They writhed like a brood of angry snakes,
Robert W. Service / The Ballad of the Northern Lights/
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 existance of these sounds. Here we describe the auroral sounds and introduce some of the most popular theories that explain them.
Most probably we will not know the right answer so soon. Scientists of northern lights are just too busy with the ionosphere where the lights originate, the Earth mangetosphere where the charged particles that create aurora are accelerated, and the solar wind that drives the whole thing.
There are no tape recordings available so we should try to imagine the sounds from descriptions of eyewitnesses. Here aer some witness reports
- It could compare to the sound of a radio left on a station that has gone off the air
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.
To imagine how skies may look like when auroral sounds are heard you can watch a movie composed of pictures taken by the all-sky camera in Poker-Flat, Alaska. The picture includes the view of all skies, where the North is downwards, the South is upwards, and the skies above the head are in the middle of the picture. The pictures are black and white, in reality white would correspond to a greenish color of aurora. From movie you can see that the time of the most powerful aurora display longs for just about 20 minutes.
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.
There are not many studies of the auroral sounds in the scientific literature. There are not many attempts to register auroral sounds either. One of the explainations is the large scepticism about the subject which dominates among the space scientists.
The most thourough studies of auroral sounds has been done by Silverman and Tuan. They have presented a large list of observations, they have done statistical, geographical and other type of analysis on these data, and they have pointed out possible theories that may explain at least parts of auroral sound observations. Silverman and Tuan favor the brush discharge theory even though they are not certain about it. Another large contribution has been done by Colin Keay. He favors the theory that electromagnetic waves are transduced into acoustic waves. He even points out that sounds from meteorites have similar character as the auroral sounds. Below are most popular theories that explain the auroral sounds
This theory is appreciated by those who have seen a lot of aurora but have heard nothing. Tinnitus is the noise in the ears which you can hear anytime, just find some silent place. Tinnitus explains why many 'reliable' persons do not hear auroral sounds - they simply can differ the ringing in the ears from the 'real thing'. The difficulty of this theory is that the character of tinnitus sounds differs from that of auroral sounds. In addition, tinnitus theory does not explain why similar sounds are not observed in connection with other nature phenomena.
This theory was proposed by Keay in 1980. It shows that audio-frequency electromagnetic waves can partially transfer their energy into acoustic waves with the help of appropriate objects in the vicinity of the observer. These objects can be hair, grass, trees etc. The laboratory experiments show that the 'most sensitive' subjects could perceive electric field peak-to-peak variations as low as 160 V/m for waves with frequency 4 and 8 kHz. However, the sensitivity among subjects could vary thousand times.
Electromagnetic waves with audio frequencies are observed during the aurora both in the space with satellites and on the ground with radio antenna. Strong waves are observed also in connection with fireballs (very strong meteors falling on the earth) and nuclear explosions. In both cases sounds have the same character as auroral sounds.
The auroral lights originate at heights from 60 to 400 km. One may assume that sounds are produced inside the aurora and afterwards propagate down to the earth. Then the sound will attenuate because of the spreading. In addition, the attenuation of high frequency sounds makes that there will be only 0.1% of 40 Hz wave energy reaching the ground level from the height 60 km, and even less for higher frequency waves. As a result, high frequency sounds will not be audible. Only ion acoustic waves with frequency around 1 Hz and less (infra sound) can propagate practically unattenuated. The infra sounds have been registered by scientific instruments, however, they are too weak to be audible.
There is an additional problem with the theory of direct sound transmission. It takes a few minutes for the sound to propagate from the aurora altitude down to the ground. However, most of the observations report the simultaneous aurora movement and sound appearance.
This theory is favored by Silverman and Tuan. Brush discharge occurs from point-like electrodes where sharp potential gradients exist. It can be trees, bushes, hair and other sharp protruding objects. The discharge can produce sounds from rustling up to hissing. The necessary electrical field to trigger the brush discharge is around 1500 V/m. Remember that the electrical field of the order 100 V/m in the open air is present permanently. During the periods of the strong aurora the electrical field can reach values of 1000 V/m and more. However, the changes in the electric field are not instaneous but with the rise time of the order ten seconds. Again, it is difficult to explain observations of instantenous changes in the sound which follow the development of the aurora.
Author: Andris Vaivads, firstname.lastname@example.org, updated 2002.