Can light make noise

Can you hear the northern lights?

When charged particles from the sun get into the earth's atmosphere, they trigger a light show in the sky. How such auroras are created has now been clarified. But whether you can not only see them, but also hear them, has long been a matter of dispute. In recent years, scientific studies have provided new insights into this phenomenon.

The dancing colored lights in the sky have impressed people for a long time. In addition to the northern lights - also called aurora borealis - in northern latitudes, similar phenomena also occur over the southern hemisphere. Here they are called the southern lights or aurora australis. Both luminous phenomena have the same origin: charged particles from the sun.

Colors of an aurora

The solar particles race towards the earth at speeds of around three hundred kilometers per second. Mainly it concerns electrons and protons, which finally hit the magnetic field of the earth during their journey and are guided along the field lines in the direction of the poles. There the charged particles enter the earth's atmosphere, where they collide with air molecules. In doing so, they transfer energy to the molecules, which they release again a little later by lighting up. These processes usually take place at an altitude of eighty to three hundred kilometers.

The color of the polar lights depends on the type of molecule with which the charged particles collide. Excited oxygen atoms produce a red and the most frequently visible green color. Nitrogen atoms can also produce violet and blue light.

Reports from people in Scandinavia and other countries near the Arctic Circle that at the same time as the light show could be heard crackling and cracking were long dismissed as fairy tales. The northern lights, more than a hundred kilometers away, seemed far too far away for their sounds to be heard over such a distance. Without evaluable recordings and without an exact localization of the source of the noise, neither its existence nor an explanation could be found.

Evidence of the noises

A group of researchers led by Unto Laine from Aalto University in Finland began making audio recordings of the sounds of the northern lights in 2000. Their goal was to first detect the sound at all and then to localize the source. At different locations and on several nights when the northern lights could be seen, the scientists tried to record the sounds they were looking for - with success. They also examined the magnetic field in the respective areas. In addition to the noises, there should also be a sudden change in the magnetic field at the source, which should be measurable in the form of a magnetic pulse. In fact, the team succeeded in measuring corresponding fluctuations in the magnetic field.

Inversion layer

The magnetic pulses travel at the speed of light, while the sounds travel at the speed of sound. The two signals therefore need different times to travel the same distance - this difference can be used to determine the distance from the earth to the noise source. “The delay between the fluctuation in the magnetic field and the sound gives you the height of the source. Based on these measurements, it must be at a height of around seventy to eighty meters, ”says Laine. The researcher and his colleagues evaluated data from the Finnish Meteorological Institute on the geomagnetic activity of the earth and were thus able to establish a concrete connection between the noises and the light phenomena. “You are more likely to hear the sounds associated with the Northern Lights when geomagnetic activity is high,” explains Laine.

Inversion layer creates tones

However, at this point in time, the researchers were unable to provide a precise explanation of how the tones are generated. The question of why the northern lights can only be heard on some nights and not on others also remained unanswered. The scientists made up for this in a new study that they presented in 2016. The weather conditions are therefore an important factor for the fact that noises can arise at all. "Clear and calm weather is necessary to hear the sound of the aurora," says Laine. The reason for this is a so-called inversion layer, which only forms under such conditions.

Usually the air temperature decreases with increasing altitude. However, this temperature gradient can be reversed on clear and windless winter nights: If the ground cools down significantly at night, the air layer close to the ground becomes colder than the air layer above. The inversion layer is formed between the two. This acts like a kind of lid that prevents the upper and lower layers of air from mixing. In such weather conditions, Laine says, negative ions can accumulate at the bottom of the boundary layer, at a height of seventy to eighty meters. Due to the increased conductivity in the upper atmosphere of the northern lights, more positive ions reach the upper edge of the inversion layer at the same time. “As a result, this layer forms a huge, charged capacitor, so to speak,” says Laine. A capacitor consists of a negatively and a positively charged surface, between which an electrical voltage is applied.

Northern lights during an audio recording

This electrical voltage increases due to additional charge carriers. The higher the voltage, the more likely a sudden discharge will occur. Such a discharge creates a crackling and crackling sound - and the magnetic pulse measured by Laine and his colleagues. In addition to the geomagnetic activity that occurs during the Northern Lights, the weather conditions are also crucial for noise to be generated. “A small gust of wind can destroy this construction. This also explains why I am not able to record the sounds of the northern lights when there is already a mild wind, ”says Laine.

It is true that Laine and his colleagues have now been able to use scientific methods to prove the noises of northern lights. However, the researchers are still puzzling over the details of their production. So far they have not been able to find out what is causing the discharge of the inversion layer. “The physics behind such layers appears very complex and surprising,” is Laine's conclusion. In the future, the scientists want to work on this issue as well.