Kennelly and Heaviside Theorize Existence of the Ionosphere

Date March and June, 1902

When Arthur Edwin Kennelly and Oliver Heaviside independently theorized the existence of an electrified layer in the upper atmosphere that reflects radio waves around the curved surface of the earth, their work stimulated new scientific understanding.

Locale Cambridge, Massachusetts; Newton Abbot, England

Key Figures

• Arthur Edwin Kennelly (1861-1939), British American electrical engineer and Harvard professor
• Oliver Heaviside (1850-1925), English physicist and electrical engineer
• Balfour Steward (1828-1887), Scottish physicist
• Guglielmo Marconi (1874-1937), Italian electrical engineer and cowinner of the 1909 Nobel Prize in Physics for demonstrating radio transmission across the Atlantic Ocean
• Edward Victor Appleton (1892-1965), English physicist who won the 1947 Nobel Prize in Physics for his discovery of the ionosphere in 1924

Summary of Event

On December 12, 1901, only thirteen years after German physicistHeinrich Hertz discovered radio waves, Guglielmo Marconi succeeded in transmitting radio signals from Cornwall, England, to Newfoundland, Canada. This historic event was difficult to explain, given that it was known that radio signals consist of electromagnetic waves that, like light, travel in nearly straight lines. Considerable discussion took place among scientists as to how Marconi’s signals could propagate around the curved surface of the Atlantic Ocean. Several scientists tried to show that electromagnetic waves of sufficiently long wavelength could bend around the earth’s curvature by diffraction (the small tendency of waves to spread around obstacles). Calculations showed, however, that diffraction effects were inadequate to explain Marconi’s results.

The correct explanation of radio wave propagation around the curved surface of the earth was suggested in 1902 almost simultaneously by Arthur Edwin Kennelly in the United States and Oliver Heaviside in England. These two men independently postulated the existence of an electrically conducting layer in the upper atmosphere that would reflect radio waves back to the earth. Successive reflections between this conducting layer and the surface of the earth could guide the waves around the curvature of the globe. Heaviside also suggested that the conductivity of this region might result from the presence of positive and negative ions in the upper atmosphere caused by the ionizing action of solar radiation.

It is interesting to note that this hypothetical reflector was usually called the “Heaviside layer,” although it was Kennelly who first published the idea under the title “On the Elevation of the Electrically-Conducting Strata of the Earth’s Atmosphere” in the March, 1902, issue of Electrical World and Engineer. Heaviside was a brilliant but self-educated scientist who often had difficulty getting his work published. His ideas were more fully developed than Kennelly’s, but when he submitted an article to The Electrician, it was rejected; it then appeared in the Encyclopedia Britannica in June, 1902. In 1912, W. H. Eccles published the first theory of how charged particles affect the propagation of radio waves. Because he was aware of Heaviside’s rejected article, he apparently attempted to set the record straight by referring to the postulated reflecting region of the atmosphere as the Heaviside layer.

In fact, the idea of such a conducting shell in the upper atmosphere had already been suggested by Balfour Steward twenty years earlier. In 1882, as part of his study of terrestrial magnetism, Steward had proposed that electrical currents flowing high in the atmosphere could explain the small daily changes in the earth’s magnetic field. Such variations would be caused by the tidal movements in the surrounding “sea of air,” which were caused by solar and gravitational influences. This explanation of fluctuations in the earth’s magnetism, combined with many peculiar features of shortwave radio propagation, seemed to confirm the existence of the Kennelly-Heaviside layer, but these phenomena provided only indirect evidence.

After Marconi’s demonstration, commercial radio links were established across the Atlantic. Researchers then noticed that the strengths of the signals varied in a regular way during the day and night, as well as over seasonal and solar cycles. Furthermore, the regular daily variations were disturbed during magnetic storms. These findings suggested that the scientific study of radio propagation could lead to new knowledge about the upper atmosphere. It soon became clear that daily and seasonal variations could be explained by the changing aspects of the Sun and its effect on the charge concentration in the postulated Kennelly-Heaviside layer in what came to be called the ionosphere.

The first direct evidence for the existence of the ionosphere was obtained by Sir Edward Victor Appleton, with the assistance of Miles A. F. Barnett in 1924. At the University of Cambridge, Appleton had studied radio signals from the new British Broadcasting Company station in London and noticed the typical variations in their strength. When he took up a new position at the University of London in 1924, Appleton arranged to use the new British Broadcasting Company transmitter at Bournemouth after midnight, with receiving apparatus located at the University of Oxford. By varying the transmitter frequency, he hoped to detect a changing intensity at the receiver caused by changing interference (canceling of wave crests by the troughs of other waves) between the direct waves (along the ground) and the waves that he assumed would be reflected from the ionosphere.

On December 11, 1924, Appleton and Barnett observed the regular fading in and out of the signal as the frequency of the transmitter was slowly increased. From the lowest and highest transmitted wavelengths and the corresponding number of intensity oscillations at the receiver, they calculated that the reflection was from a height of about 100 kilometers (62.1 miles). This confirmation of the Kennelly-Heaviside prediction was published in 1925 in the journal Nature under the title “Local Reflection of Wireless Waves from the Upper Atmosphere.”

