Thermosphere

The Earth is surrounded by a blanket of air called the atmosphere. The atmosphere is a thin layer of gases that envelope the Earth. The gases are held close to the Earth by gravity and the thermal movement of air molecules. Life on Earth is supported by the atmosphere, solar energy, and the magnetic fields. Five layers have been identified in the atmosphere, using thermal characteristics, chemical composition, movement, and density.

The atmosphere is divided into the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. The thermosphere, from the Greek word for heat (thermos), is the fourth atmospheric layer from Earth, separated from the mesosphere by the mesopause. It begins about 50 mi. (80 km) above the Earth and is the layer of the atmosphere directly above the mesosphere and below the exosphere. The lower part of the thermosphere, from 50 to 342 mi. (80 to 550 km) above the Earth’s surface, contains the ionosphere, which is the region of the atmosphere that is filled with charged particles. Beyond the ionosphere, extending out to perhaps 6,214 mi. (10,000 km), is the exosphere.

Hottest Layer in the Atmosphere

The Earth’s thermosphere is the layer of the atmosphere that is first exposed to the sun’s radiation, and so is first heated by the sun; it is the hottest layer of the atmosphere. Within the thermosphere, temperatures rise continually to well beyond 1,832 degrees F (1,000 degrees C). In the thermosphere, ultraviolet radiation causes ionization. At these high altitudes, the residual atmospheric gases sort into strata according to their molecular mass. Thermospheric temperatures increase with altitude as a result of the absorption of highly energetic solar radiation by the small amount of residual oxygen present. Temperatures in the thermosphere are highly dependent on solar activity. Radiation causes the air particles in this layer to become electrically charged, enabling radio waves to bounce off and be received beyond the horizon.

The few molecules that are present in the thermosphere receive extraordinary amounts of energy from the sun, causing the layer to warm to high temperatures. Air temperature, however, is a measure of the kinetic energy of air molecules—not of the total energy stored by the air. The air is so thin that a small increase in energy can cause a large increase in temperature. Because the air is so thin within the thermosphere, such temperature values are not comparable to those of the troposphere or stratosphere. Again, because of the thin air in the thermosphere, scientists cannot measure the temperature directly. Instead, they measure the density of the air by how much drag it puts on satellites, and then use the density to determine the temperature.

Although the measured temperature is very hot, the thermosphere would actually feel very cold to humans because the total energy of the few air molecules residing there would not be enough to transfer any appreciable heat to our skin. In addition, it is so near vacuum that there is not enough contact with the few atoms of gas to transfer much heat. A normal thermometer would read significantly below 32 degrees F (0 degree C). The dynamics of the lower thermosphere are dominated by the atmospheric tide, which is driven in part by the very significant diurnal heating.

The atmospheric tide dissipates above this level because molecular concentrations do not support the coherent motion needed for fluid flow. The International Space Station has a stable orbit within the upper part of the thermosphere, between 199 and 236 mi. (320 and 380 km). The northern lights also occur in the thermosphere.

Atmospheric Layers

The atmosphere is classified into five different layers,These layers are the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. Each layer differs in composition and temperature profile.

• Troposphere The layer closest to Earth’s surface, the troposphere, contains most of the mass of the atmosphere. Weather occurs in the troposphere. In the troposphere, air temperature decreases as altitude increases. The altitude at which the temperature stops decreasing is called the tropopause. The height of the tropopause varies from about 16 km above Earth’s surface in the tropics to about 9 km above it at the poles. Temperatures at the tropopause can be as low as –60°C.

• Stratosphere Above the tropopause is the stratosphere, a layer in which the air temperature mainly increases with altitude and contains the ozone layer. In the lower stratosphere below the ozone layer, the temperature stays constant with altitude. However, starting at the bottom of the ozone layer, the temperature in the stratosphere increases as altitude increases. This heating is caused by ozone molecules, which absorb ultraviolet radiation from the Sun. At the stratopause, air temperature stops increasing with altitude. The stratopause is about 48 km above Earth’s surface. About 99.9 percent of the mass of Earth’s atmosphere is below the stratopause.

• Mesosphere Above the stratopause is the mesosphere, which is about 50 km to 100 km above Earth’s surface. In the mesosphere, air temperature decreases with altitude. This temperature decrease occurs because very little solar radiation is absorbed in this layer. The top of the mesosphere, where temperatures stop decreasing with altitude, is called the mesopause.

• Thermosphere The thermosphere is the layer between about 100 km and 500 km above Earth’s surface. In this layer, the extremely low density of air causes the temperature to rise. Temperatures in this layer can be more than 1000°C. The ionosphere, which is made of electrically charged particles, is part of the thermosphere.

• Exosphere The exosphere is the outermost layer of Earth’s atmosphere. The exosphere extends from about 500 km to more than 10,000 km above Earth’s surface. There is no clear boundary at the top of the exosphere. Instead, the exosphere can be thought of as the transitional region between Earth’s atmosphere and outer space. The number of atoms and molecules in the exosphere becomes very small as altitude increases.

In the exosphere, atoms and molecules are so far apart that they rarely collide with each other. In this layer, some atoms and molecules are moving fast enough that they are able to escape into outer space.