Northern lights

Northern lights, or scientifically also known as Aurora Borealis in the northern hemisphere and Aurora Australis in the southern hemisphere, have been occupying humanity for a very long time, to be precise for around 2500 years. While people in Lapland, Alaska and Siberia for a long time believed that these were signs from their gods, harbingers of war or disaster, today the science is much further advanced, even if there are still a few ambiguities. Northern lights mainly occur in the polar regions around the poles, i.e. around the northern and southern polar circles.

Their occurrence and frequency depend on solar activity. While the Northern Lights can be seen practically every day around 70 degrees northern latitude under clear skies during the polar nights, in Central Europe in the middle latitudes they are only visible when there is stronger solar activity.

Each cycle of the sun's activity lasts an average of 11 years. A so-called solar minimum, which stands at the beginning and end of each cycle, initiates and ends the cycle, with the solar maximum in the middle.

The frequency of the occurrence of northern lights can also be observed accordingly. The last solar maximum was in 2013/2014. In these years, strong solar eruptions, also known as coronal mass ejections (CMEs), were particularly common. They are the prerequisite for the appearance of northern lights in Central Europe. At the beginning and towards the end of each sunspot cycle, significantly fewer and less strong eruptions can be observed, so that the visibility of the northern lights retreats to the regions around the polar circles.

In average, in Central Europe you can observe the northern lights on about 5-15 nights per year. However, these can usually only be seen in the sky in the north. After a coronal mass ejection has taken place in the direction of the Earth, the solar wind needs around one to four days until it hits the Earth, so that auroras can then be expected and can therefore be predicted relatively well. During a solar maximum, northern lights can very rarely be observed even as far away as the Mediterranean. This was for example the case in 2003.

The sun constantly throws huge amounts of matter into space; here we speak of the solar wind. This is made up of protons and electrons; the Earth's magnetic field protects us from these particles and radiation that would make life on earth impossible. These eruptions occur more frequently at times of a solar maximum, so that entire “solar storms” can arise. The normal speed of a solar wind is around 300 kilometers per second, but in October 2003 a very strong solar storm reached 2000 kilometers per second. If the speed of the solar wind is more than 300 kilometers per second, this indicates impressive northern lights; here, up to 700 kilometers per second can be reached.

The Earth's magnetic field is deformed by the strong pressure of the solar wind so that it practically collapses on the day side, while on the night side it extends several million kilometers into space. As soon as the solar wind approaches the Earth's magnetic field, it is deflected by it and flies around the Earth. The particles flying past - between the Earth's magnetic area and the solar wind - convert their kinetic energy into electrical energy. In the Earth's magnetic field, positive protons and negative electrons are separated as charged particles of the solar wind, causing enormous electrical voltage to build up above the Earth.

The voltage discharge takes place in part due to the electrons flowing in a spiral along the field lines towards the Earth, where the field lines enter and exit. The polar regions then act as a kind of funnel for the magnetic field lines. Electrons from the solar wind then collide with molecules in the atmosphere at an altitude of around 150 km, causing such a violent exchange of energy that the molecules become electrically charged and therefore glow. These are the northern lights we know.

Aurora Borealis comes in six different colors: green, red, yellow, purple, white and blue. Their color depends on both the different components of the atmosphere and the altitude. Green auroras, usually caused by oxygen, are the most common and occur at an altitude of approximately 80-150 km. Red and blue auroras are caused by nitrogen atoms and occur at an altitude of between 150 and 600 kilometers. Orange and magenta occur at an altitude of 70-90 kilometers, while red auroras occur at an altitude of 200-320 km. Yellow auroras only appear as an additive superposition; Yellow-green northern lights can be found at an altitude of 120-140 km. The latter only takes one second to form.

Despite the magnetic field that protects the Earth, auroras can sometimes distort measurement data with their strong voltage fields or have an influence on satellites. In 1989, entire power grids failed in Canada during a solar maximum. Auroras can occur in four different ways, each dependent on the solar wind:

Corona
ribbons
curtains
quiet arches

The best time to observe the Northern Lights is between the beginning of September and mid-April, when the nights get dark again in the far north. On clear nights in the northernmost part of Scandinavia you can observe the northern lights nine out of ten nights, even during a solar minimum. The further north you go, the brighter the nights are between mid-April and early September, so that during this time you can only observe the northern lights much further south, but this requires a larger coronal mass ejection towards the Earth.

The basics for night photography also apply to the camera settings here. Depending on the desired section, you will also need a very fast wide-angle or ultra-wide-angle lens with an aperture of f2.8 or smaller to photograph the northern lights. The ISO varies between 800 and 3200 depending on the intensity of the auroras, while the exposure time depends on the intensity and speed of the auroras. You usually need exposure times of 6-20 seconds.