The secret sound of Aurora

Whole summer long in Berlin’s planetaria the stunning 360° images of Auroras shot by South Korean photographer Kwon O Chul are shown. The images are supported by computer graphics and stunning views from International Space Station. Below we will talk about the shows, explain briefly the origin of Auroras and elaborate on the mysterious sounds, which are heard during these magnificent light plays in the sky.

What?

Aurora – Wonders of the Northern Lights. Zeiss Großplanetarium.

 Prices?

8€ adult / 6€ child

 

 

 

Introduction

During the summer – with the last viewing scheduled for 28th of September, the show “Aurora – Wonders of the Northern Lights” is presented in two out of three Berlin planetaria. It is shown in the Zeiss Großplanetarium and the Planetarium am Insulaner. The show consists of two parts – 30 min of live presentation about the stars and the planetarium in German. And 30 min projection and explanation of Auroras in German (both sites) or English, French, Japanese, Korean or Italian (Zeiss Planetarium).

If you don’t speak German, don’t be afraid of the German part. The 360° projection of the stars on the dome is fascinating by itself. As you have to look up for one hour the chairs can be put in half-laying position (in the row 11, seat 14 the chair can be set almost horizontal), in which even German becomes a soothing lullaby language.

After half an hour explanation of stars, hemispheres and planetarium itself, the actual show begins. It was shot with a special camera by the Korean astrophotographer Kwon O Chul and is probably the only possibility to see the Northern Lights without actually travelling up to Polar circle and freezing your ass off.

“AURORA – wondwer of the Northern Lights” Zeiss-Großplanetarium (c) Kwon O Chul

During the presentation you will see not only the images from earth, but also spectacular view from the space shot from International Space Station (ISS). Also the explanatory part, where the origin of the Auroras is explained, is supported by stunning images – real fotos with insets of simulated graphics.

Discussion

The Auroras as nature phenomenon are well known for a long time. What is the cause of this light show?

The sun is the answer. The appearance and bightness of the Northern Lights correlate with the activity of the sun, which can be measured by the number of the spots visible on the surface of the star. The period between two sun activity maximas is roughly 11 years. However, the peak activity is different from peak to peak and therefore the strength of corresponding Northern Lights also varies.

Depiction of the solar wind and it’s deflection by the magnetic fiel of the earth (c) Breanne Sewards Travelmanitoba.com

The higher the solar activity the more charged high-energy particles are sent from the sun in the direction of the Earth.

These particles (mostly electrons and protons), as they are carried by solar wind, are mostly deflected by the magnetic field of the Earth, but some of them are directed towards the magnetic poles of the planet, hit the ionosphere there and create the Auroras (Borealis and Australis). Here the high-energy particle hit the upper layers of the atmosphere and excite the molecules in the air.

The molecules and atoms of oxygen and nitrogen, which are two most dominant gases in the atmosphere consist of cores, made up from protons and neutrons as well as electrons, which are circling the cores at orbitals. If another particle like electron or proton hits such an atom, they can exchange enrgy, like e.g. the balls during the pool game. However the energy goes mostly to the electron, which results usually of the electon going to the higher orbit aka being excited. As excited state is not natural for an electron in an atom, after some time it gets disexcited by radiating the excessive energy in form of photon aka light.

Ähnliches Foto
Diagram of an electron disexitation from a higher orbital to a lower one with emittance of a photon (c) Wikicommons

The disexcitation happens by light emission. If oxygen is excited with high energy electrons it emits green light. The oxygen excitation with low energy electrons results in red light. Disexcitation of nitrogen emits blue. The mixes of these three colors result in purple, pink or white (NASA, 2003).

The emitted spectrum of oxygen and nitrogen (c) Les Cowley 

However the Auroras are apparently not only delight for the eyes, but also for the ears.

For more than three hundred years there were reports on the misterious sounds, which are apparently made by Auroras (Journal of the Royal Astronomical Society of Canada, 1933). Recently Unto Laine – a finnish researcher, was able to record this accoustic phenomena and to propose a cogent theory to explain their origin (TKK, 2005). The surprising thing about the sound of the Northern Lights is that is described as coming from the air itself, above the listeners, not from the treetops or the ground, which excludes the possibility of the objects connected to the ground acting as antennas for sounds of any kind.

Most researchers discarded the accounts of the sounds as improbable or as background noise, especially as the first experiments aimed at the recording of the sounds failed. However Prof. Laine and his teams believed that human ear is very well suitable to locate the sources of the faint noises and continued the search. With several experimental setups, which were changed during the experiments, which started in 2000 the group was able to record the sounds several times and to locate the source as coming from the air ca. 70-80 meters above the ground. The results collected during the geomagnetic storm in March 2013 were evaluated with 99.9% probability to be true.

Based on this observation a plausible physical explanation of the cause was proposed, which took from the critics one of their last arguments. The source of the sounds in the sky during an Aurora event is considered to be a corona discharge caused by inversion layer (BNAM, 2016; ICSV24, 2017).

Безымянный
Illustration of the inversion layer, which creates the sound of Aurora (c) U.K. Laine

On cool and calm evenings the frozen ground cools the air directly above it. This can result in a layer of cold air up to several hundreds meters in height trapped below a layer of warm air. This is called the “inversion layer”. Simultaneously, the negative ions formed at the surface rise up to the lower boundary of the inversion layer, where they are prevented from ascending farther. The negative charge on the lower surface of the inversion layer attract the positive charges from the atmosphere above the inversion layer, which accumulate on the upper border of the layer. This process results in electrostatic potential, which is increased by the processes causing the Northern Lights until the charge is high enough to provoke a sudden discharge. This discharge results in ultraviolet light, magnetic field pulses and sounds – the sounds of the Auroras.

Summary

Inspired by the show currently running in planetaria in Berlin the Auroras as well as their origin were discussed here. The mysterious sounds appearing in the air during Northern Lights were introduced and the latest theory – the inversion layer – explaining the source of these sounds was presented.

References

  1. C. S. Beals, The Audibility of the Aurora and its Appearance at Low Atmospheric Levels. Journal of the Royal Astronomical Society of Canada, 27:184-200 (1933)
  2. J. Hautsalo, Study of Aurora Related Sound and Electric Field Effects. M.Sc. Thesis, TKK, (2005)
  3. NASA, Aurora… fabled glowing lights of the Sun – Earth Connection. NASA, (2003)
  4. U. K. Laine, Auroral Acoustics project – a progress report with a new hypothesis. BNAM, (2016)
  5. U. K. Laine, Localization of sound sources in temperature inversion layer during a geomagnetic storm. ICSV24, (2017)

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