Aurora and the International Space Station: What Happens in the Thermosphere?

FROM THE LECTURE SERIES: A Field Guide to the Planets

By Sabine Stanley, Ph.D., Johns Hopkins University

The thermosphere is one of the layers in the atmosphere. Too thin for planes to fly and too cold for humans to survive. However, it is home to the beautiful northern lights–the aurora. However, they are not the only things that happen in the thermosphere. The International Space Station also orbits the Earth in this layer.

Icelandic spiral northern lights in autumn time.
Aurora is one of the phenomenon happening in the thermosphere layer.

The thermosphere is one of the higher layers of the atmosphere, with loosely flying gas atoms. The atoms are so far away that collisions rarely happen, and they cannot behave like a gas. However, the electrons flying through this layer from the Sun are enough to collide with the atoms and create the aurora. To understand the thermosphere and what happens there, first we should know the atmosphere.

The first two layers of the atmosphere are troposphere and stratosphere. The third layer is the mesosphere, extending from about 50 to 80 kilometers in altitude.

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The Mesosphere

The mesosphere has no ozone atoms or other heating source; thus, its temperature decreases significantly with height. The layer ends at the mesopause, with a temperature of -120°F, making it the coldest layer of the atmosphere. The mesosphere is the least studied layer, due to its conditions: the pressure is too low for anything to fly and gather data, but it is too high for satellites to orbit.

Some electric phenomena happen in the mesosphere: red sprites and blue jets, both due to lightning in the mesosphere. Besides, most meteors burn up here, making the mesosphere home to shooting stars. The next layer is the thermosphere.

This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.

The Ionosphere in the Thermosphere

The high layer above the mesosphere is called the thermosphere. It starts 80 kilometers above the surface, varying from 500 kilometers up to 1000 kilometers in altitude. Like in the stratosphere, temperatures in the thermosphere increase with altitude. Ultraviolet and X-rays are absorbed by molecules, ionizing them, and also creating the heat. Where electrically charged ions dominate is called the ionosphere, and thermosphere covers the largest part of it.

Even though temperatures can reach several thousands of degrees, the low density of air can make people freeze in the thermosphere. The conditions are not suitable for humans to survive but are very suitable for the aurora to be born.

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What Creates Aurora?

Colorful northern lights in Iceland.
Different colors of aurora depend on the atoms or molecules involved in the photon-emission process. (Image: Simon’s passion 4 Travel/Shutterstock)

The aurora is created when the high-energy particles around Earth’s magnetic field lines collide with atmospheric atoms. The incoming energy excites the electrons in the particles into higher energy levels, but cannot keep them there. Once the electrons go back to their normal state, they release photons, i.e., light. The color depends on the type of atom or molecule hit by energy: Oxygen creates red and green aurora, while nitrogen creates blue.

Not all aurorais visible because some occur in the ultraviolet and infrared ranges of the electromagnetic spectrum. However, there is another visible inhabitant of the thermosphere: The International Space Station.

The International Space Station

The most beautiful photos of aurora come from the International Space Station, orbiting in the thermosphere in 330 to 420 kilometers of altitude. The station is the size of a football field – the largest structure put into space – and has hosted over 230 astronauts since the year 2000.

It weighs almost a million pounds and orbits the Earth at over 17,000 miles per hour, thus, orbiting the Earth every 93 minutes. With its 15 orbits per day and the orbit inclination about 50° from Earth’s equator, the station gets pretty good coverage of the globe. It can be seen best before sunrise or right after sunset, as a bright white dot speeding through the sky. Normally, two to six astronauts live in the station.

Gravity in the International Space Station

Astronauts float in the space station, but not because gravity is very weak there. The gravitational acceleration at these altitudes is only about 10% weaker than gravity on Earth’s surface. However, the station is constantly freefalling, so people inside can float in the air.

International Space Station over the planet Earth. Elements of this image furnished by NASA
The International Space Station is another inhabitant of the thermosphere, orbiting the Earth 15 times a day. (Image: Vadim Sadovski/Shutterstock)

They do not feel gravity, as the station falls at the same rate they do. The horizontal velocity is enough to keep the station in orbit. This is the status of all objects orbiting the Earth. Living in space is a different subject, too long to be covered here.

In the end, aurora and the International Space Station are both in the thermosphere, whose super-low pressure creates the perfect conditions for orbiting and magnetic phenomena.

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Common Questions about Aurora

Q: What is an aurora, and what causes it?

An aurora is, in fact, photons released from electrons. Charged particles from the Sun collide with gas atoms in Earth’s atmosphere, exciting them to a state of higher energy. However, the electrons soon go back to their lower-energy state, emitting a photon. Photons are ‘particles’ of light; hence, the aurora is created.

Q: Where can I see an aurora?

An aurora occurs as a result of Earth’s magnetic field, and the collision of particles from the Sun with gas atoms in the Earth’s atmosphere. Thus, they happen around the Poles. In places that are closer to either Pole, there is a chance of seeing the northern lights. For example, Alaska, Sweden, and Northern Canada are places to spot aurora.

Q: Are auroras dangerous?

For people watching from the ground, an aurora has no danger. In general, they are not harmful to humans but can have potentially negative effects on infrastructure and technology. The reason is that they contain charged particles and excited atoms, which can affect the electricity-based technology under the circumstances.

Q: What causes aurora colors?

When charged particles from the Sun collide with gas atoms in Earth’s atmosphere, an aurora happens. The visibility of the phenomenon is due to the released photons in the process, which are in the range of visible light. The color depends on the type of atom involved in the process: Oxygen creates the green or red aurora, and nitrogen creates blue. However, some auroras happen in the UV range and are not visible to the human eye.

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