The northern lights attract millions of people. Ever since aurora has been seen there have been people able to hear their sounds. It is not known where the sounds come from. There is no accepted theory which can approve and explain or disprove crackling, rustling, and swishing sounds that are sometimes heard in connection with aurora. There are even doubts about the existence of these sounds. It is not clear which conditions should be satisfied for the auroral sounds to be heard. However, most of the observations indicate that sounds are heard during periods lasting a few minutes or more during which a powerful display of the aurora can be seen right above the head.
It also seems that not everybody can hear sounds. There are many people that have seen hundreds of powerful aurora displays without any signs of sounds. At the same time, there are observations where in a group of people everybody have heard the sounds (in observation list almost half of all cases). Common to all observations is the fact that sounds have been heard only once or twice in a life time. The observations indicate that the auroral sounds are real but can be heard on very rare occasions.
The sounds usually resemble a swishing or crackling similar to that heard as waves of static on the radio. The level of intensity is said to change along with the rapidity of movement of the auroral forms overhead. If a physical reason does exist for auroral sounds to be audible on the ground, it would most likely be due to the electrical field produced by the aurora. Power fluctuations in long transmission lines have been linked to auroral activity. The northern (or southern) lights generate VLF radio waves as well as light.
There are generally two types of sounds reported to accompany the aurora. The first is a swishing sound that change with movements in that auroral display. The second type is a crackling sound, like static electricity makes.
The problem with claims of sound that changes in time with auroral movements is that the aurora is a long ways away (100 km/60 miles), so if sound was coming from the aurora, there would be a long delay between the auroral movement and the sound arriving at the listener's ears, just as thunder arrives long after a distant lightning flash is observed. There is also the small detail that the air between us and the aurora is far too thin to carry sound over such long distances. Therefore, if this kind of sound exists, it must be created very near to the observer.
The most likely explanation for this type of sound is that it is created inside the observer's head: not a figment of the imagination, but rather leakage of the electrical impulses from the nerves in the eye (carrying images of the aurora to the brain) into the part of the brain the processes sound. In a very quiet environment, there are no sound signals for the brain to process, so it notices these tiny leakage signals and the result is sounds that change in time with the aurora. This explanation was actually tested by some early explorers, who found that the sound went away if their eyes were covered.
