Ionosphere

About Ionosphere

• This is the zone containing charged particles called ions. It lies from the upper mesosphere to the thermosphere.
• The charged particles are ionized by absorption of cosmic rays, gamma rays, X-rays and shorter wavelengths of ultraviolet rays.
• It is in this layer that incoming space vehicles and meteorites begin to heat due to friction.
• Above this layer i.e. above 480km, atomic oxygen is prevalent and beyond that first helium is more common and then hydrogen atoms predominate.
• The ionosphere is a deep layer of electrically charged molecules and atoms (which are called ions) in the middle and upper mesosphere and the lower thermosphere, between about 60 and 400 kilometers (40 and 250 miles). The ionosphere is significant because it aids long-distance communication by reflecting radio waves back to Earth.
• It is also known for its auroral displays, such as the “northern lights” that develop when charged atomic particles from the Sun are trapped by the magnetic field of Earth near the poles. In the ionosphere, these particles “excite” the nitrogen molecules and oxygen atoms, causing them to emit light, not unlike a neon light bulb.

Ionospheric Irregularities

• Ionosphere has been divided into different sets of layers during day and night 

D Layer

• The D layer explains why the AM Radio gets disturbed during day time, but quite smooth in night time. 
• We see in the above graphics that the D layer is the innermost layer, 60 km to 90 km above the surface of the Earth.
• At this layer, the net ionization effect is low, but loss of wave energy is great due to frequent collisions of the electrons. 
• This is the reason that the high-frequency (HF) radio waves are not reflected by the D layer but suffer loss of energy therein. 
• The absorption is small at night and greatest about midday. 
• This causes the disappearance of distant AM broadcast band stations in the daytime.

E-Layer

• The E layer is the middle layer, 90 km to 120 km above the surface of the Earth, with the primary source of ionization being soft X-ray (1-10 nm) and far ultraviolet (UV) solar radiation ionization of molecular oxygen (O2). 
• This layer disappears in the night because the primary source of ionization is no longer present. 
• The practical value of this layer is that it reflects long radio-waves back to earth, which enables them to be received at a distance, rather than disappear into space. It is also known as HEAVISIDE-KENNELLY LAYER.

F-Layer

• The F LAYER extends from about 200 km to more than 500 km above the surface of Earth. 
• The E-layer allows the penetration of short-radio waves, which continue until they reach the APPLETON LAYER. 
• The Appleton layer reflects short-radio waves (which have penetrated the HEAVISIDE-KENNELLY LAYER) back to earth. 
• This is also supposed to be the region where polar AURORAS occur and where most of the meteors burn themselves out.

Ionospheric Electron Density (IED)

• The ionosphere exists between about 90 and 1000 km above the earth’s surface.
• Radiation from the sun ionizes atoms and molecules here, liberating electrons from molecules and creating a space of free electrons and ions.

Studying IED

• The ionospheric variability is greatly influenced by both solar originated processes and the neutral atmosphere origin.
• Scientists have tried to model the ionosphere using theoretical and empirical techniques; however, the accurate prediction of electron density is still a challenging task.
• In recent years, Artificial Neural Networks (ANNs) are showing potential to handle more complex and non-linear problems.

What is Significance of IED

• Due to the ability of ionized atmospheric gases to refract high frequency (HF, or shortwave) radio waves, the ionosphere can reflect radio waves directed into the sky back toward the Earth.
• Radio waves directed at an angle into the sky can return to Earth beyond the horizon.
• This technique, called “skip” or “skywave” propagation, has been used since the 1920s to communicate at international or intercontinental distances.