The secret of the heavenly lights

Polar lights in view

Northern Lights (Aurora borealis) near Anchorage NGDC / NOAA (Collection of Dr. Herbert Kroehl)
Read out

Northern Lights, Aurora Borealis, Northern Lights - the mysterious light phenomena of the North have long since lost their mystical aura, but even today they still give many a mystery to science.

Up to 240 nights a year, the residents of the so-called "Polar Light Oval" can admire the sky spectacle: Green bands of light, sometimes red or with any color in between, like a fog wafting over the starry sky at night. Similar to a lightning in the mountains, this is completely silent - however polar lights sometimes stand for several minutes completely motionless in the sky. Time enough to marvel breathlessly at her beauty or to think about her origins. So how come these bizarre heavenly lights?

Mystic messengers

Already in the early legends and myths of the Nordland inhabitants, the Northern Lights are described as messengers of long-dead ancestors or fallen warriors. Even in the Middle Ages, the northern lights were considered a sign of God, which called for conversion, and even in our century see many people in the sky spectacle a menetekel, a harbinger of evil and war. Galileo Galilei, himself fascinated by the sky lights, once coined the visual comparison with the dawn, still perpetuated in the scientific name "Aurora borealis".

Although the Northern Lights were observed from time immemorial, their scientific cause remained hidden in the dark for a long time. At first, it had been suggested that the light bands were created by scattering or mirroring the sunlight on clouds, ice crystals or atmospheric gases. Only in connection with the exploration of the comets and the phenomenon of the cometary tails researchers of the explanation came on the track: The sun emits not only light and heat from, emanates from it an incessant stream of electrically charged particles, the so-called solar wind.

Earth under attack

Northern Lights (Aurora borealis) near Anchorage ┬ę NGDC / NOAA (Collection of Dr. Herbert Kroehl)

At speeds between 300 and 800 kilometers per second, the charged particles shoot as a continuous stream through space. If this solar wind reaches the earth, the particles do not penetrate to the earth's surface, but are deflected by the magnetic field of our planet. However, the force of the solar wind is so strong that it literally deforms the terrestrial magnetic field: on the sun-facing side of the earth it is compressed, on the other hand, the field lines form a kind of tail behind the earth. Nevertheless, the symmetrical magnetic lines protect the earth from the dangerous radiation like a kind of Faradayan cage - but not completely. display

The actual weak point for the solar wind are the poles. Here the field lines stream out vertically and into the earth. If the charged particles hit the magnetosphere at these points, they can reach the upper layers of the atmosphere along the magnetic field lines. At the latest, 100 kilometers above the earth's surface, the time has come: they collide with the oxygen and nitrogen molecules in the air. The released energy is emitted as light of different wavelengths Oxygen atoms generate green and red, nitrogen atoms blue and violet light.

Brilliant Central Europe

Although the polar light most often occurs in a roughly 400-kilometer-wide strip around the poles, it can also be observed in Central Europe statistically, with a frequency of one to three nights in the year. This occurs when, in times of increased solar activity, the magnetic field of the earth is particularly strongly deformed under the onslaught of the charged particles. Only then can solar wind particles penetrate into the atmosphere even in the temperate latitudes.

Normally, however, the northern reaches of the Northern Lights to northern Alaska. In the south, on the other hand, it runs partly over the S├╗dpolarmeer and partly over the uninhabited Antarctic, which is why the phenomenon of the "Aurora australis", the southern light, far less well-known than its northern variant Aurora borealis .

Crackling light bands

By the way, it is still disputed whether polar lights are a purely optical phenomenon: Eyewitnesses repeatedly report that they also heard noises during a northern light. For the scientists, this remains a mystery, as the thin air of the ionosphere, where the aurora arises, does not conduct sound waves. In addition, the sound would take several minutes to reach the Earth's surface.

Some researchers suspect that the sounds are not caused directly by the northern lights but rather by the impact of electromagnetic waves of the polar light on the ground. Others believe that the brain itself is responsible for the sounds by converting the electromagnetic waves of the aurora directly into sound. How and whenever this riddle is solved, the auroras remain fascinating and mysterious even without sound.

(AWI / NASA / go.de, 26.04.2005 - AHE)