What is magnetic field strength and “Bz” have to do with the aurora?

By Sean McCloat

In the first part of this special space weather blog post series, we introduce the basic relationship between the solar wind and Earth’s magnetosphere that causes aurora and start to discuss the different properties of the solar wind (like density and speed) that directly affect production of the aurora.  Here, we go into more detail about the role of magnetism and what “Bz” refers to.

Magnetism plays one of the most significant roles in determining the interactions between the solar wind and Earth.  In addition to carrying solar particles away from the Sun, the solar wind blows the magnetic field of the Sun out to the rest of the solar system. The Sun’s extended magnetic field is called the interplanetary magnetic field (IMF), and it can interact with the region of space around Earth influenced by our magnetic field, called the magnetosphere.  If you have ever played with magnets, then you know the way magnets behave depends on how they are lined up to one another. Interactions between the IMF carried by the solar wind and Earth’s magnetosphere are similarly dependent on how the directions of their magnetic field lines line up relative to each other.

Earth’s magnetic field lines “point” towards the north pole, and this direction from south to north is referred to as the z-direction.  The x-direction points from Earth towards the Sun, and the y-direction points from east to west. (see below)

Bxyz Earth Sun

Magnetic field strength in general is denoted as B, and Bz refers to the strength of the magnetic field in the north-south direction. If the Bz of the Coronal Mass Ejection is northward (recorded as positive values in the red ACE data) then the IMF lines are pointing in the same direction as Earth’s magnetic field lines.  As a result, the interaction between the two sets of field lines is minimal and auroral activity is also minimal. The effect is kind of like two magnets lined up and repelling each other.

If the Bz is southward (recorded as negative values in the red ACE data), then the IMF field lines are pointing in the opposite direction as Earth’s magnetic field lines. When this alignment occurs, it allows for the reconfiguration of Earth’s magnetic field lines by an energy transfer process called magnetic reconnection.

Southward Bz acts in such a way to “peel” the magnetic field lines from the Sun-side of Earth’s magnetosphere and layer them on the night-side, in the magnetosphere’s long tail .  A larger southward Bz value allows for a more effective energy transfer from the Sun’s magnetic field lines to the Earth’s and this creates a more vibrant aurora.

This all sounds like a lot to keep track of.  Is there any way to simplify it?

As a matter of fact, there is!  The scientists working behind the scenes on the Aurorasaurus project combine some of these ACE measurements (the speed, strength and direction of the IMF) into one handy dandy value called SOLAR WIND POWER! By checking the solar wind power, you will quickly be able to get a good idea of how strongly the solar wind that is currently blowing past Earth will generate aurora.

Solar wind power page

In part 3: stay tuned for more detail on our Solar Wind Power and how it compares to other measurements of geomagnetic activity, like the Kp index!


Sean McCloat interned with Aurorasaurus in the summer of 2015 while pursuing his masters degree in Space Studies at the University of North Dakota with a focus on the planetary sciences and astrobiology. He helped analyze the project’s data, contributed to scientific papers, presentations, and blog posts, and became good friends with Rory, the Aurorasaurus plush doll mascot.

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