What is Ionosphere?

          The story of the discovery of the Ionosphere is very interesting. On December 12, 1901, G. Marconi demonstrated with his newly discovered wireless equipment that radio waves could travel across the Atlantic Ocean. This demonstration could not be explained with radio waves travelling in straight lines over the spherical shaped earth for such a long distance. So the scientists came to believe in the existence of some layer in the upper atmosphere, which acts like a mirror for radio waves and reflects them.

          In 1902, O Heaviside of England and A.E. Kennelly of the United States confirmed the existence of such a layer. This was called Heaviside-Kennelly layer. Watson Watt, the inventor of radar, coined the name ionosphere to this layer. Later in 1925, E.V. Appleton and M.A.F. Barnett of England experimentally proved the existence of the ionosphere.

          How did the ionosphere come into existence?

 

          The ionosphere owes its origin mainly to the ultra-violet radiations coming from the sun and the cosmic rays. We know that our upper atmosphere contains a thin layer of air and ozone gas in the atomic state. So when the ultra-violet rays and cosmic rays hit the atoms of these gases, electrons and positive ions are produced and the gases get ionized. This ionized region of upper atmosphere is called ionosphere. The ionosphere is electrically neutral because the number of negatively charged electrons and positive ions is equal. This state is known as the plasma state. The ionosphere extends from about 50 km to 500 km above the earth’s surface.

          When a radio-wave below a certain frequency (30 MHz) is incident on the ionosphere, it is reflected back to the earth, just as a beam of light is reflected by a mirror and can be received by radio receivers. In fact, the ionosphere is mainly responsible for short-wave global communications. Without it radio-waves would not have been reflected back to the earth, and consequently reception would have been impossible.

           There are three distinct ionized layers in the ionosphere — D, E and F. Usually D layer exists between 50 to 90 km above the earth, E layer 100 to 120 km and F layers at 130 to 450 km. It does not end at F layer, but extends many thousand kilometres beyond F layer. The F layer is made up of two layers F1 and F2.

          When a signal from a transmitting station passes through the different layers of the ionosphere, the electric field of the electromagnetic waves interacts with the electrons present in the ionosphere. As a result of this interaction, the signal gets reflected in a similar way, as light gets reflected from a mirror. There is a critical frequency beyond which the signals are not reflected by the ionosphere, but get transmitted through it. Television frequencies are high enough not to get reflected from the ionosphere. That is why; ionosphere is not used for TV transmission.

          The ionosphere is thus nature’s boon which is highly useful for global communications.