In February 2016, I had the opportunity to participate in a course of the University of Umeå with the title Arctic Science. In the course, that was held for one week in the snow-covered landscape of Lappland, we learned about the physics of northern lights, snowflakes and climate change within the context of the arctic. In this series of posts, I want to share with you my impressions and what I learned during the lectures and lab sessions.

# The Aurora Borealis

An aurora borealis is a natural light display, that can regularly be observed in the polar regions of our planet. Under certain conditions, the green and red lights in the sky can even be seen in the more densely populated parts of northern and central Europe. In recent posts, I wrote about auroral observations in Dalarna and Stockholm. If occurring in the southern hemisphere, it is called aurora australis instead.

Observing the sky brighten up in the middle of the night displaying colorful and vivid formations of light has fascinated people for centuries. However, a clear understanding of what is happening up in the atmosphere could only be gained thanks to modern physics within the last hundred years. And even today, many details about the aurora are still unclear.

During an aurora, about 1.000.000 MW of energy are released to the atmosphere. This power, that is greater than the total U.S. electric power consumption, originates from the Sun. In so called solar winds, charged particles like electrons travel from the Sun to Earth, where they hit and interact with its magnetosphere. The magnetosphere acts as a protection shield and only lets particles pass close to the poles. There, they deposit their energy in the atmosphere by interaction with molecules, resulting in beautiful lights in red and green colours.

## The Solar Wind

The Sun is the major source of energy in our solar system. It is a gigantic burning ball of hydrogen gas at a temperature of about 6,000 Kelvin at its surface. Not only emits the Sun light, that makes living possible on Earth. In gigantic eruptions, it also fires a lot of its hydrogen into space, resulting in the so called solar wind.

Because of the high temperature of the solar wind’s origin (about 1,000,000 Kelvin), hydrogen ions split into their constituents: electrons and protons. Such an ionized gas is called plasma. With speeds of around 150 to 300 km/s the charged particles travel towards Earth and reaches our planet after around a week (the distance between Sun and Earth is $1~\text{au} = 10^8~\text{km}$).

## The Magnetosphere

Arriving at Earth, the solar wind has to pass another hurdle before it can generate beautiful lights in the sky: the magnetosphere.

Illustration of Earth’s Magnetosphere (image from NASA/public domain)

The magnetosphere is a magnetic shielding against the solar wind that arises from Earth’s intrinsic magnetic field. Because of interactions between the solar wind and the magnetic field, the magnetosphere is compressed to 10 Earth radii at the side facing the Sun and extended to 100 Earth radii at the opposite site as shown in the illustration on the right.

For the solar wind, the equipotential lines of the magnetosphere (blue in the illustration) act as if they were wires for the current. This results in effective blocking of the solar wind on the day side and a deflection towards the night side. The solar wind can then flow back and enter the Earth’s atmosphere, but only close to the poles, where the equipotential lines originate.

That’s why an aurora is more likely to be observed close to the poles, but it doesn’t mean, that an aurora cannot be observed further south. Depending on the strength of the solar wind they can even be visible as far south as Germany or Poland. In fact, the highest likelihood for auroral observations is at latitudes around the north of Sweden, where the cities Kiruna and Abisko are located and these pictures were taken.

## Auroral Observations

When the protons and electrons of the solar wind hit the particles of the atmosphere, we finally see an aurora. The fast and charged particles carry a lot of energy and when they hit the oxygen and nitrogen molecules of the atmosphere, they can transfer this energy. The important result is not, that the oxygen starts to move (because especially an electron is comparably light), but the oxygen will start to vibrate and more importantly transfer energy to its electrons.

After a while, the electrons that surround the oxygen molecule will relax. One important conclusion of modern physics is, that they will not do this gradually, but instantaneous under the emission of (in this case) visible light. This light will have a predetermined wavelength depending on which molecule was excited. By means of this we can even identify which molecules of the atmosphere cause which colours.

• Red light is emitted by atomic oxygen (O) at high altitudes.
• Green light is present in lower altitudes and emitted by a different excitation of atomic oxygen and molecular oxygen (O2).
• Blue light is emitted at yet lower altitudes by molecular nitrogen (N2).

This results in colourful auroral displays in the night sky, where green is most dominant as there are more excitations for green light and additionally the human eye is most sensitive for this colour. The forms of the aurora are given by the magnetic field lines, that guide the particles through the atmosphere. Because the electrons and protons also change the magnetic field, a strong aurora will also move just like it would be blown by wind. Depending on from where the aurora is observed, one can either see a huge corona above or long lines and circles close to the horizon.

Making use of modern physics, auroral displays can be explained today after they fascinated people for centuries. And even if one doesn’t know whey they occur, the northern lights are something magical. I hope you found the text interesting and enjoyed the pictures. Feel free to have a look at my Facebook page or subscribe for future updates by mail. More information about auroral displays can be found in the book “The Northern Lights” by Dr. Syun-Ichi Akasofu.