Mesosphere (literally mean sphere) is the third highest layer in our atmosphere, occupying the region 50 km to 80 km above the surface of the Earth, above the troposphere and stratosphere, and below the thermosphere. It is separated from the stratosphere by the stratopause and from the thermosphere by the mesopause.

Location of the Mesosphere

Temperatures in the mesosphere drop with increasing altitude up to about -148°F (-100°C). The mesosphere is the coldest atmospheric layers. In fact it is colder than the lowest temperatures recorded in Antarctica. It is cold enough to freeze water vapor in clouds of ice. You can see these clouds if sunlight hits them after sunset. They are called "Noctilucent Clouds" (NLC). NLC are more easily visible when the sun is 4-16° below the horizon.

The mesosphere is the layer in which many meteors burn up when entering the Earth's atmosphere. From Earth they are seen as shooting tars.

Dark blue layer next to the blackness of space is the
mesosphere. The upper atmosphere extends much further.


The highest layer is the thermosphere. The thermosphere begins at 90-100 km and extends to 1280 km altitude. Pressure there is almost zero and the air molecules are very rare. Solar ultraviolet very short wavelengths (between 100 and 200 nm) is absorbed between 100 and 150 km altitude with molecular oxygen. The temperature increases with altitude and maintains to a level called "thermopause" located from 250 km to 500 km along the solar activity as after thermopause temperature between 572°F (300°C) and 2912°F (1600°C) depending on the energy received by the Sun. Temperatures are high, but as the matter density is extremely low it would be very cold for us since the few air molecules are not enough to transfer heat suitable for us. The thermosphere is the region near the poles where the northern and southern lights take form.

Location of the Thermosphere

The lower part of the ionosphere is called thermosphère. The ionosphere reflects short waves (radio waves). These waves emitted by a transmitter, bounce off the ionosphere and are returned to Earth. If they are turned at a certain angle, they can do almost around the globe. The ionosphere can therefore communicate with faraway regions.

The separation between the mesosphere the lower limit of the thermosphere is called the mesopause.

Thermosphere is composed of two parts :


Ionosphere is submerged in the very thin upper layer of our atmosphere called the thermosphere. It is an ionized air in the atmosphere layer extending from 50-60 kilometers above the surface of the Earth to approximately 640 km. At the magnetic equator, a phenomenon is observed called equatorial électrojet which results in significant movements of convection in the ionosphere. Movements of the ionosphere are complex and depend on many parameters such as atmospheric conditions, solar activity, season ie...

Ionosphere is divided into four parts characterized by a relative maximum of electron density :

    • D Region extends 50-60 km to 90 km altitude. It behaves like a sponge to face high frequency waves passing therethrough. Much present during the day, its ionization is directly proportional to the solar flux, it is formed at sunrise and sundown immediately disappears. It consists essentially of heavy ions ( nitrogen oxide). As its absorption is inversely proportional to the frequency bands of 160 and 80 meters are completely absorbed during the daylight hours.

    • E Region region extends from 90 to 140 km altitude. It is the lowest layer used by radio waves to think about it. This is a very special kind of mirror used in its two faces, reflecting upwards and downwards. It appears and disappears from dawn to sunset . This layer has , at minimum solar activity, known as the sporadic E that we will observe in excess of 21 MHz frequency phenomena.

    • F Region is the more ionized primarily responsible for long-distance communication . When solar cycle is at maximum it creates more of ionization of F layer, and allows the ionosphere refract higher frequencies (15, 12 , 10 and even 6 meters ) to the earth for contact DX. Around the minimum of the cycle, the sunspot number is so low that the higher frequencies pass through the ionosphere and disappear into space. The large number of free electrons in the ionosphere allows the propagation of electromagnetic waves. Radio signals - a form of electromagnetic radiation - can " bounce " off the ionosphere allowing radio communication over long distances. The F layer is ionized at sunrise, peaked very quickly to gradually decrease at sunset and reach its minimum just before sunrise. During the day, the F region is divided into two :

      • F1 Region extending from 140 to 200 km altitude is not an important means of propagation and whose training is directly dependent on the sunrise and sunset. After sunset, the F1 layer decreases sharply to leave place for the F2 layer.

      • F2 Region extends from 200 to 250-600 km depending on solar activity. This is the first layer that supports the high-frequency communications. During the day, it is relatively thin due to the presence of F1. On the other hand, during the night, this layer doubles its dimensions, being directly under the influence of solar radiation, it is very dense and allows communications to more than 1 500 km in a single bound.

Region of the superior atmosphere

Simplified view of the ionosphere around the Earth

Click here to enlarge the two schemas

energy and dynamic of the thermosphere are strongly coupled to the ionosphere as that of the lower layers of the atmosphere. The ionosphere plays likewise, coupled with the the magnetosphere, a special role in the mechanisms of loss of atmospheric chemical species into the interplanetary environment, and as such participates in the chemical evolution of our atmosphere. In this region of the atmosphere the sun's energy is so strong that it breaks the molecules and atoms of air, leaving the ions (atoms with missing electrons) and free electrons floating.

The ionosphere is the region of the atmosphere where auroras occur following the Sun's activity. They occur mostly in the F layer.

aurora borealis

Click here to see an animated Aurora Borealis


Composite image of a beautiful aurora observed from Polar space
by the satellite in the visible portion and ultraviolet of the spectrum.


The ionization of air molecules in the ionosphere is produced by ultraviolet radiation from the sun, and to a lesser extent by high-energy particles from the sun and cosmic rays.

The ionization of the upper atmosphere is continuously measured in the visible spectrum by observing the number of spots appearing daily on the Sun , the result of this observation is reported by Wolf number (subjective measurement that takes into account a factor of scale appropriate to the observer and instrument used, finally used very little). Measurements are also performed outside the visible band in the UV and X bands normally absorbed by the atmosphere. The indication of interest to the radio ham and indicates the degree of ionization of the upper atmosphere is a measure of solar radio flux Fs. Its direct measurement is performed every day at 17.00 UTC on 2.800 MHz band by the Algonquin Observatory located in Ontario ( the least subjective indication that the number of Wolf, is available daily from the WWV from 18.18 UTC). F is expressed in units of flux : 10-22 Wm2 HZ-1. It can vary from 65/66 in years when solar activity is minimum ( 66 on June 1986) to more than 300 units at a maximum of solar activity (327 on 15 June 1989) .


Above the ionosphere to above the upper of atmosphere whether 640 kilometers to 10.000 kilometers is the exosphere. This is the outer thermosphere, where the atmosphere merges with the space in the extremely thin air. It consists of hydrogen, helium and oxygen atoms but scarce particles collide practically no more and behave like independent bodies subject under the action of gravity only. It is in this area where there are many satellites orbiting the Earth.

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