IMF Bz
Definition
The north–south component of the solar wind's magnetic field — the single most important real-time aurora number.
In plain language
Think of Bz as the position of Earth's magnetic door. When Bz points south (negative), the solar wind's field can merge with Earth's and pour energy in. When it points north, the door is mostly closed no matter how fast the wind blows.
The science
Bz is the GSM z-component of the interplanetary magnetic field. Sustained negative Bz enables dayside magnetic reconnection, the dominant solar wind–magnetosphere coupling mode; values below about −10 nT for several hours typically drive storm conditions.
Why it matters for aurora viewing
Watch Bz live before driving anywhere: strongly negative for an hour or more is the best short-term "go" signal in aurora chasing.
See this in action on tonight's data.