The aurora borealis: chasing colorful sky data

The myths and legends surrounding the aurora borealis are almost as diverse as their manifestations. They range from the idea that they are the spirits of the dead to the assumption that this is how the gods communicate their messages. Initial research into lights in the sky initially led to the conclusion that auroras are reflections.

The spectacle of light got its name in 1621 from the French mathematician and astronomer Pierre Gassendi: “Aurora Borealis,” the northern dawn. On the other hand, the northern lights over the South Pole are referred to as the “aurora borealis”, that is, the southern dawn. In 1774, French astronomer and geophysicist Jean-Jacques Doritos de Meyran for the first time discovered a connection between the northern lights and solar activity. In this way, the mystery of the colored sky data was gradually resolved.

It starts with the solar wind

The Northern Lights journey begins approximately 150 million kilometers from Earth: on the Sun. Solar charged particles are thrown into space at high speed by the solar wind. After one to three days, the Sun’s particles hit the Earth at a speed of up to eight million kilometers per hour.

The electromagnetic field that extends around the Earth initially protects the planet from the bombardment of solar particles. When the particles hit the Earth after their long journey, they are deflected by the magnetic field. But it can happen that some particles move along the magnetic field towards the geomagnetic poles. Some of the particles enter Earth’s atmosphere, which is where the spectacle begins.

Once particles from the Sun hit oxygen atoms or nitrogen molecules in Earth’s atmosphere, they emit light. Then the northern lights appear in the twilight region around the north and south geomagnetic poles. Depending on the intensity of the solar wind, the area is sometimes wider, which is why the northern lights can also be seen at atypical latitudes.

Data in many colors and shapes

The celestial scene can appear in different colors and shapes. Each color and shape combination provides information about space weather, the Earth’s electromagnetic field and its association with the sun’s particles. In a way, this makes the aurora borealis a kind of natural data visualization.

The most common color of the northern lights is light green. They form at an altitude of about 100 to 200 km when particles from the Sun collide with oxygen atoms. Higher – at an altitude of 200 to 300 km – this combination leads to the red aurora. There can also be overlaps between color levels, resulting in mixed colors.

When solar particles collide with nitrogen molecules, pink-violet lights are formed at an altitude of less than 100 km, while blue or dark violet auroras decorate the sky above 300 km.

The shapes of the northern lights also vary – depending on the altitude. While low auroras often appear as ribbons or arcs, they can also be curtains and rays at higher elevations. If a person stands directly under the active and very high northern lights, then he can form a wreath – a kind of ring.

Solar activity and its attendant geomagnetic activities are critical factors in the brightness, speed, and diversity of the colors of the northern lights. They are measured using the Kp index, which describes the magnetic activity generated by solar storms. The higher the Kp, the wider the Aurora zone.

On the trail of lights

“It can roughly be said that from Kp 7, there is also the possibility of seeing the aurora borealis in Austria,” says Roman Leonhardt, head of the Konrad Observatory of the Central Institute of Meteorology and Geodynamics (ZAMG). But the weather and light conditions should also be suitable.

“Of course, a clear and dark night is important,” Leonhardt told ORF.at. Because: The northern lights are higher than any cloud cover. “And in large cities with light disturbances, observations of the aurora borealis are very difficult.” In addition, the weak aurora is often difficult or impossible to see with the naked eye in particular. Only the view through the viewfinder reveals the full extent of the celestial scene.

If you want to see the northern lights in Austria, you not only need the right weather and the right lighting conditions, you also need a clear view of the north. The horizon must be flat, otherwise large mountains or hills can block the view of light. Under these conditions, it is in principle possible to see the northern lights anywhere in Austria.

Basically, the frequency and intensity of the aurora borealis follows a pattern of about eleven years. The sunspot cycle – the cycle that the sun’s magnetic field goes through – lasts the same length. When the cycle reaches its maximum, solar winds or solar flares occur frequently, and the geomagnetic activity also increases.

Exceptionally weak solar activity

But solar storms have more unpleasant effects in addition to the appearance of the northern lights. After very violent solar eruptions, there can be blackouts or disruptions to satellite navigation or communication networks. If the Earth was not protected by the surrounding magnetic field, the consequences would be dire.

More recently, in 2003, after a strong sunspot cycle, a particularly large number of polar lights could be seen. Sky views stretched as far as Austria. “The immediate past sunspot cycle from 2008 to 2019 was the weakest for 100 years, which means that there were almost no northern lights in Austria,” meteorologist and geophysicist Andreas Pföser told ORF.at.

A link between relatively low auroral activity and climate change can be ruled out according to the current situation. While climate change affects the lower atmosphere, the aurora borealis is a phenomenon in the upper atmosphere. “According to the current state of knowledge, there is no evidence to date that would allow to conclude that current significant changes in the lower atmosphere also have an effect on the upper atmosphere,” explains Pfoser.

Hope in stormy times

However, it is difficult to predict in detail when the northern lights will increase again in Austria. According to the forecasts of the US National Meteorological and Oceanographic Administration (NOAA), the next maximum sunspot in 2025 will reach approximately the same level as the last maximum in 2014.

But: “December 2019 minimum sunspot was two years ago, so things are getting better,” says Pfoser. In principle, an increase in auroral activity can be expected in the next few years. “The current development gives at least hope that the next increase in activity could be a little better than expected.”

Despite all the predictions and information hidden behind the sky data, the auroral observations are a unique experience – even for experienced scientists like Pfoser: “Even if I try to imagine what happens just above us when observing the aurora borealis, I then often escape in scenic admiration for this the scene “.