Ionospheric absorption

Ionospheric absorption (or ISAB) is the scientific name for absorption occurring as a result of the interaction between various types of electromagnetic waves and the free electrons in the ionosphere, which can interfere with radio transmissions.

Description

Ionosphere absorption is of critical importance when radio networks, telecommunication systems or interlinked radio systems are being planned, particularly when trying to determine propagation conditions. Walker, J.K. ; Bhatnagar, V.P. Ionospheric absorption, typical ionization, conductivity, and possible synoptic heating parameters in the upper atmosphere. Geological Survey of Canada.http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=6068491]

The ionosphere can be described as an area of the atmosphere in which radio waves on shortwave bands are refracted or reflected back to earth. As a result of this reflection, which is often key in the long-distance propagation of radio waves,some of the shortwave signal strength is decreased. In this regard, ISAB is the primary limiting factor in radio propagation. Davies, Kenneth. Ionospheric Radio. ISBN 086341186X]

Causes

("See ionosphere for more information on the D Layer")

Roughly 70% of any possible signal attenuation associated with ISAB is believed to happen in the D layer of the ionosphere. The remaining 30% attenuation that is seen transpires in the E and F layers, but this varies in terms of signal loss and is often insignificant next to the amount of attenuation in the D layer. Signal attenuation is measured in potential loss in dB.Davies, Kenneth. Ionospheric Radio. ISBN 086341186X]

A signal enters the D layer, which is suffused with free electrons. As the signals propagate through the layer, they begin to transfer energy to these electrons, setting them in motion. This results in the electrons vibrating , which can cause them to collide with other molecules, ions, or electrons. R. D. Hunsucker, J.K. Hargreaves. The high-latitude ionosphere and its effects on radio propagation.2003 Cambridge University Press. ISBN 0521330831]

Collisions can happen many times per second, and each one dissipates a small amount of signal strength as energy is transferred from wave to particle. The specific amount of energy, and thus signal strength, that is lost depends on a number of factors:

* The number of collisions. The more collisions, the more signal attenuation.Walker, J.K. ; Bhatnagar, V.P. Ionospheric absorption, typical ionization, conductivity, and possible synoptic heating parameters in the upper atmosphere. Geological Survey of Canada.http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=6068491]
* The frequency of the signal. As this decreases, the wavelength becomes longer, and more collisions take place. A lower wavelength transmits less energy, but there is less energy to retain signal cohesion with as well.
* The number of particles. This is mostly a factor in the lower layers of the D layer, where density and pressure of the atmosphere is higher.R. D. Hunsucker, J.K. Hargreaves. The high-latitude ionosphere and its effects on radio propagation.2003 Cambridge University Press. ISBN 0521330831]

Attenuation mechanics

ISAB is only a factor in the period of the day where radio signals travel through the portion of the ionosphere facing the sun. The solar wind and radiation cause the ionosphere to become charged with electrons in the first place. At night, the atmosphere becomes drained of its charge, and radio signals can go much further with less loss of signal. In particular, low wavelength signals that would be attenuated to nothing during the day will be received much further away at night.John Keith Hargreaves. The Solar-Terrestrial Environment: An Introduction to Geospace. 1992 Cambridge University Press. ISBN 0521427371] Leo F. McNamara. (1994) ISBN 0-89464-804-7 Radio Amateurs Guide to the Ionosphere.]

The specific amount of attenuation can be derived as a function of the inverse square-law. The lower the frequency, the greater the attenuation. Davies, Kenneth. Ionospheric Radio. ISBN 086341186X]

ee also

*Radio horizon
*Sudden Ionospheric Disturbance

Resources