The polar lights (as northern lights in the northern hemisphere scientifically aurora borealis , as southern lights in the southern hemisphere Aurora australis ) is a luminous phenomenon caused by excited nitrogen and oxygen atoms of the high atmosphere ( electrometeor ), which in polar regions when accelerated charged particles from the earth's magnetosphere hit the atmosphere is caused. Northern lights can usually be seen in two bands of about 3 to 6 degrees of latitude near the magnetic poles .
Aurora borealis arise when electrically charged particles of the solar wind from the magnetosphere (mainly electrons , but also protons ) meet oxygen and nitrogen atoms in the upper layers of the earth's atmosphere and ionize them. When the recombination takes place again after a short time , light is emitted. Due to the energy transfer, the electrons reach a higher energy level , but then fall back again ( fluorescence ).
The energy comes from the sun. It sends out the solar wind, a plasma with a density of approx. 5 particles / cm³ and an average speed of 500 to 800 km / s, which takes about 2 to 3½ days to reach the earth's orbit. The largest solar wind outbursts occur through magnetic reconnection in the area of sunspots during the turbulent, speckled phase of the solar cycle. Solar wind particles hit the terrestrial magnetosphere and interact with it.
The incident solar wind particles compress the earth's magnetosphere on the side facing the sun and pull it out into a long tail on the side facing away. Due to their charge, the solar wind particles are mainly deflected along the direction of the earth's magnetic field and flow around the earth's magnetosphere, which protects the biosphere below from the solar wind. The magnetosphere is continuously moved by the unsteady solar wind. The movement of the magnetic field towards the charged particles induces currents in it. The greatest energy releases occur through magnetic reconnections in the tail area of the terrestrial magnetosphere. Within the earth's magnetosphere there is therefore a complex system of moving electrical charges, which move around the earth in sometimes large, global currents such as the ring current , the Birkeland currents , the Pedersen currents and the polar electric jet . When the plasma particles flow down into the atmosphere, they excite the diluted gases in high layers of the atmosphere when they collide. These emit fluorescent light when the excitation drops.
Local focus on the poles
Northern lights occur mainly in the polar regions , where the field lines penetrate the atmosphere. They occur both in northern latitudes ( northern lights , also aurora borealis ) and in the southern hemisphere ( southern lights , also aurora australis ).
These phenomena are also observed on other planets in the solar system . The prerequisite for this is that the planet has its own magnetic field and atmosphere. In 2015, astronomers were able to observe the northern lights outside the solar system for the first time. The activities observed on the low mass star LSR J1835 + 3259, 18 light-years away, were about 10,000 times more powerful than auroras on Jupiter .
The frequency of the auroras in the middle latitudes (Central Europe) depends on solar activity . The sun goes through an activity cycle ( sunspot cycle ) that lasts an average of eleven years from the beginning (solar minimum) through the middle (solar maximum) to the end (renewed minimum). The frequency of auroras fluctuates with this cycle. Strong eruptions on the sun occur particularly frequently during the peak of activity (also known as solar max; last occurred in 2013/14) . The large coronal mass ejections are essential for Northern Lights in Central Europe. In the early and late phases of the solar cycle, close to the solar minimum, there are far fewer of these eruptions and thus there is a much lower probability of auroras. Nevertheless, strong events can also be observed in the descending and ascending solar cycle.
In autumn 2003, for example, northern lights were sighted in Greece and also on the Canary Islands . On average, around 10 to 20 of these luminous phenomena can be observed per year during the phase of the solar maximum in German-speaking countries. In general they can be seen in the northern sky, only when the solar wind is particularly strong they can also appear in a southerly direction. Aurora borealis can be predicted at short notice through earth-based, visual solar observation . However, this works better by using freely available data from the various space missions of ESA and NASA to research the sun and solar wind. Since the solar wind travels from the sun to the earth for two to four days, auroras can be expected in this time interval after a strong, earth-facing solar flare.
The statistical deduction that auroras occur mainly in autumn / early winter, from late October to mid-December, and in late winter / spring, from late February to early April, is not absolutely certain. Here the magnetic fields of the earth and the sun are particularly favorable to one another, but this effect can be neglected due to the low inclination of the earth's axis towards the ecliptic . It is more likely that especially on the coldest winter nights, observations are very rare due to the weather. The same applies to the lack of summer observations, because at this time of year the midnight sun prevails in the north and makes it practically impossible to see the aurora borealis. In general, it can be said that with increasing distance to the respective pole, e.g. from Germany, Austria, Switzerland and Italy, polar lights can usually only be observed during the maximum activity of the sun, which is relatively rare. Using modern digital cameras, however, it is still possible to document individual events - from Central Europe - even during the less active phases of the solar cycle.
The intensity of the northern lights has increased again since 2007 and had its last peak in 2013/14. According to the American aerospace authority NASA, it should be the strongest polar lights in 50 years. In fact, the current sunspot cycle was weaker than its predecessor, which means that aurora activity is rather subdued.
Northern lights can be of different colors. Green light (557.7 nm wavelength) predominates, it is created by oxygen atoms that are excited at a height of over 100 km. Oxygen atoms also emit red light (630.3 and 636.3 nm), which is mainly created in the thinner atmosphere in higher layers around 200 km altitude.
Excited ionized nitrogen emits violet to blue light (427.8 nm and 391.4 nm). However, very high energies are required to excite nitrogen atoms, which is why these colors can only be observed in the event of strong magnetospheric disturbances.
Because of the eye's high sensitivity to green light and the relatively high concentration of oxygen, green auroras are the most commonly observed.
