The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an important role in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on Earth. Travel through this layer also affects GPS signals, deflecting their paths and delaying their arrival.
History of discovery As early as 1839, the German mathematician and physicist Carl Friedrich Gauss postulated that an electrically conducting region of the atmosphere could account for observed variations of Earth's magnetic field. Gauss speculated that magnetic forces might be generated not only by electrical currents flowing through the Earth's interior but also by some sort of electrical current(s) flowing through the atmosphere. From p. 50: "§ 36. Ein anderer Theil unserer Theorie, über welchen ein Zweifel Statt finden kann, ist die Voraussetzung, … zu untersuchen, wie die aus denselben hervorgehende magnetische Wirkung auf der Erdoberfläche sich gestalten würde." (Another part of our theory about which doubt may arise is the assumption that the agents of terrestrial magnetic force have their source exclusively in the interior of the Earth. If the immediate causes [of terrestrial magnetism] should be sought entirely or in part outside [the Earth's interior], then we can — in so far as we exclude baseless fantasies and we want to restrict ourselves to the scientifically known [facts] — consider only galvanic currents. Atmospheric air is not a conductor of such currents; empty space also is not: thus our knowledge fails us when we seek a carrier for galvanic currents in the upper regions [of the atmosphere]. Only the enigmatic phenomena of the northern lights — in which by all appearances electricity in motion plays a major role — prohibits us from simply denying the possibility of such currents just on account of that ignorance, and in any case it remains interesting to investigate how the magnetic effect resulting from [those currents] would manifest itself on the Earth's surface.) English translation: See p. 229. English translation: Sixty years later, Guglielmo Marconi received the first trans-Atlantic radio signal on December 12, 1901, in St. John's, Newfoundland (now in Canada) using a kite-supported antenna for reception. The transmitting station in Poldhu, Cornwall, used a spark-gap transmitter to produce a signal with a frequency of approximately 500 kHz and a power of 100 times more than any radio signal previously produced. The message received was three dits, the Morse code for the letter S. To reach Newfoundland the signal would have to bounce off the ionosphere twice. Dr. Jack Belrose has contested this, however, based on theoretical and experimental work.John S. Belrose, "Fessenden and Marconi: Their Differing Technologies and Transatlantic Experiments During the First Decade of this Century ". International Conference on 100 Years of Radio, 5–7 September 1995. However, Marconi did achieve transatlantic wireless communications in Glace Bay, Nova Scotia, one year later.
In 1902, Oliver Heaviside proposed the existence of the Kennelly–Heaviside layer of the ionosphere which bears his name. Speaking of wireless telegraphy, Heaviside speculated about the propagation of Hertzian (radio) waves through the atmosphere. From p. 215: "There may possibly be a sufficiently conducting layer in the upper air. If so, the waves will, so to speak, catch on to it more or less. Then the guidance will be the sea on one side and the upper layer on the other." Heaviside's proposal included means by which radio signals are transmitted around the Earth's curvature. Also in 1902, Arthur Edwin Kennelly discovered some of the ionosphere's radio-electrical properties.
In 1912, the U.S. Congress imposed the Radio Act of 1912 on amateur radio operators, limiting their operations to frequencies above 1.5 MHz (wavelength 200 meters or smaller). The government thought those frequencies were useless. This led to the discovery of HF radio propagation via the ionosphere in 1923.worldradiohistory.com: Broadcast listening in the pioneer days of radio on the short waves, 1923 1945 Jerome S. Berg Quote: "...In addition to having to obtain licenses - a constraint to which they adapted only slowly - the amateurs were, with some exceptions, restricted to the range below 200 meters (that is, above 1500 kc.), bands that were largely unexplored and thought to be of little value. The navy attributed most interference to the amateurs, and was happy to see them on the road to a hoped - for extinction. From the amateurs' point of view, their development of the shortwave spectrum began less as a love affair than a shotgun marriage. However, all that would change...It took several years before experimenters ventured above 2-3 mc. and started to understand such things as shortwave propagation and directionality. The short waves, as they were called, were surrounded with mystery...Also in 1928 Radio News publisher Hugo Gernsback began shortwave broadcasting on 9700 kc. from his station, WRNY, New York, using the call W2XAL. "A reader in New South Wales, Aus- tralia," reported Gernsback, "writes us that while he was writing his letter he was listening to WRNY's short-wave transmitter, 2XAL, on a three-tube set; and had to turn down the volume, otherwise he would wake up his family. All this at a distance of some 10,000 miles! Yet 2XAL ...uses less than 500 watts; a quite negligible amount of power. "6...The 1930s were the golden age of shortwave broadcasting...Shortwave also facilitated communication with people in remote areas. Amateur radio became a basic ingredient of all expeditions...The term shortwave was generally taken to refer to anything above 1.5 mc., without upper limit...", backup
In 1925, observations during a solar eclipse in New York by Dr. Alfred N. Goldsmith and his team demonstrated the influence of sunlight on radio wave propagation, revealing that short waves became weak or inaudible while long waves steadied during the eclipse, thus contributing to the understanding of the ionosphere's role in radio transmission.
