Interplanetary Magnetic Field (IMF)

The solar wind doesn't travel empty-handed — it drags the Sun's magnetic field with it. The strength and direction of that traveling field when it hits Earth largely decide whether an aurora happens.

The IMF is the coronal field frozen into the solar wind plasma, wound into the Parker spiral by solar rotation. Its components (Bx, By, Bz) and magnitude (Bt) modulate magnetospheric coupling; southward Bz dominates energy transfer efficiency.

Two numbers from the IMF — Bt (strength) and Bz (direction) — headline every serious aurora dashboard for good reason.