What is the cause of auroral sound that sometimes occurs with an aurora?

What is the cause of auroral sound the sometimes occurs with an aurora? This question is for a homework assignment...I hope you have the answer for it! Thanks a lot!

First, what is an aurora? It is a ghostly glow seen mostly in the extreme northern or southern latitudes (near the North and South poles) that is caused by high-speed particles from the Sun slamming into the Earth's atmosphere. The Earth's magnetic field guides the particles to the north or south pole, where they hit air molecules. This ionizes (strips an electron off) the air. When the electron recombines with the air molecule, energy is released in the form of light.

So aurorae are an electrical/magnetic phenomenon. They occur very high in the atmosphere, at least 90 or so kilometers high. This high, the air is extremely thin and not capable of supporting sound. Yet people claim to hear a crackling sound, like static electricity. There are several possibilities: one is that the sound is simply not real, and people are mistaken. Another is that the sound is real, and is caused by something closer to the Earth that is being sparked by the aurora (maybe a local magnetic effect). No one knows for sure!

There are lots of great sites about aurorae on the Web. Here are a few: