The thermosphere is a layer of Earth’s atmosphere characterized by incredibly high temperatures yet a cold environment. It begins around 85 kilometers (53 miles) above the surface and extends to about 600 kilometers (372 miles), where it fades into the exosphere and then into space. Temperatures in this region range between 500°C and 2,000°C (932°F and 3,632°F), sometimes rising higher during intense solar activity. Despite this heat, the air density is so low that the thermosphere is often considered part of outer space, and an object within it would feel intensely cold.
Absorption of Extreme Solar Radiation
The thermosphere’s high temperature results from its direct interaction with the sun’s most energetic radiation. This layer is the first to encounter and absorb high-energy, short-wavelength radiation, specifically X-rays and Extreme Ultraviolet (EUV) light. When these photons strike the sparse gas molecules (primarily oxygen and nitrogen), the energy is absorbed directly.
This absorption converts the sun’s radiant energy into the kinetic energy of the gas particles. The molecules gain enough energy to break apart (photodissociation) and lose electrons (photoionization). The resulting individual atoms and ions move at extremely high velocities, which is the physical definition of high temperature. This continuous energy input keeps the few particles highly energetic, causing the temperature to increase sharply with altitude.
The Distinction Between Temperature and Heat
The high temperature recorded in the thermosphere measures the average speed of its individual particles, not the amount of thermal energy available. Temperature reflects the kinetic energy of the particles, which is very high because the few atoms present are moving extremely fast. Heat, however, is the transfer of thermal energy, requiring a dense medium where particles frequently collide to share energy.
The thermosphere is an almost perfect vacuum, with an air density less than one-trillionth of the density at sea level. Because the particles are so spread out, the rapidly moving atoms rarely collide with each other or with any solid object. While the temperature reading is high, the overall thermal energy content is low, and very little of that energy can be transferred to an object.
An object placed here would not feel hot; instead, it would lose heat rapidly through thermal radiation into the cold vacuum of space. This loss far outpaces the negligible heat gain from the sparse, fast-moving particles. This is why the International Space Station and satellites, which orbit within the thermosphere, must be heavily insulated to protect against the cold.
Unique Atmospheric Phenomena
The ionization caused by solar radiation absorption creates the ionosphere, a region of electrically charged particles within the thermosphere. This layer reflects certain radio waves back to Earth, enabling long-distance radio communication. The density of these charged particles varies significantly with solar activity, affecting the reliability of global positioning systems and satellite communications.
The high-energy particles also cause the Aurora Borealis (Northern Lights) and Aurora Australis (Southern Lights). These lights occur when charged particles from the solar wind are channeled toward the Earth’s poles by the magnetic field. When these particles collide with oxygen and nitrogen atoms, the atoms become excited and release their excess energy as photons of light.
The slight, varying density of the thermosphere creates atmospheric drag, affecting objects in low Earth orbit. Even though the air is extremely thin, this constant friction slowly causes orbiting spacecraft to lose altitude. Engineers must account for this drag by periodically performing orbital reboost maneuvers to push satellites back to a higher altitude.