It may be one of the outermost layers of the Earth’s atmosphere, but the importance of the thermosphere cannot be underestimated. In this article, we take a closer look at this atmospheric layer and its characteristics.

Situated close to the boundary between the atmosphere and space, the thermosphere is only separated from outer space by the exosphere, Earth’s fifth and outermost layer.

Although Earth’s fourth atmospheric layer has very few characteristics in common with the three layers closer to the planet’s surface, it still has a valuable role to play.

We examine not only what the defining characteristics of the thermosphere are but also the facts that separate it from the other four layers of the atmosphere.

Before we look at specific facts about the thermosphere and its defining characteristics, we first need to establish exactly what the thermosphere is:

Thermosphere Definition

The thermosphere is the fourth of Earth’s five atmospheric layers, situated above the mesosphere and below the exosphere. It begins at an altitude of 90 km (56 miles) and extends to 1000 km (621 miles).

It is the hottest atmospheric layer and the part of the atmosphere where the Aurora Borealis and Aurora Australis occur.

“Thermosphere” is derived from the Greek word thermos, meaning “heat.” It is a very apt description since the thermosphere is characterized by extremely high temperatures.

Scientists know less about this layer than those below it, as it lies above the upper limits of conventional aircraft but overlaps with the region where low-orbiting satellites operate. One of the ways to study this part of the atmosphere is by using sounding rockets.

The air is extremely thin (with almost zero air density), and gravity is still present in the thermosphere. The properties of the air closely resemble those of the vacuum of space as a result.

Since space is seen to start at 100 km (62 miles), known as the Kármán line, above the Earth by many definitions, it is not surprising that the thermosphere is seen as part of space in many circles.

The little air present in the thermosphere mainly consists of helium, atomic nitrogen, and atomic oxygen.

Ions are also created in the thermosphere when ultraviolet radiation causes photoionization of molecules. (This process takes place in the ionosphere, which is spread over the thermosphere and stretches over parts of the mesosphere and exosphere.)

The thermosphere starts at a height of around 90 km (56 miles) and extends up to heights of between 500–1 000 km (311 to 621 miles). This makes the thermosphere thicker than all the other layers combined.

One of the main characteristics of the thermosphere is the extremely high temperatures that occur within this layer (as the name of the layer would suggest). With temperatures reaching around 2000° Celsius (3632° Fahrenheit), the thermosphere is the hottest of all the layers in the atmosphere by a huge margin.

The temperature is not constant, though. Between day and night, an average difference of 200° Celsius (360° Fahrenheit) can occur. The amount of solar radiation also has a direct influence on the temperature, causing as much as 500° Celsius (900° Fahrenheit) variation, depending on the amount of radiation.

Ironically, you would not be able to feel these extremely high temperatures. As the air is so thin that it basically resembles a vacuum, there are very few particles/atoms in the air to conduct the heat.

As a result of this lack of conduction, you will actually experience cold temperatures. It very often drops to below 0° Celsius (32° Fahrenheit), especially at night.

One of the most well-known characteristics of the thermosphere is the presence of the Aurora Borealis (Northern Lights), the spectacular meteorological phenomenon that occurs over regions in the Arctic Circle.

The Aurora Borealis is a result of charged particles from the sun colliding with gaseous particles in the thermosphere. This causes the colorful light display that observers in the Northern Hemisphere are so familiar with. (Green is one of the common colors created.)

The thermosphere has three main benefits, with one being a valuable side-effect.

1) Protection Against The Sun’s Radiation

It supports and protects all life on Earth by absorbing the majority of the Sun’s X-rays and extreme ultraviolet radiation. A byproduct of the absorption of solar radiation is the creation of the ionosphere.

The ionosphere is a direct result of the vast amount of ions that are formed within the thermosphere. When X-rays and UV radiation collide with gas molecules, some electrons are knocked free to form electrically charged ions. And herein lies the side-benefit…

2) Creation Of The Ionosphere

The great benefit of the ionosphere is its ability to make long-distance radio communication possible.

Before satellite and other forms of wireless communication emerged, radio waves were the only way to communicate over long distances. As it requires “line-of-sight” to communicate, radio waves are limited by the natural curvature of the Earth.

Radio operators then discovered the unique characteristics of the ionosphere. The electrically charged ions act as a giant mirror for radio waves. This simply means radio waves can now travel vast distances by simply bouncing them off the ionosphere.

(There are still limitations, and the use of radio waves has been replaced by digital forms of communication in most cases, but it still remains very important and relevant.)

3) The Ideal Environment For Space Utilization & Exploration

As already mentioned, the thermosphere is home to the ISS (International Space Station) and almost a thousand active low-orbit satellites around the Earth.

The thermosphere’s location and environment make it ideal for us to be able to put objects in a permanent (or semi-permanent) orbit in space. 

It is high enough for gravity to have very little effect on a spacecraft, yet still close enough to the Earth’s surface to use less powerful rockets to reach the thermosphere. This makes it much more affordable and economically viable to use the advantages of space.

(In order for a spacecraft to break completely free from Earth’s atmosphere and travel into outer space, it needs much more powerful rockets that would have been too expensive to make the launch and maintenance of satellites and NASA’s Space Shuttle Program economically viable.)

In the future, platforms may be built in the thermosphere that will serve as launching pads for deep space exploration. The thermosphere really provides endless possibilities, especially with the increase of companies from the private sector entering the space arena.

Like all the layers below it, the thermosphere is separated from the exosphere above it by a thin layer. The thermopause forms the boundary between the two layers.

Above the thermopause, you will find the atmosphere’s fifth and last layer, the point where the atmosphere truly turns into space.

The following list highlights the characteristics and facts of the thermosphere in more detail.

1. 1. The thermosphere is the fourth layer of the atmosphere (above the troposphere, stratosphere, and mesosphere).
   2. It extends from a height of approximately 90 km (56 miles) to 1000 km (621 miles) above the Earth’s surface.
   3. It borders the mesosphere below through a thin transitional space called the mesopause.
   4. It borders the exosphere above through a thin transitional space called the thermopause.
   5. It is the part of the atmosphere where low-orbiting satellites and the International Space Station are found.
   6. The thermosphere is the hottest of the five atmospheric layers, with temperatures reaching up to 2 500° Celsius (4 530° Fahrenheit).
   7. It is home to the phenomena known as the Aurora Borealis (Northern Lights) and Aurora Australis (Southern Lights).
   8. A large part of the ionosphere is located in the thermosphere.
   9. Like the stratosphere, the thermosphere plays an important part in protecting the planet from the Sun’s dangerous UV and X-ray radiation through absorption.
   10. It is the thickest of the four inner atmospheric layers at 513 km (319 miles). 
   11. The layer is characterized by the presence of atmospheric waves (similar to those experienced in our oceans).
   12. It makes long-distance radio communication possible by allowing radio waves to bounce off the ions in the layer and travel over longer distances.

Although this list does not contain all the data available about the thermosphere, it highlights the key facts and characteristics of this layer.

Like the three atmospheric layers below it, the thermosphere has a vital role to play in protecting the planet and all life in it, as this article illustrates.

Although it only has a fraction of the gas and other particles present in lower layers, the thermosphere is situated at the ideal height for low-orbit space utilization and contains enough gas molecules to absorb a significant amount of dangerous solar radiation.

If you are interested in the complete structure and make-up of the atmosphere, this article covers all five atmospheric layers and their relation to each other in more detail.

Until next time, keep your eye on the weather!