For the first time, researchers have located where the sounds made by the aurora borealis are created.
The sounds that accompany the Northern Lights have long been described in folktales and by wilderness wanderers, but confirmation of their existence has been hard to come by.
Researchers from Aalto University in Finland located exactly where the sounds associated with the lights are created, about 70 meters above the ground level in the measured case.
“Researchers located the sound sources by installing three separate microphones in an observation site where the auroral sounds were recorded. They then compared sounds captured by the microphones and determined the location of the sound source. The aurora borealis was seen at the observation site. The simultaneous measurements of the geomagnetic disturbances, made by the Finnish Meteorological Institute, showed a typical pattern of the northern lights episodes.”
“Our research proved that, during the occurrence of the northern lights, people can hear natural auroral sounds related to what they see. In the past, researchers thought that the aurora borealis was too far away for people to hear the sounds it made. This is true. However, our research proves that the source of the sounds that are associated with the aurora borealis we see is likely caused by the same energetic particles from the sun that create the northern lights far away in the sky. These particles or the geomagnetic disturbance produced by them seem to create sound much closer to the ground,” said Professor Unto K. Laine from Aalto University.
“The recorded, unamplified sounds can be similar to crackles or muffled bangs which last for only a short period of time. Other people who have heard the auroral sounds have described them as distant noise and sputter. Because of these different descriptions, researchers suspect that there are several mechanisms behind the formation of these auroral sounds. These sounds are so soft that one has to listen very carefully to hear them and to distinguish them from the ambient noise.”
An overview of the Northern Lights:
“Auroras result from emissions of photons in the Earth’s upper atmosphere, above 80 km (50 mi), from ionized nitrogen atoms regaining an electron, and oxygen and nitrogen atoms returning from an excited state to ground state.”
“They are ionized or excited by the collision of solar wind and magnetospheric particles being funneled down and accelerated along the Earth’s magnetic field lines; excitation energy is lost by the emission of a photon, or by collision with another atom or molecule.”
“Oxygen emissions; green or brownish-red, depending on the amount of energy absorbed.”
“Nitrogen emissions; blue or red; blue if the atom regains an electron after it has been ionized, red if returning to ground state from an excited state.”
“Oxygen is unusual in terms of its return to ground state: it can take three quarters of a second to emit green light and up to two minutes to emit red.”
“Collisions with other atoms or molecules will absorb the excitation energy and prevent emission. Because the very top of the atmosphere has a higher percentage of oxygen and is sparsely distributed such collisions are rare enough to allow time for oxygen to emit red. Collisions become more frequent progressing down into the atmosphere, so that red emissions do not have time to happen, and eventually even green light emissions are prevented.”
“This is why there is a color differential with altitude; at high altitude oxygen red dominates, then oxygen green and nitrogen blue/red, then finally nitrogen blue/red when collisions prevent oxygen from emitting anything. Green is the most common of all auroras. Behind it is pink, a mixture of light green and red, followed by pure red, yellow (a mixture of red and green), and lastly, pure blue.”
“Auroras are associated with the solar wind, a flow of ions continuously flowing outward from the Sun. The Earth’s magnetic field traps these particles, many of which travel toward the poles where they are accelerated toward Earth. Collisions between these ions and atmospheric atoms and molecules cause energy releases in the form of auroras appearing in large circles around the poles. Auroras are more frequent and brighter during the intense phase of the solar cycle when coronal mass ejections increase the intensity of the solar wind.”
Source and Images: Aalto University, Wikipedia via Creative Commons