Mesosphere is the layer of Earth's atmosphere located approximately between 50 and 85 kilometers, positioned between the stratosphere and the thermosphere. This region is the coldest layer of the atmosphere, with temperature decreasing with altitude, dropping to about -90°C at its upper boundary, called the mesopause.
The mesosphere is a region where dynamic processes such as gravity waves, atmospheric tides, and planetary waves are effective. Furthermore, as it is the layer where meteoroids burn up and are destroyed in the atmosphere, it functions as Earth's natural shield against objects coming from space. Studying this layer is important for understanding the general dynamics and energy transfer mechanisms of the atmosphere.
Earth's Atmosphere and Layers (TUA)
Physical Properties and Thermal Structure
Temperature Profile
The mesosphere is an important layer in Earth's atmospheric temperature profile, exhibiting an inverse gradient. In this layer, which begins from the stratopause, the upper boundary of the stratosphere, temperature continuously decreases as altitude increases. This decrease is due to the absence of any permanent heat source, unlike other layers of the atmosphere.
At the lower boundary of the mesosphere (at an altitude of approximately 50 km), temperatures are around -5°C on average. However, as altitude increases, the temperature decreases dramatically, dropping to values between -90°C and -125°C at the mesopause, its upper boundary, located in the range of approximately 85–100 kilometers. This makes the mesosphere the coldest layer of the atmosphere.
The temperature drop is related to the inability of electromagnetic energy from the sun to reach this altitude and its insufficient absorption due to the sparse molecular density. While temperature increases in the stratosphere due to the ozone layer absorbing ultraviolet radiation, there is no similar heat source in the mesosphere. Additionally, the emission of energy into space via infrared radiation by carbon dioxide gas, which is the primary cause of heat loss, plays a significant role in the cooling of the mesosphere.
The temperature structure of the mesosphere shows significant variations according to the seasons of the year and geographical latitudes. Especially in summer, temperatures in polar regions can drop to minimums of -125°C, and these extreme colds provide the basis for the formation of polar mesospheric clouds (noctilucent clouds). Therefore, the thermal structure of the mesosphere is shaped not only by altitude but also by seasonal circulation and atmospheric wave activity.
Radar and lidar observations have shown distinct semiannual oscillations and temperature changes due to atmospheric wave breaking in the upper mesosphere. This variability interacts with wind systems and contributes to the dynamic balance of the mesosphere.
Temperature Conditions in the Mesosphere (Generated by artificial intelligence.)
Pressure and Density
In the mesosphere, atmospheric pressure and air density are considerably lower compared to the lower atmospheric layers, the troposphere and stratosphere. This low level is directly related to the mesosphere's upward positioning and the sparseness of air molecules.
Atmospheric pressure decreases logarithmically with altitude. At the lower boundary of the mesosphere, at approximately 50 kilometers, the pressure is only about 1 millibar (1 hPa) of the standard atmospheric pressure at sea level (1013 hPa). At the mesopause, the upper boundary (approximately 85–100 km), this value drops below 0.01 hPa. This indicates a pressure 100,000 times lower than at sea level.
Similarly, air density also decreases significantly with altitude. While air density in the troposphere is approximately 1.2 kg per cubic meter, it can drop to the order of 10⁻⁵ kg/m³ in the upper parts of the mesosphere. This low density causes an increase in the mean free path between gas molecules and a decrease in collision frequency. As a result, the effect of molecular diffusion increases, which significantly influences the dynamic and chemical properties of the mesosphere.
This sparse environment is of decisive importance, especially for the propagation of electromagnetic waves, heat transfer, and the kinetics of chemical reactions. At the same time, the heating and burning of meteoroids due to friction in this layer also occur thanks to the long mean free path caused by low density.
The sparse and low-pressure nature of the mesosphere also makes observations in this layer difficult. For example, traditional weather balloons cannot reach this altitude because the air density does not provide sufficient medium for lift. Therefore, pressure and density measurements related to the mesosphere are generally obtained using sounding rockets or indirect lidar/radar methods.
