Ionosphere
The ionosphere is a region of the Earth's upper atmosphere with high concentrations of electrically charged particles, or ions. These ions are produced by the intense ionizing radiation from the sun, which strikes molecules of oxygen and nitrogen present in the upper atmosphere, yielding positively charged ions and negatively charged free electrons. The ionosphere forms an electrically conducting layer capable of reflecting radio waves transmitted from the Earth's surface. Because of this unique property, the ionosphere enables radio and television broadcasts to be received at far distances over the Earth's horizon as the radio waves "bounce" between the ionosphere and the surface. The ionosphere is found at an altitude of about 50 miles (80 km).
Within the ionosphere are several layers, identified by letters, which are distinguished by how they interact with radio waves. Layer D is between 50 and 60 miles (80 and 100 km). With the loss of solar radiation at night, layer D nearly disappears. Layer E is located between 60 and 90 miles (100 and 150 km) and is referred to as the Kennelly-Heaviside layer. The two F layers, F1 and F2, are located between 90 and 190 miles (150 and 300 km) upward respectively, and are referred to as the Appleton layers. These upper layers rise at night and extend the reception of radio signals. This is why radio stations from distant locations can be picked up with clarity during the night.
The ionosphere was used for radio transmissions before it was known to exist. In 1888 Heinrich Rudolph Hertz made the first radio transmissions. In 1901 Guglielmo Marconi (1874-1939) signaled the letter "S" from Cornwall, England, to St. John's, Newfoundland. He proved that signals could be transmitted beyond the Earth's curvature but failed to explain how it was done. Until then it was thought that radio waves could only be transmitted in a straight line and over only a short distance from any given point.
Marconi's transmission resulted in a flurry of debate. The theory of a reflective layer in the upper atmosphere was postulated almost simultaneously in 1902 by British-American Arthur Kennelly (1861-1939) and Englishman Oliver Heaviside (1850-1925). These two men had parallel careers, but their personal lives were quite different. Kennelly was born in India, and was educated in England and Continental Europe, where he developed an interest in electrical engineering. He immigrated to the United States in 1887 and became an assistant to Thomas Edison. He was a recent appointee to a professorship at Harvard University when he developed his reflective layer theory. Heaviside, on the other hand, was a self-taught scientist who spent little time outside of England and had difficulty, because of his lack of formal training, getting his writings published. Yet his theory on the reflective layers came so close in timing with Kennelly's theory that both names were applied to it. Heaviside also made contributions to the field of electromagnetism.
The one individual who contributed the most to current knowledge of the ionosphere was Edward Appleton (1892-1965). Appleton was an extremely intelligent individual who passed through England's finest schools with honors. He became interested in radio science while a signal officer during World War I. In 1924 Appleton became a professor at King's College, University of London. It was here that he and graduate student Miles Barnett experimented with the fading in and out of radio signals. They were able to calculate the distance to the Kennelly-Heaviside Layer and discovered its diurnal (daily) fluctuations. Appleton discovered in 1926 that reflections were being received off layers that were higher and lower than the known layer. These were the D and F layers. Appleton's illustrious career included a professorship at Cambridge University and was capped in 1947 with his being awarded the Nobel Prize in physics for his atmospheric research. Alexander Watson-Watt, one of Appleton's research associates, was the first to refer to the reflective layers as the ionosphere for its characteristic highly charged ions.
Sunspot activity and solar flares are directly responsible for perturbations of the radio-reflective layers. Sunspots and solar flares are accompanied by bursts of X-rays, which strike the upper atmosphere and increase the concentration of ions farther downward in the ionosphere. Increased ion densities in the D layer act to absorb, rather than reflect, radio waves, so intense solar events can severely disrupt global radio communications.
As research continues, scientists are learning about the ionosphere's cycles as well as the effect that solar changes have upon it. In 1997 it became possible for the first time to issue an ionospheric "weather forecast": by monitoring the activity of the sun and the near Earth space environment with satellites, it was possible to predict a solar event that impacted the ionosphere and affected radio communications. Advances in satellite coverage and computer modeling may work to make such forecasts of "space weather" more common in the coming years
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