Thermosphere
Introduction
The thermosphere is the fourth layer of the Earth's atmosphere, situated above the mesosphere and below the exosphere. It extends from about 90 kilometers (56 miles) to between 500 and 1,000 kilometers (311 to 621 miles) above our planet. The thermosphere is characterized by steadily increasing temperatures with height, reaching up to 2,500 degrees Celsius (4,532 degrees Fahrenheit) or higher, though it would not feel hot to a human in direct contact due to the low pressure and density of the molecules.
Characteristics
The thermosphere is distinguished by several unique characteristics. It is within this layer that the aurora borealis and aurora australis occur, due to the interaction of solar particles with the Earth's magnetic field. The thermosphere also contains the ionosphere, a region populated by ions and free electrons, making it highly conductive and crucial for radio communication.
Temperature
Despite its name, the thermosphere would not feel hot to a human touch because its high temperature is a measure of the energy acquired by a few molecules moving at high speeds, rather than the average kinetic energy of an ensemble of molecules. The thermosphere's temperature increases with altitude due to absorption of highly energetic solar radiation by the small amount of residual oxygen still present. The temperature range is wide, varying from 500 degrees Celsius (932 degrees Fahrenheit) at night to 2,000 degrees Celsius (3,632 degrees Fahrenheit) during the day, and reaching up to 2,500 degrees Celsius (4,532 degrees Fahrenheit) during periods of high solar activity.
Composition
The thermosphere is composed primarily of atoms of nitrogen and oxygen, along with smaller quantities of helium and hydrogen. The ionosphere, which is part of the thermosphere, contains a high concentration of ions and free electrons, which allows it to reflect radio waves. This ionization is primarily caused by extreme ultraviolet radiation from the Sun.
Density and Pressure
The thermosphere has a very low density, with the atmospheric pressure at the bottom of the thermosphere being about one-millionth of the pressure at sea level. This low density means that an individual molecule travels an average of 1 kilometer (0.62 miles) between collisions, compared to just 68 nanometers (0.000000068 meters) at sea level.
Solar Interaction
The thermosphere is the layer of the Earth's atmosphere that is first to receive the Sun's radiation, making it the first line of defense against the Sun's harmful ultraviolet and X-ray waves. The Sun's extreme ultraviolet radiation ionizes the atoms and molecules in the thermosphere, creating a layer of charged particles, the ionosphere, which is capable of reflecting radio waves, thus enabling long-distance radio communication.
Satellites
Many satellites, including the International Space Station (ISS), orbit within the thermosphere. The drag experienced by these satellites as they move through the thermosphere requires regular adjustment of their orbits.