The thermosphere is a vast, high-altitude layer of Earth’s atmosphere, serving as the transition zone between the denser atmosphere below and the vacuum of space above. It begins above the mesosphere, at an altitude of approximately 80 to 90 kilometers, and stretches upward to between 500 and 1,000 kilometers, where it gradually merges into the exosphere. This layer is unique because the atmosphere is so thin that the traditional concept of “flight” gives way to orbital mechanics. The objects and phenomena that exist here result directly from the thermosphere’s extreme physical characteristics and its interaction with the Sun’s energy.
Defining the Thermosphere Environment
The thermosphere is named for its high temperatures, which increase dramatically with altitude due to the absorption of high-energy solar radiation, specifically ultraviolet and X-rays. Temperatures can soar from around -120°C at the bottom to over 2,000°C near the top, often reaching 2,500°C during periods of high solar activity. Despite these extreme temperature readings, an object or person within the thermosphere would feel intensely cold, a contradiction explained by the layer’s extremely low density.
The air in this region is so sparse that most of the thermosphere is technically considered outer space, with density many trillion times lower than at sea level. The high temperature reflects only the kinetic energy of the individual, widely spaced gas molecules, primarily atomic oxygen and nitrogen. Since molecules are few and far between, they do not collide often enough to transfer significant heat energy, making conventional heat transfer impossible. This layer also contains the ionosphere, an electrically charged region created by solar radiation stripping electrons from gas atoms.
Artificial Satellites and Orbital Habitats
The primary “fliers” in the thermosphere are objects in Low Earth Orbit (LEO), the region most utilized by human space activity. These objects are not flying in the aerodynamic sense, but are orbiting the Earth at immense speeds, generally between 340 and 600 kilometers in altitude. This places them firmly within the upper thermosphere. The largest and most prominent object is the International Space Station (ISS), which maintains an orbit averaging around 400 kilometers.
A vast number of functional satellites also reside in this layer, performing roles that include Earth observation, weather monitoring, and communication. The orbital paths of these spacecraft allow them to take advantage of close proximity to Earth while remaining above the bulk of the atmosphere. Constellations of internet satellites typically operate in the upper reaches of the thermosphere for global coverage.
Natural Phenomena in the Thermosphere
The thermosphere is the stage for Earth’s most stunning natural light shows: the Auroras Borealis (Northern Lights) and Australis (Southern Lights). These displays occur when charged particles from the solar wind, guided by Earth’s magnetic field, precipitate into the upper atmosphere. The collisions between these energetic particles and the sparse gas atoms cause the atoms to become excited.
When the atoms return to their normal, lower-energy state, they release the absorbed energy as photons of light. The color of the aurora depends on the type of gas atom and the altitude of the collision. Collisions with atomic oxygen, which is abundant, typically produce the common green light, usually between 100 and 150 kilometers in altitude. Red auroras, produced by oxygen at higher altitudes, and blue/purple hues, created by nitrogen, also occur within this dynamic layer.
The Role of Atmospheric Drag and Orbital Decay
Despite the extremely low density of the thermosphere, it is still dense enough to exert a measurable drag force on orbiting objects. This atmospheric friction, though slight, slowly saps orbital energy from satellites traveling at speeds of approximately 7 to 8 kilometers per second. The consequence of this constant drag is orbital decay, a gradual reduction in altitude.
Engineers must account for this drag when calculating satellite trajectories. Spacecraft operating in LEO, like the ISS, require periodic reboosts to counteract the decay and maintain their intended altitude. The density of the thermosphere is highly variable, expanding and contracting significantly due to the 11-year solar cycle. During periods of high solar activity, the atmosphere heats up and expands, increasing the drag on satellites and accelerating the rate of orbital decay.