Exosphere
Introduction
The Exosphere is the outermost layer of the Earth's atmosphere, extending from the exobase upward. It is primarily composed of extremely low densities of hydrogen, helium, and several heavier molecules including nitrogen, oxygen and carbon dioxide closer to the exobase. The exosphere is located too far above Earth for any meteorological phenomena to be possible. However, the exosphere plays a crucial role in the phenomena of space weather, and it is where most of the satellites orbit the Earth.
Composition
The exosphere is composed mainly of hydrogen and helium, with trace amounts of atomic oxygen near the exobase. The atoms and molecules are so far apart that they can travel hundreds of kilometers without colliding with one another. Thus, the exosphere no longer behaves like a gas, and the particles constantly escape into space. These free-moving particles follow ballistic trajectories and may migrate in and out of the magnetosphere or the solar wind.
Characteristics
The exosphere is the lightest layer of the Earth's atmosphere. It is the layer with the lowest density and pressure, and the highest temperature. The temperature in the exosphere can vary depending on solar activity, but it typically ranges from 500 to 1000 K. However, the concept of temperature becomes less useful in such low densities. The exosphere is also characterized by the large-scale motions of particles, where the particles follow ballistic trajectories rather than diffusing and colliding as they do in the lower atmosphere.
Boundary
The lower boundary of the exosphere, called the exobase, is defined as the height at which the mean free path of a particle is equal to the scale height - the point at which the atmosphere becomes collisionless. The altitude of the exobase varies but typically lies between 500 and 1000 km above the Earth's surface. Above the exobase, the atmosphere becomes the exosphere and extends until it merges with the solar wind, a stream of charged particles emitted by the sun.
Role in Space Weather
The exosphere plays a crucial role in the phenomena of space weather. The charged particles in the exosphere can be carried away by the solar wind, leading to a loss of atmosphere. This atmospheric loss is a significant factor in the evolution of a planet's atmosphere. The exosphere is also where the Earth's magnetosphere interacts with the solar wind, leading to phenomena such as the aurora borealis and aurora australis.
Satellites
Most of the satellites orbiting the Earth are in the exosphere. The low density of the exosphere provides an environment with low drag, allowing satellites to maintain their orbits with minimal fuel consumption. However, the exosphere is not completely devoid of matter, and satellites experience a small but measurable atmospheric drag. This drag can cause a gradual decay of satellite orbits and eventually lead to their re-entry into the Earth's denser atmosphere.