What is Bz and how does it affect the Northern Lights?

The solar wind has embedded up-down magnetic field direction called Bz. Tracking the Bz direction is one of the most important and useful tool for aurora chasers. A negative Bz value of south (minus) is like an "on" switch for the aurora, while a Bz of positive (north) is like an "off" switch). Generally speaking, the lower the Bz, the better. 

When the Interplanetary Magnetic Field (IMF), essentially the magnetic field of the solar wind itself, points south, conditions are favourable to connect Earth’s magnetosphere, with the solar wind and inject charged particle into the upper atmosphere. When the IMF points south it has a negative Bz and when it points north a positive Bz. 

Earth's magnetic field naturally points north and since opposites attract, we want an incoming stream of charged solar particles to have a south pointing magnetic field. North-north coupling will repel and deflect the solar wind away from Earth.

When the Bz points south, a crack opens up in Earth’s magnetic field. This happens on the daytime side of the Earth, where the magnetopause connects with the incoming solar wind. The field lines then reconnect on the nightside, in the magnetotail where the particles are then accelerated along the field lines towards the polar regions to cause aurora. This phenomena is called the Dungey cycle and happens when Bz is south. The video below shows the process.

The Interplanetary Magnetic Field data published here on Iceland at Night comes from the DSCOVR satellite, located 1.5 million km from Earth, between the Earth and the Sun. This satellite senses incoming solar wind 15 to 60 minutes before a blizzard of charged particles rains over Earth. It’s like a buoy at sea that warns of an oncoming tsunami, as NASA writes.

And that's the caveat: When you see the Bz suddenly turn south, you’ll need to wait from 15 to 60 minutes or so for the Northern Lights to appear in the sky.

In Iceland, under the aurora oval, a Bz strength of –1 to –5 is fine and good Northern Lights are very likely to appear in the sky. When the direction drops below –5 or even –10 or lower and stays there for an hour or more, beautiful Northern Lights dance in the sky. If it plunges below –20, an unforgettable evening of worldwide aurora displays ensues, like happened on October 10-11, 2024.

So when you are outside, focus on Bz and skip the almost completely useless Kp-index. 

Faster solar wind expands the aurora oval and moves it further south. Higher density leads to more colourful displays.