In 1926, Scottish physicist Robert Watson-Watt introduced the term ionosphere in a letter published only in 1969 in Nature:The letter, dated 8 November 1926, was addressed to the Secretary of the Radio Research Board. The letter was quoted in: See also: See p. 554.
In the early 1930s, test transmissions of Radio Luxembourg inadvertently provided evidence of the first radio modification of the ionosphere; HAARP ran a series of experiments in 2017 using the eponymous Luxembourg Effect.
Edward V. Appleton was awarded a Nobel Prize in 1947 for his confirmation in 1927 of the existence of the ionosphere. Lloyd Berkner first measured the height and density of the ionosphere. This permitted the first complete theory of short-wave radio propagation. Maurice V. Wilkes and J. A. Ratcliffe researched the topic of radio propagation of very long radio waves in the ionosphere. Vitaly Ginzburg has developed a theory of electromagnetic wave propagation in plasmas such as the ionosphere.
In 1962, the Canadian satellite Alouette 1 was launched to study the ionosphere. Following its success were Alouette 2 in 1965 and the two ISIS satellites in 1969 and 1971, further AEROS-A and -B in 1972 and 1975, all for measuring the ionosphere.
On July 26, 1963, the first operational geosynchronous satellite Syncom 2 was launched. On board radio beacons on this satellite (and its successors) enabled – for the first time – the measurement of total electron content (TEC) variation along a radio beam from geostationary orbit to an earth receiver. (The rotation of the plane of polarization directly measures TEC along the path.) Australian geophysicist Elizabeth Essex-Cohen from 1969 onwards was using this technique to monitor the atmosphere above Australia and Antarctica.
Geophysics The ionosphere is a shell of electrons and electrically charged atoms and molecules that surrounds the Earth, stretching from a height of about to more than . It exists primarily due to ultraviolet radiation from the Sun.
Heaviside and Kennelly proposed atmospheric reflection as a model for long-range radio wave propagation, in which the electrically conductive earth and an upper atmospheric layer acted as a wave guide. Heaviside used his Telegrapher's equations, and the conductivity of seawater, to propose "There may possibly be a sufficiently conducting layer in the upper air. Then the guidance will be by the sea on one side and the upper layer on the other." Kennelly proposed a physical cause, calculating that at 80 kilometers air conductivity was 20 times that of seawater. This meant that radio signals propagated as cylindrical waves, and the slower energy divergence meant they could be detected at long distances. In 1919, G. N. Watson used the earth as a conducting sphere, and a spherical atmospheric reflective layer concentric to the earth, to mathematically reproduce the Austin-Cohen formula and other long-distance propagation experiments. In 1924, Joseph Larmor published a theory of ionic refraction, quantitatively showing how ionized air bent radio waves' direction of propagation.
Between 1923 and 1925, Appleton, Miles Barnett, Reginald Smith-Rose, and R.H. Barfield conducted radio sounding experiments while looking for skywaves. They established the height of an ionized Kennelly-Heaviside layer, or "E layer" for electricity, in the upper atmosphere at between 80 and 90 km. Appleton and Barnett manipulated the frequency of transmitting waves and measured the fading caused by the interference with ground waves. They noted sky waves were weak in the day when stronger solar rays made the ionized layer thicker and closer to the ground. Skywaves also dominated at longer transmitter-receiver distances but were superpositioned with ground waves at closer distances. The skywaves also did not attenuate as fast as ground waves. Smith-Rose and Barfield measured the polarization of skywaves, incident angles, and the height of the wave-deflections.
In 1925, Gregory Breit and Merle Tuve used pulse-echo, sharp periodically repetitive pulses, to measure the ionosphere height. However, their height measurements varied from 88 km to 211 km, and varied with wavelength, indicating the ionosphere was not a sharp reflective surface, but behaved according to Larmor's ionic refraction theory. In 1927, Appleton's frequency change experiments established the higher "F layer", which indicated Breit and Tuve measurements corresponded to the bottom of the E layer and the top of the F layer. Since the E layer electron density was lower than the F layer, it refracted the longer waves more. Appleton then proposed longer wavelengths could sound the E layer, while shorted wavelengths could sound the F layer. Appleton also showed solar radiation generated the most ions around noon, pushing the F layer lower, while low ion generation at midnight raised it higher.