Above the cold mesosphere lies the thermosphere — the hottest layer of the Earth's atmosphere, with temperatures that can exceed 2,500°C during periods of high solar activity. The name comes from the Greek "thermos" (hot), and the heat is real. Yet a person somehow placed in the thermosphere would feel cold rather than hot, because the gas is so thin that there is not enough mass to transfer significant heat. The "temperature" of the thermosphere is a kinetic temperature — a measure of the average speed of the individual gas molecules — but at the densities present, there are too few molecules around to actually warm anything substantial. The thermosphere extends from about 85 kilometres altitude (the top of the mesosphere) up to between 500 and 1,000 kilometres altitude, depending on conditions. Almost all human spaceflight and most satellites operate in the thermosphere or the exosphere just above it.
The Temperature Structure
The temperature in the thermosphere increases sharply with altitude, rising from about -100°C at the bottom (the mesopause) to between 500°C and 2,500°C at the top, depending on solar activity. The lower temperatures occur during solar minimum and the higher temperatures during solar maximum, when the Sun emits much more extreme ultraviolet (EUV) and X-ray radiation. EUV photons are absorbed by nitrogen and oxygen molecules in the thermosphere, breaking apart molecular bonds and giving the resulting atoms and ions high kinetic energies. The temperature gradient is steepest in the lower thermosphere and gradually levels off; above about 400 kilometres, the thermosphere is essentially isothermal at whatever temperature the solar input has produced. The temperature itself is highly variable — varying by 1,000°C or more between solar minimum and solar maximum is normal.
Why It Would Feel Cold
The thermosphere's "hot" temperatures are deceiving. Temperature in physics is a measure of the kinetic energy of particles, but heat is the transfer of that energy from particle to particle. In the thermosphere, the gas density is about a billion times less than at sea level — there are very few molecules around to actually conduct or convect heat into anything you brought into the layer. The total thermal energy per cubic metre is much less than at sea level, even though each individual molecule is moving faster. A spacecraft or astronaut in the thermosphere would lose heat to space (through infrared radiation) much faster than it could gain heat from collisions with the sparse hot gas. This is why thermal management for spacecraft is dominated by radiative balance, not by atmospheric heating, even when the spacecraft is "in" the hot thermosphere.
Composition and the Heterosphere
The thermosphere marks the beginning of the "heterosphere" — the part of the atmosphere where the composition varies with altitude due to differential diffusion. Below about 100 kilometres altitude, the atmosphere is well-mixed and has roughly the same proportions of gases at all altitudes (the "homosphere"). Above 100 kilometres, turbulent mixing is no longer fast enough to overcome the tendency of each gas to settle according to its own scale height (which depends on molecular weight and temperature). Lighter gases (hydrogen, helium, atomic oxygen) become progressively more dominant with altitude. By the upper thermosphere, atomic oxygen is the dominant species. Higher still, hydrogen takes over. The composition is no longer the familiar 78 percent nitrogen / 21 percent oxygen / etc. — it varies with altitude in complex ways.
The Ionosphere
Overlapping with the thermosphere is the ionosphere — a region of ionised gas (plasma) produced by solar UV and X-ray radiation. The ionosphere extends from about 60 kilometres altitude through the thermosphere and into the exosphere, with several distinct layers (D, E, F1, F2) at different altitudes. The ionosphere is electrically conductive because of its high content of free electrons and ions, and it plays a critical role in radio communications by reflecting some radio frequencies back to Earth. The "skywave" propagation that allows shortwave radio to be heard at very long distances depends on ionospheric reflection. The ionosphere is highly variable — its electron density changes substantially between day and night, with solar activity, and with geomagnetic conditions. Space weather forecasting is largely about predicting ionospheric variability.
