The Northern Lights
The Northern Lights
Aurora Borealis," a name meaning "northern dawn" in Latin, is a phenomenon that occurs once every seven years in the Northern Hemisphere of Earth. Also known as the Northern Lights, it originates in the atmosphere very high above the Earth's surface. The light is emitted when charged particles from the Sun are guided by the earth's magnetic field into the atmosphere near the magnetic poles. When these particles collide with atmospheric molecules, primarily oxygen and nitrogen, at 100-300 kilometers altitude, some of the energy in these collisions transforms to visible light.
(http://www.northern-lights.no/english/pages/facts/intro.shtml)
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...