There are four different types of northern lights, which are dependent on the solar winds. These are: corona, curtains, calm arches and ribbons. Scientifically, they are divided according to the Vallance-Jones Classification:
|HA||Homogeneous Arc||Even arc|
|HB||Homogeneous band||Uniform band|
|RA||Rays Arc||Radial arc|
|RB||Ray's band||Radial band|
|DS||Diffuse surface||Diffuse surface|
|PS||Pulsating surface||Pulsating surface|
|PA||Pulsating Arc||Pulsating arc|
|C.||corona||Corona (ring-shaped rays)|
|F.||Flaming||Zenith-directed, pulsating rays|
Furthermore, there are clearly dark areas within the lights, the so-called anti-aurora. There are areas in which the flow of electrons in the direction of the high atmosphere comes to a standstill. In such an area, satellites have detected electron currents directed upwards, i.e. away from the atmosphere.
Possibly the earliest datable report on the northern lights can be found in a Babylonian cuneiform script that is over 2500 years old. She reports of an unusual red glow in the night sky, which precisely relates to the night of March 12th to March 13th, 567 BC. Is dated.
Numerous legends and sagas existed before or outside the scientific explanations. The peoples of Lapland , Siberia and Alaska believed they saw signs of their gods here, who wanted to connect with them in this way. The signs were often seen as harbingers of bad times such as war, plague, and famine. The Sami, Scandinavia's natives, also suspected an approaching disaster, especially when the red veils of color lit up. In their case, however, the dead were suspected to be the trigger, the children were forbidden to go on the streets and they waited in silence for the dead to go back to rest. The Inuits also believed in the activities of the spirits of their dead . However, these assumed that the glow was caused by a walrus skull tossed back and forth while playing ball. The Inuit also believed that the lights were a bridge to the afterlife, which was lit by torches from the dead in order to offer orientation for the newly deceased. The indigenous peoples of Canada believed that their shining God wanted to make sure of the well-being of his tribes. A little story served as an explanation for the Vikings . They saw in the northern lights the sign that a great battle had been fought somewhere in the world. According to their imagination, the Valkyries rode across the sky after each battle and chose the heroes who would henceforth dine at Odin's table. The light of the moon was reflected on their shimmering armor and the colorful northern lights emerged. In Europe especially in the Middle Ages , polar lights, like comets , were seen as harbingers of coming disaster ( e.g. wars , epidemics , famines ). Central European Christians in the Middle Ages are said to have seen signs of the Apocalypse in it, which is likely to be related to the fire-red appearance most common in their latitudes. For the Maori in the southern hemisphere , the southern lights, which are extremely rare in New Zealand, were considered to be a fire that their ancestors had lit on their way to Antarctica to remember the warm days in New Zealand.
On September 2, 1859, northern lights could be seen in the northern hemisphere as far as the tropics . The cause was a geomagnetic storm of exceptional strength caused by coronal mass ejection . It is believed that a similarly powerful eruption today would lead to a global blackout .
Historical scientific theories
In the 18th century the first attempts were made to scientifically explain the origin of the northern lights. The researchers initially assumed that the northern lights were reflections of sunlight on clouds or ice crystals. It was not until some time later that the English astronomer and mathematician Edmond Halley recognized - probably the first - the connection between the earth's magnetic field and auroras. But he couldn't explain the glow. This was not achieved until 1867 by the Swedish astronomer and physicist Anders Jonas Ångström , who was able to show that the northern lights are self-luminous gases . The Norwegian physicist Kristian Birkeland proposed a theory for the cause of the glow in 1896: He assumed that electrons from the sun stimulate the gas mixture in the upper atmosphere to glow. However, since the existence of the solar wind was not yet known at that time - this was only proven in 1959 by the Soviet Lunik 1 probe - its theory was often questioned. Although the origin of light is no longer controversial today, it has not yet been fully clarified why polar lights can be observed in certain places.
A modern theory, which has been upheld for many decades and is still widespread, states that auroras are created by the direct impact of charged particles of the solar wind on the earth's atmosphere. Solar wind particles would be funneled along the earth's magnetic field lines to the near-polar atmosphere, where the field lines penetrate the atmosphere almost perpendicularly. This theory must at least be viewed as greatly simplified, as it suppresses the complex processes between the absorption of the energy of the solar wind and the emergence of the luminous phenomena.
In science, at least since Burritt (1845), the connection between the luminous phenomena and activities in the earth's magnetic field was known.
The connection with sunspots became known in the 19th century.
In the mid-19th century, in the early years of telegraphy, induced pulses were observed on telegraph lines.
In the early 20th century, Birkeland demonstrated the plausibility of an explanation about electricity, plasma and solar activity.
In the late 1950s, Churchill's first research rocket detected electron currents in the atmosphere.
Influence on technical facilities
The high-energy, electrically charged particles of the solar wind, which are responsible for the creation of auroras, generate electromagnetic fields that can have harmful effects on electronic equipment. Satellites and aircraft are particularly at risk. For safety reasons, during times of increased aurora activity in air traffic, flights are flown at a lower altitude or flight routes are chosen that are away from the polar regions.
In addition, induction can cause voltage fluctuations in power grids . For example, the power outage in Canada in 1989 was attributed to a strong solar wind.
During the occurrence of auroras, radio waves above the shortwave range are also reflected in the ionized areas of the atmosphere ( ionosphere ) due to partial reflection. Radio amateurs use this effect in the amateur radio service to increase the range of their signals. However, since the reflection of the radio signals caused by the auroras results in a disturbance and falsification of the audio modulation in a characteristic way, namely hum and harshness in the sound, the connections are often carried out in the Morse code (CW, A1A) operating mode .
Interpretations in Fiction
In science fiction novels and films, the phenomenon of the northern lights is occasionally used as an accompaniment to supernatural events as a special effect; for example in the novella Langoliers by Stephen King and in the film Frequency . They are a central theme of the first part of the His-Dark-Materials trilogy by Philip Pullman .
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