IMF Bt
Definition
The total strength of the interplanetary magnetic field, regardless of direction.
In plain language
Bt tells you how much magnetic muscle the solar wind is carrying. A big Bt is potential: if even part of that strong field tilts southward, a lot of energy can flow into Earth's system.
The science
Bt is the vector magnitude of the IMF (√(Bx²+By²+Bz²)), typically 3–8 nT in quiet wind and 10–40+ nT inside CMEs. High Bt with strongly negative Bz is the classic signature of a storm-driving CME passage.
Why it matters for aurora viewing
Read Bt and Bz together: high Bt sets the ceiling, negative Bz decides how much of it actually reaches the aurora.
See this in action on tonight's data.