The sun is a big magnet. Our star has an internal dynamo that generates a strong magnetic field. The solar wind carries this magnetic field throughout the solar system.

Earth has a magnetic field, too. It forms a bubble around our planet called the magnetosphere, which deflects solar wind gusts. Earth's magnetic field and the sun's magnetic field come into contact at the magnetopause: a place where the magnetosphere meets the solar wind. Earth's magnetic field points north at the magnetopause. If the sun's magnetic field points south -- a condition scientists call "southward B_z" -- then the sun's magnetic field can partially cancel Earth's magnetic field at the point of contact.

Above: A cartoon of Earth's magnetosphere, from the Oulu Space Physics Textbook.

When B_z is south, that is, opposite Earth's magnetic field, the two fields link up. You can then follow a field line from Earth directly into the solar wind and eventually back to the sun. South-pointing B_z's open a crack through which energy from the solar wind can reach Earth's atmosphere!

South-pointing B_z's often herald widespread auroras, triggered by solar wind gusts or coronal mass ejections that are able to inject energy into our planet's magnetosphere.

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