The thermosphere is the atmospheric layer located above the mesosphere and below the exosphere. It generally begins at about 85 kilometres altitude and extends upward to several hundred kilometres, with the exact upper boundary depending on atmospheric conditions and the gradual transition into the exosphere. The name thermosphere comes from the Greek thermos, meaning heat, referring to the very high kinetic temperatures reached by individual atmospheric particles in this region.
Temperature can rise to well above 1000 degrees Celsius and can become considerably higher during strong solar activity. However, the thermosphere would not feel hot to a person in the ordinary sense because its density is extraordinarily low. A small number of molecules moving at high speeds cannot transfer heat to an object in the same way that dense air near the surface can. The thermosphere is therefore a region of high particle energy but extremely low atmospheric density, a distinction that is essential for understanding its physical behaviour.
Temperature rises strongly with altitude through much of the thermosphere because sparse atmospheric particles absorb high energy solar radiation, particularly extreme ultraviolet and X ray radiation. Oxygen and nitrogen molecules and atoms absorb this energy and become electronically excited or ionised. The absorbed energy increases the kinetic energy of particles, which corresponds to higher temperature. Solar activity therefore has a major effect on thermospheric temperature and density.
During periods of strong solar ultraviolet and X ray output, the thermosphere can expand, increasing atmospheric density at a given altitude. During quieter solar conditions, it contracts. These changes matter for satellites because increased thermospheric density creates greater aerodynamic drag on spacecraft in low Earth orbit. Even though the density is extremely low compared with surface air, it can still significantly affect objects travelling at several kilometres per second.
The thermosphere overlaps extensively with the ionosphere, but the two concepts describe different properties. The thermosphere is primarily defined as a temperature based atmospheric layer, whereas the ionosphere is a region characterised by significant concentrations of electrically charged particles produced by ionisation. The ionosphere extends roughly from the upper mesosphere through the thermosphere and into the lower exosphere, so there is no exact one to one boundary between the two.
Ionisation occurs when energetic solar radiation removes electrons from atoms or molecules, creating positive ions and free electrons. These charged particles respond to electric and magnetic fields and can participate in electrical currents and plasma processes. The ionosphere is crucial to radio communication because the distribution of charged particles can refract, reflect or absorb certain radio frequencies. Thus the thermosphere is both a thermally defined layer and a major environment for ionospheric physics.
Auroras are among the most visible phenomena associated with the thermosphere and ionosphere. Auroras occur when energetic charged particles associated with the solar wind and Earths magnetosphere are guided towards the polar upper atmosphere by magnetic field lines. These particles collide with atmospheric atoms and molecules, transferring energy to them. When the excited particles return to lower energy states, they emit photons at characteristic wavelengths.
Oxygen and nitrogen contribute to different auroral colours depending on altitude, particle energy and atmospheric composition. Auroras are therefore not simply clouds of light but visible evidence of interactions among the solar wind, Earths magnetic field and upper atmospheric gases. Strong geomagnetic storms can produce auroras at unusually low latitudes and can also modify ionospheric conditions. Such disturbances may affect radio navigation, satellite operations, communications and electrical infrastructure.
The thermosphere contains extremely low atmospheric pressure and density. At hundreds of kilometres above the surface, the mean distance between particles becomes very large compared with conditions near sea level. The atmosphere is therefore not uniformly mixed in the same way as the lower atmosphere. Individual atomic species become increasingly important with altitude because lighter particles are more likely to dominate at greater heights.
Collisions become less frequent, and some processes that are collision dominated near the surface become controlled by radiation and electromagnetic interactions. The transition to the exosphere occurs gradually as collisions become sufficiently rare that particles can travel long distances without interacting with other atmospheric particles. The thermosphere is consequently a region in which conventional concepts such as air pressure, wind and temperature must be interpreted in the context of a highly rarefied gas and, increasingly, a plasma environment.
The thermosphere is important for spaceflight because many low Earth orbit satellites travel through it. Although low Earth orbit is often described as space, spacecraft at several hundred kilometres altitude still encounter a very thin atmosphere. Aerodynamic drag gradually removes orbital energy, causing spacecraft to descend unless their orbits are periodically corrected. Solar activity can increase this drag by heating and expanding the thermosphere. The International Space Station operates within the upper thermosphere and therefore experiences measurable atmospheric resistance.
Satellites must account for variations in thermospheric density when predicting orbital trajectories. Atmospheric drag also contributes to the eventual natural reentry of many low Earth orbit objects. Consequently, thermospheric research is directly relevant to spacecraft tracking, collision avoidance, satellite lifetime prediction and management of the increasingly crowded near Earth orbital environment.
Scientific investigation of the thermosphere uses satellites, rockets, ground based radar, optical instruments and measurements of ionospheric properties. Researchers monitor temperature, density, composition, electric fields, magnetic fields, charged particle populations and responses to solar activity. Models combine observations of solar radiation and geomagnetic conditions with atmospheric physics to predict changes in thermospheric density. The thermosphere also participates in complex coupling between the lower atmosphere and near Earth space.
Atmospheric tides and waves can propagate upward from lower layers, while solar and geomagnetic activity can drive disturbances downward through the upper atmosphere. This bidirectional coupling means that the thermosphere cannot be treated as completely independent of the atmosphere below or the space environment above.
The thermosphere is therefore a high altitude atmospheric region characterised by very low density, strong absorption of energetic solar radiation, high kinetic temperatures, variable density and extensive interaction with the ionosphere and magnetosphere. It differs fundamentally from the troposphere because the processes dominating it are not ordinary weather and convection but radiation, ionisation, electromagnetic forces, upper atmospheric waves and interactions with solar activity.
It also differs from the exosphere because collisions, although infrequent, remain more significant within the thermosphere than in the increasingly collisionless exosphere. Understanding the thermosphere is important for atmospheric science, space weather, satellite operations, radio communication, navigation, auroral science and studies of how Earths atmosphere interacts with the Sun.