The Aurora
The visible auroral displays — the Northern and Southern Lights — happen in the thermosphere, primarily between 100 and 300 kilometres altitude. The mechanism: charged particles from the magnetosphere are accelerated downward along magnetic field lines, enter the upper atmosphere, and collide with atoms and molecules. The collisions excite the atmospheric atoms (transfer energy to them); the atoms then release this energy as light when they return to their ground states. The colours depend on which atoms are excited and at which altitudes. Oxygen produces green light at 100-200 km altitudes (the most common auroral colour) and red light at higher altitudes (300+ km). Nitrogen produces blue and purple. The auroral oval — the rough ring of typical auroral activity around each magnetic pole — extends to lower latitudes during periods of high geomagnetic activity. During major storms, auroras can be visible at mid-latitudes; the largest ever, the 1859 Carrington Event, produced aurora visible as far south as the Caribbean.
The ISS and Low-Earth Orbit
The International Space Station orbits at about 400 kilometres altitude — comfortably within the thermosphere. The ISS experiences detectable atmospheric drag at this altitude, requiring periodic reboosts to maintain its orbit (about once a month on average). Many other satellites in low Earth orbit (LEO) — including most weather satellites, communications satellites, and reconnaissance satellites — are in similar altitude ranges of 300-1,000 kilometres. The drag at these altitudes is enough to gradually lower satellite orbits over time; most LEO satellites eventually re-enter the atmosphere as their orbits decay. Higher altitudes have less drag (and less atmosphere), so satellites there have longer orbital lifetimes. The atmospheric drag in the thermosphere is highly variable — it increases substantially during solar maximum (when the thermosphere is warmer and expanded) and decreases during solar minimum. This complicates satellite operations and orbital prediction.
The Karman Line
The Kármán line, often defined as the boundary between Earth's atmosphere and outer space, sits at 100 kilometres altitude — at the boundary between the mesosphere and thermosphere. The line is named after the Hungarian-American engineer Theodore von Kármán, who proposed it in 1957 as the altitude above which aerodynamic flight would require orbital speeds — meaning conventional aircraft physics breaks down because the air is too thin. The Kármán line is used by the FAI (International Aeronautical Federation) and most national space programs as the formal definition of where space begins for purposes of awarding astronaut status. The US uses a slightly different boundary (80 kilometres, the top of the mesosphere), which is why some US Air Force pilots earned astronaut wings in the 1960s for flights that the FAI would not have considered space flight.
Re-entry
The thermosphere is where atmospheric re-entry begins for returning spacecraft. The thin air at 100-200 kilometres altitude is enough to start producing significant drag and heating on a spacecraft moving at orbital speed (about 8 km/s). The compression of the air in front of a re-entering spacecraft heats it (and the surrounding air) to several thousand degrees, producing the well-known fireball effect. The heat shield of a re-entering spacecraft has to withstand and dissipate this heat. The deepest part of the heating occurs in the upper mesosphere and lower thermosphere, roughly 50-80 kilometres altitude. By the time the spacecraft has slowed to subsonic speed (in the stratosphere or upper troposphere), the heating phase is largely over and parachutes or other landing systems can be deployed.
Solar Influence
The thermosphere is the part of the atmosphere most directly and dramatically influenced by the Sun. Solar UV and X-ray output varies enormously over the 11-year solar cycle, and even more dramatically during solar flares and coronal mass ejections. The thermosphere expands during solar maxima (warmer and puffier), and contracts during solar minima. The geomagnetic storms produced by solar events can heat the thermosphere by hundreds of degrees within hours, with consequences for satellite drag, communications, and various other systems. The thermosphere is therefore a key part of the "space weather" system that affects modern technology, including GPS, satellite communications, electric power grids, and human spaceflight.
What We Know and Don't
The thermosphere is studied through a combination of satellite measurements, ionospheric radars, sounding rockets, and modelling. The fundamentals of its temperature, density, and composition are well-understood. The dynamic responses to solar variability are less well-predicted than would be ideal, and improving thermospheric forecasting is an active research area. Long-term trends, including the cooling and contraction associated with greenhouse gas increases, are documented but their full implications for satellite operations and other concerns are still being worked out. The thermosphere is in many ways the frontier between the atmosphere we live in and the space we explore — and it is a frontier whose study continues to mature.