Thermosphere

Thermosphere is the layer of Earth’s atmosphere that begins at approximately 80 kilometers above the surface and extends to heights of 300 to 600 kilometers. This layer reaches very high temperatures due to the absorption of ultraviolet (UV) and X-ray radiation from the Sun; these temperatures can vary between 500 °C and 2,000 °C depending on solar activity. However, the temperature measured here is not temperature in the classical sense of being felt, because gas density is extremely low and molecular collisions occur very rarely.

The thermosphere also encompasses a significant portion of the ionosphere. In this region, ionization of gas molecules leads to the emergence of electromagnetic properties that affect the propagation of radio waves. Atmospheric emissions such as auroras also occur in this layer. Additionally, the thermosphere is of importance for space research and communication technologies because it is the region where space weather effects are observed and satellite orbits are regulated.

Physical Characteristics of the Thermosphere

Vertical Temperature Profile and Thermal Structure

The thermosphere begins immediately above the mesopause at approximately 80 kilometers and can extend up to 600 kilometers. Temperature in this layer increases rapidly due to the absorption of ultraviolet and X-ray radiation from the Sun. Temperature can vary between 500 °C and 2,000 °C depending on solar activity. The primary cause of this temperature increase is the absorption of high-energy photons by gas molecules in the low-density environment, which increases their kinetic energy. However, these temperature values do not represent perceptible heat in the classical sense because molecular collisions are extremely infrequent. Additionally, the propagation of atmospheric gravity waves in the upper atmospheric layers can cause fluctuations in local temperature and density distributions within the thermosphere. These waves have the potential to create disturbances in the ionospheric structure.

Gas Composition and Density Profile

The gas composition of the thermosphere consists largely of nitrogen (N₂) and oxygen (O₂) molecules. However, these molecules become ionized under the influence of photons to form the ionosphere. Density decreases exponentially with altitude. The gas density in the thermosphere is millions of times lower than in the troposphere. Therefore, in the thermosphere, the individual motion of particles is more significant than the concept of “air” as a continuous medium. Moreover, aerosols detected in the upper thermosphere can influence the region’s energy balance through the absorption and scattering of solar radiation, creating indirect effects on atmospheric chemistry and thermal structure.

Boundaries and Sub-Layers

The lower boundary of the thermosphere is the mesopause and its upper boundary is the exosphere. The exosphere is the region where molecules can escape Earth’s gravity and enter space. While some studies suggest that the thermosphere may be subdivided into internal layers, this distinction is not definitive. However, it overlaps with the ionospheric regions known as the D, E and F layers based on ion density. The F layer, particularly located between 200 and 400 kilometers, plays a role in reflecting radio waves.

Ionospheric Interactions and Electromagnetic Phenomena

Structure of the Ionosphere

The thermosphere contains a major portion of the ionosphere. The ionosphere is a region formed by the ionization of gases due to high-energy radiation from the Sun, consisting of electrically charged particles. This region is significant for the propagation reflection and refraction of electromagnetic waves. In particular, the F2 layer is used for high-frequency (HF) radio communication. The degree of ionization in the ionosphere is directly related to solar radiation and the day-night cycle.

Auroral Phenomena

The thermosphere is the layer in which visual natural phenomena such as aurora borealis (northern lights) and aurora australis (southern lights) occur. These phenomena result from charged particles from the Sun moving along Earth’s magnetic field lines and colliding with atmospheric molecules near the polar regions. These collisions produce the emission of high-energy photons, which are observed as visible light displays.

Space Weather Interactions

The thermosphere is the atmospheric layer where space weather processes are most distinctly felt. Events such as solar flares and coronal mass ejections directly affect particle density and temperature in the thermosphere. Such events can cause deviations in satellite orbits, disruptions in communication systems, and errors in global navigation systems.

Practical Importance and Observation of the Thermosphere

Satellite and Spacecraft Operations

The thermosphere is the layer where artificial satellites are placed in orbit and where structures such as the International Space Station (ISS) operate. Therefore, frictional forces in this layer are important for orbital calculations. Although gas density in the thermosphere is low, even this minimal density has a measurable effect over long-term orbital predictions and can cause satellites to gradually lose altitude.

Observational Instruments and Measurement Methods

The thermosphere cannot be observed directly from Earth’s surface. It is studied using instruments such as lidar systems, satellite-based spectrometers, ionospheric sounders, and rockets. NASA’s TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics) satellite and ESA’s Swarm satellites provide critical data for understanding the dynamic structure of the thermosphere. Furthermore, inter-hemispheric dynamic coupling processes occurring between the thermosphere and mesosphere are significant in terms of energy and mass transfer. These interactions contribute to a better understanding of global circulation and climate models at upper atmospheric levels.

Climate Change and the Thermosphere

Current research demonstrates that the thermosphere is affected by climate change. Increasing carbon dioxide levels enhance radiative cooling in the thermosphere, leading to its overall contraction and cooling. This condition can directly influence satellite orbital longevity and their interactions with the atmosphere.