The thermosphere is a high-altitude layer of Earth’s atmosphere positioned directly above the mesosphere and beneath the exosphere, which transitions into outer space. This region begins at approximately 80 to 90 kilometers and can extend upward to between 500 and 1,000 kilometers, with its upper boundary fluctuating significantly due to solar activity. Although it is considered part of the atmosphere, the air density is incredibly low, meaning the environment within the thermosphere is functionally a vacuum. This layer absorbs the most energetic radiation from the Sun, which causes the temperature of the sparse gas particles to soar.
The Elemental Composition
The air within the thermosphere is highly rarified, but its composition differs dramatically from the nitrogen and oxygen molecules found closer to the ground. Above 90 kilometers, known as the turbopause, gases separate based on their atomic weight. This process is called diffusive separation, and it means the lighter elements dominate the higher altitudes. Solar ultraviolet and X-ray radiation bombard the few molecules present, causing them to break apart in a process called photodissociation. Molecular nitrogen and molecular oxygen break down into their atomic forms, leading to a layer dominated by atomic oxygen and atomic nitrogen. At the very top of the thermosphere, the lightest elements, helium and hydrogen, become the most abundant species.
The Ionized State of the Atmosphere
A defining feature of the thermosphere is the presence of the ionosphere, an electrically charged region that largely overlaps with it. The absorption of intense solar radiation, specifically ultraviolet and X-ray photons, strips electrons from the neutral gas atoms and molecules. This process, photoionization, creates a collection of positively charged ions and negatively charged free electrons, forming a plasma. The kinetic energy of these charged particles is extremely high, which is why the thermosphere is named for heat, with temperatures potentially reaching 2,000° C or more.
However, the extreme thinness of the air means there are too few particles to transfer this energy to an object, so an astronaut would feel intensely cold despite the high kinetic temperature. This plasma is layered into distinct regions, labeled D, E, and F, with the F-layer possessing the highest plasma density. This ionized state has a practical application, as the layers of the ionosphere can reflect certain radio waves back toward Earth, allowing for long-distance communication. The density and effectiveness of the ionosphere for radio communication vary constantly, changing between day and night as well as with the 11-year solar cycle.
Phenomena of Light and Energy
The interaction between the thermosphere’s gas particles and energy from space produces the auroras, spectacular visual displays known as the Aurora Borealis in the north and the Aurora Australis in the south. These light shows are concentrated in oval-shaped zones around the magnetic poles. They are caused by the solar wind, a stream of charged particles ejected from the Sun. Earth’s magnetosphere captures and channels these charged particles down the magnetic field lines toward the polar regions, where they plunge into the upper atmosphere.
The incoming particles collide with the atomic oxygen and nitrogen within the thermosphere, exciting these atoms to a higher energy state. When the excited atoms return to their normal state, they release the excess energy in the form of light. The color of the light depends on the type of atom and its altitude of interaction. Collisions with atomic oxygen at lower altitudes (around 100 to 300 kilometers) typically produce the most common green glow. Higher-altitude oxygen collisions, above 300 kilometers, can result in a red color, while nitrogen atoms produce blue and reddish-purple hues.
Objects Orbiting or Entering the Layer
The thermosphere is a heavily utilized region for space operations, hosting many Low Earth Orbit (LEO) satellites and the International Space Station (ISS). The ISS orbits well within this layer, typically at an altitude between 370 and 460 kilometers. Though the atmosphere is extremely thin, the residual air particles create friction called atmospheric drag. This drag constantly acts to slow down orbiting objects, gradually pulling them back toward Earth. To counteract this effect, the ISS and other LEO satellites require periodic orbital boosts. The density of the thermosphere, and thus the strength of the drag, is highly sensitive to solar activity, requiring more frequent boosts when the Sun is active.
The thermosphere also serves as a protective shield against space debris and meteoroids. When small chunks of rock and metal enter the atmosphere at extremely high speeds, the friction causes them to heat up and disintegrate. This process, known as ablation, creates the visible streaks of light commonly called shooting stars, with most burning up completely within this high-altitude layer.