If conditions are right, you may hear some unusual noises. Earwitnesses have said the sound is like radio static, a small animal rustling through dry grass and leaves, or the crinkling of a cellophane wrapper. Inuit folklore says it’s the sound of the spirits of the dead, either playing a game or trying to communicate with the living.

It’s the sound of the aurora itself. And the cause is currently unknown. Understanding the phenomenon is made more difficult by the fact that though there are many anecdotal reports, the sound has yet to be recorded.

Aurora displays are caused by the solar wind interacting with the Earth’s magnetic field and atmosphere. Because these interactions happen at altitudes of at least sixty kilometers, the sounds heard cannot be made by the aurora directly. Even if the air up there were dense enough to support sound waves, they would disperse and fade long before they reached the ground.

The sounds aren’t common, and there doesn’t seem to be any consistency in their occurrences. What’s more, one observer of an aurora may hear the sounds distinctly, while another observer of the same display– even at the same location– may not.

The inconsistency makes it difficult to determine the underlying cause of the sounds. As with any faint phenomenon that is difficult to observe and study, theories abound. One hypothesis claims it’s all in the observer’s head. Modern media has made us used to hearing sound along with visual display, so we sometimes believe we are hearing things even when there is no actual sound. But this doesn’t account for those Inuit legends that predate the technological era, nor does it account for observations made by blindfolded or indoor observers.