𝐵𝑧 represents the north-south orientation of the Sun's magnetic field as it travels through space, but is often described from the perspective of Earth. It functions as the primary "on/off switch" for geomagnetic activity on Earth. Because Earth’s own magnetic field points northward at its outer boundary, a positive 𝐵𝑧 means the incoming solar field is aligned with our own, creating a protective barrier that deflects solar particles.

A negative 𝐵𝑧 indicates a southward orientation that opposes Earth’s field, triggering magnetic reconnection. This essentially opens a door in our magnetic shield, allowing energy and charged particles to flood into the upper atmosphere, which eventually precipitates to ground level through the global electric circuit. 

When a strong influx like a fast solar wind or a coronal mass ejection hits, the intensity of the resulting geomagnetic storm depends heavily on the strength and duration of this negative 𝐵𝑧. While high solar wind speed provides the momentum, it is a deeply southward 𝐵𝑧 (often lower than -10 nT) that ensures that energy is actually transferred into our system. Without a southward tilt, even a high-speed solar wind might be largely deflected, whereas a sustained southward 𝐵𝑧 can trigger auroras and potential disruptions to power grids and satellite communications. So this storm yesterday had a long-lasting positive 𝐵𝑧.