Ionosphere Facts and role in Radio Transmission and Aurora
Ionosphere is called so because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. Ionosphere stretches from 50 to 1,000 km and typically overlaps both the exosphere and the thermosphere. It has practical importance because it influences, for example, radio propagation on the Earth. It is also responsible for auroras.
Temperature in Ionosphere
Ionosphere is also known as THERMOSPHERE because of the high temperatures because of the high temperatures prevailing there as much as 870°C over the equator and 1427°C over the north pole, the temperature near the upper boundary of the thermosphere may become higher than 1000-1500°C. Along with temperature rise sharp changes caused by the corpuscular and ultraviolet solar radiation are observed in it.
Various Layers in Ionosphere
We note that the ionization depends primarily on the Sun and its activity. This means that the amount of ionization in the ionosphere varies greatly with the amount of radiation received from the Sun. This is the reason that there are changes in the Ionosphere and there are diurnal effect and seasonal effects. The activity of the Sun is associated with the position of earth in the revolutionary orbit, sunspot cycle, with more radiation occurring with more sunspots. Radiation received also varies with geographical location (polar, auroral zones, mid-latitudes, and equatorial regions). There are also mechanisms that disturb the ionosphere and decrease the ionization. There are disturbances such as solar flares and the associated release of charged particles into the solar wind which reaches the Earth and interacts with its geomagnetic field.
Accordingly, Ionosphere has been divided into different sets of layers during day and night which are shown in this graphic:
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 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 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 HEAVYSIDE-KENNELY LAYER.
Importance of E-Layer
The E layer is a region of the ionosphere which influences long-distance communications by strongly reflecting radio waves in the 1-3 megahertz. It is also called E region, Heaviside layer, or Kennelly-Heaviside layer. This region reflects radio waves of medium wavelength and allows their reception around the surface of the Earth. The layer approaches the Earth by day and recedes from it at night. In technical terms, it is a cylinder of relativistic electrons gyrating in the magnetic field, which produces a self field strong enough to dominate the externally applied field and produces half reversal in the system. Since the mid ’20s, another connection regarding the ionosphere has been hypothesized that lightning can interact with the lower ionosphere. According to this theory, thunderstorms could modulate the transient, localized patches of relatively high-electron density in the mid-ionosphere E layer, which significantly affects radio wave propagation.
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. . Appleton layer reflects short-radio waves (which have penetrated the HEAVYSIDE-KENNELY 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.
Concept of Aurora
The luminous effect of electro-magnetic phenomena in the ionosphere is known as Aurora, visible in high latitudes as red, green and white arcs, draperies, streamers, rays and sheets in the night sky, best developed at a height of about 90 km. Probably, aurora is the result of magnetic storms and of electrical discharges from the sun during periods of sun-spot activity, causing ionization of gases, though this is still a matter of research. It is called the Aurora Borealis (or northern lights) in the northern hemisphere and the Aurora Australis in the southern hemisphere. Occasionally the Aurora borealis is seen in England, but it is more common in northern Scotland, presents a magnificent spectacle in northern Scandinavia and northern Canada.
Exosphere
The exosphere lies above the altitude of 800 kilometer and it needs further studies. Characteristic of exosphere is an extreme rarefaction of the air; gas particles, moving with tremendous velocities, nearly fail to meet one another and there takes place an outflow of gas particles into the interpreter space.