Chemical Composition
In terms of chemical composition, the mesosphere primarily contains nitrogen (78%) and oxygen (21%) gases, similar to the troposphere and stratosphere. However, the ozone density is lower in this layer. Additionally, iron, magnesium, and silicon-based metal atoms and ions, formed as a result of meteoroids burning at these altitudes, are also present. These metallic particles play an important role in chemical reactions in the mesosphere and especially in the formation of polar mesospheric clouds (noctilucent clouds, NLCs).
Mesosphere (Generated by artificial intelligence.)
Dynamic Structures and Waves
Gravity Waves and Atmospheric Waves
The mesosphere is a region where gravity waves and atmospheric tides (thermal tides) are effective. These waves can propagate upwards from the lower atmosphere, break there, create turbulence, and cause momentum transfer in wind systems. Particularly small-scale gravity waves play a critical role in determining the mesosphere's wind structure and temperature distribution.
Seasonal and Latitudinal Differences
The dynamic structures observed in the mesosphere vary depending on the seasons of the year and geographical latitude. Noctilucent clouds seen in polar regions during summer months are a result of the extremely cold and humid conditions in this layer. These clouds form at an altitude of approximately 82 kilometers and can only be observed when the sun is 6°–16° below the horizon, i.e., during twilight hours. Furthermore, methane gas, increased by anthropogenic effects, can turn into water vapor through oxidation, thereby increasing the frequency of these cloud formations.
Electrical Properties
In the mesosphere, especially at high latitudes, vertical electric fields have been observed. These fields are directly related to geomagnetic activity. During solar flares and proton events, the strength of these electric fields increases, and ion conductivity also rises. These electrical processes are thought to be effective in the transfer of energy and matter between the ionosphere and the mesosphere.
Observations and Research Methods
Observational Challenges
The mesosphere is one of the least observationally accessible layers of the atmosphere. This difficulty stems from technical limitations: weather balloons cannot reach the mesosphere as they can only ascend to approximately 40 kilometers. On the other hand, most satellites operate above the mesosphere, in orbits of approximately 200 kilometers and higher. Therefore, collecting direct data about this layer is quite challenging. Information regarding the mesosphere's properties is mostly based on indirect observation techniques.
Primary Observation Methods
Sounding Rockets: Sounding rockets are vehicles that collect direct data from the mesosphere during short atmospheric transits. These rockets can measure physical parameters such as temperature, pressure, ion density, and electric fields in the atmospheric layers between 50 and 150 kilometers. They provide temporary but high-resolution data.
Lidar (Light Detection and Ranging) Systems: Lidar systems use laser beams to study structures such as particle density, temperature profiles, and sodium layers in the mesosphere. These ground-based observations provide high-precision data, especially in the 80–100 kilometer altitude range. Rayleigh and Na lidars are widely used for this purpose.
Radar Observations (Meteor Radar and MF/VHF Radars): Meteor radars track the ionized trails created by meteoroids in the atmosphere to reveal the mesosphere's wind profiles. This method is particularly effective for measuring zonal (east-west) and meridional (north-south) wind components in the 75–110 km range. It is also used for tracking atmospheric tides and gravity waves.
Satellite Observations: Satellites are generally positioned above the mesosphere, so data related to the mesosphere is indirect. However, instruments such as HRDI (High Resolution Doppler Imager) and WINDII (Wind Imaging Interferometer) on the Upper Atmosphere Research Satellite (UARS) can provide temperature and wind measurements in the 65–105 kilometer range. These data are usually collected during daylight hours and contain local time-dependent variabilities. Therefore, the data are evaluated with long-term averages.
Seasonal and Latitudinal Observation Programs
Various long-term observation programs are being conducted to study the seasonal changes of the mesosphere. For example, over 10 years of data have been collected with meteor radars at Germany's Andenes (69°N) and Juliusruh (54°N) stations, and annual variation patterns of atmospheric tides have been derived from this data.
