always at least as large as any single component including Bz. When Bz equals Bt (or very close to it), that means almost all of the field is pointed directly north-south.