In later experiments, Appleton used an improved technique of rapid frequency changes so that variations in signal strength could be distinguished more clearly from natural fading because of changes in the ionosphere. In 1926, he found that the ionization of the Heaviside layer (E layer) was sufficiently reduced before dawn by recombination of electrons with positive ions to allow penetration by radio waves. Reflection, however, was still observed from a higher layer, where the air was too thin for efficient recombination. The height of what is now called the Appleton layer (F layer) was measured at about 230 kilometers (142.9 miles) above the earth. This result was published in Nature in 1927 under the title “The Existence of More than One Ionized Layer in the Upper Atmosphere.” Observations during a solar eclipse in 1927 indicated that the height of the Heaviside layer changed during that event, revealing that ionization is caused by solar radiation, as Heaviside had suggested.

Significance

The theorization and discovery of the ionosphere were important in stimulating new scientific understanding and advances in technology. Appleton’s magnetoionic theory of the ionosphere showed that electron density and the magnetic field at any layer in the ionosphere can be calculated from the critical frequency for penetrating that layer. In 1931, scientists began systematic experiments to determine the variation of electron densities in the ionosphere, revealing an increase in ionization as the Sun rises and low ionization at night, except for sporadic increases possibly caused by meteoric activity. Noon ionization was found to increase as the sunspot maximum of 1937 was approached, suggesting a correlation between sunspots and increases in the ultraviolet radiation that ionizes the upper layers of the atmosphere. This made it possible to measure the ultraviolet radiation from the Sun even though little of it reaches the ground. During the sunspot maximum of 1957-1958, the International Geophysical Year was established to study geophysical phenomena on a worldwide scale, including their relation to ionospheric variations. The study of the ionosphere has thus contributed to developments in other sciences, such as astronomy, meteorology, and geophysics.

Appleton’s methods proved especially valuable in the development of radio communications, radar systems, and their applications in meteorology. Long-distance radio communications, in which the waves are guided around the earth by the ionosphere, became especially important during World War II. A worldwide network of more than fifty stations was established to monitor the ionosphere and to determine the most suitable frequencies for radio transmissions as ionospheric conditions changed. Discovery of the influence of the sunspot cycle made it possible to forecast ionospheric weather and thus to improve the reliability of radio communications. On the standard broadcast band (500-1,500 kilohertz), ground waves travel about 500 kilometers (310.7 miles), whereas sky waves are absorbed during the day but travel by reflection several thousand kilometers at night. At frequencies between 5 and 25 megahertz and distances greater than about 100 kilometers, radio transmission depends almost entirely on ionospheric reflections. On the 20-meter amateur radio band (14 megahertz), it is possible to reach some part of the world at almost any time of the day or night.

The early development of radar was closely associated with studies of the ionosphere. The most powerful radar systems use the over-the-horizon technique of reflecting radar signals from the ionosphere to cover distances up to about 3,000 kilometers (1,864.1 miles), about ten times farther than conventional radar. For reliability, over-the-horizon radar systems depend on computers to chart the constantly changing intensity and thickness of the ionosphere and determine where conditions are best and which frequencies are needed for maximum performance. The ionosphere has thus become an indispensable tool for both communications and national security.

Bibliography

Aitken, Hugh G. J. Syntony and Spark: The Origins of Radio. New York: John Wiley & Sons, 1976. A good history of the early development of radio from the discovery of radio waves by Hertz to the work of Marconi. Makes only brief mention of the ionosphere proposal of Kennelly and Heaviside but provides more than one hundred references on the work of Marconi and many useful diagrams of early radio apparatuses.

Anderson, Dave, and Tim Fuller-Rowell. “The Ionosphere.” Space Environment Topics SE-14, Space Environment Center, Boulder, Colo., 1999. http://www.sel.noaa.gov/info/Iono.pdf. This brief report explains the characteristics of the ionosphere as well as ionospheric variability in a straightforward manner.

Craig, Richard. The Edge of Space: Exploring the Upper Atmosphere. Garden City, N.Y.: Doubleday, 1968. This small book is intended for secondary students and the lay public. It has a good chapter on the discovery of the ionosphere, along with helpful diagrams.

Davies, Kenneth. Ionospheric Radio Propagation. Washington, D.C.: Government Printing Office, 1965. This highly authoritative volume contains most of what is known about the ionosphere from both theory and worldwide measurements. Includes references and indexes.

Hargreaves, J. K. The Solar-Terrestrial Environment: An Introduction to Geospace, the Science of the Terrestrial Upper Atmosphere, Ionosphere, and Magnetosphere. Reprint. New York: Cambridge University Press, 1995. Begins with three chapters that provide some basic physics and then covers the neutral and ionized upper atmosphere and the magnetosphere. Suitable for readers with a basic background in engineering or physics.

Nahin, Paul J. Oliver Heaviside: Sage in Solitude. New York: IEEE Press, 1987. An interesting and well-documented biography of Heaviside, with extensive discussion of his contributions to telegraphy, applied mathematics, and electrodynamics. The concluding chapter, “The Final Years of the Hermit,” describes his ionosphere prediction.

Ratcliffe, John A. Sun, Earth, and Radio: An Introduction to the Ionosphere and Magnetosphere. New York: McGraw-Hill, 1970. A very readable and well-illustrated introduction to the history and applications of ionosphere research. An appendix provides background on electromagnetic waves, and a bibliography lists about twenty books on the ionosphere.