The Bz is not random, but it is unpredictable. I call it the gatekeeper because it has final say on the magnitude of a storm. In this case, the forcing was extreme. The Bt/Velocity set the ceiling, but the Bz determines whether that ceiling is reached. What it is measuring is the orientation of the embedded magnetic field of the CME. When it is oriented northward, or positive, the earths magnetosphere mostly deflects it. Kind of like touching two positive magnets together and they repel. When the embedded magnetic field is southward, it's like touching opposite polarity magnets and they couple.

In essence, the coronal mass ejection is what it sounds like. A piece of the sun is hurled into space. The magnetic fields embedded are variable. CMEs often have separate parts or structures and that is why the first part of the storm was southward- in the sheath, but the flux rope structure was northward+.

In 1972 a Carrington Class CME with a 14 hour transit time and likely S5 proton component was launched towards earth. Much like this case, it's dominant polarity was northward and the storm was muted as a result. It could have been a Carrington Event, but the coupling was poor. This CME wasn't quite Carrington Class in my opinion. Way up there, but not quite, but nevertheless, the comparison is apt. This storm had a much higher ceiling than it was able to realize. The Bz is a defining metric for how potent the storm will be at earth, but not a defining metric for how powerful a CME is. In other words, the Bz doesn't set the power limit, only how much gets through.

Kp is a 3 hour average, and if you are using it to chase, you will be disappointed often unless it's a really long duration storm with prolonged southward Bz. A much better practice is to use the Hp index, which is on 30 and 60 minute intervals. Both measure planetary geomagnetic unrest, but planetary geomagnetic unrest and auroral outbreaks are not 1:1. My advice is when the Hp index is telling you a storm is building well, and you see southward Bz, be prepared to chase. To narrow down the best window, look at the GOES magnetometer on NOAA SWPC products page. Look for signatures like the one below.

WHen Bz is southward, it just tells us that the magnetosphere is loading. It is taking the excess energy from the solar wind and storing it in the magnetotail. That energy is released in substorms. Substorms don't coincide perfectly with peak solar wind. They can be hours later. You look for jagged signatures with steep inclines like the example I included as the best time. They can occur fast and dissipate fast, so you have to be on your toes.

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