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20-527: The F-region is located directly above the E region (formerly the Kennelly-Heaviside layer) and below the protonosphere . It acts as a dependable reflector of HF radio signals as it is not affected by atmospheric conditions, although its ionic composition varies with the sunspot cycle. It reflects normal-incident frequencies at or below the critical frequency (approximately 10 MHz) and partially absorbs waves of higher frequency. The F1 layer

40-407: A wave is the rate at which the wave propagates in any medium . This is the velocity at which the phase of any one frequency component of the wave travels. For such a component, any given phase of the wave (for example, the crest ) will appear to travel at the phase velocity. The phase velocity is given in terms of the wavelength λ (lambda) and time period T as Equivalently, in terms of

60-418: A definition can be extended to a beat of waves, or to a signal composed of multiple waves. For this it is necessary to mathematically write the beat or signal as a low frequency envelope multiplying a carrier. Thus the phase velocity of the carrier determines the phase velocity of the wave set. The group velocity of a collection of waves is defined as When multiple sinusoidal waves are propagating together,

80-403: A product of two waves: an envelope wave formed by f 1 and a carrier wave formed by f 2 . We call the velocity of the envelope wave the group velocity. We see that the phase velocity of f 1 is In the continuous differential case, this becomes the definition of the group velocity. In the context of electromagnetics and optics, the frequency is some function ω ( k ) of

100-544: Is the lower sector of the F layer and exists from about 150 to 220 km (100 to 140 miles) above the surface of the Earth and only during daylight hours. It is composed of a mixture of molecular ions O 2 and NO, and atomic ions O. Above the F1 region, atomic oxygen becomes the dominant constituent because lighter particles tend to occupy higher altitudes above the turbopause (at ~90 km, 56 miles). This atomic oxygen provides

120-489: Is to observe We can then rearrange the above to obtain From this formula, we see that the group velocity is equal to the phase velocity only when the refractive index is independent of frequency ∂ n / ∂ ω = 0 {\textstyle \partial n/\partial \omega =0} . When this occurs, the medium is called non-dispersive, as opposed to dispersive , where various properties of

140-441: The E region . It reflects medium-frequency radio waves . Because of this reflective layer, radio waves radiated into the sky can return to Earth beyond the horizon . This " skywave " or "skip" propagation technique has been used since the 1920s for radio communication at long distances, up to transcontinental distances. Propagation is affected by the time of day. During the daytime the solar wind presses this layer closer to

160-645: The American electrical engineer Arthur Edwin Kennelly (1861–1939) and the British polymath Oliver Heaviside (1850–1925), as an explanation for the propagation of radio waves beyond the horizon observed by Guglielmo Marconi in 1901. However, it was not until 1924 that its existence was shown by British scientist Edward V. Appleton , for which he received the 1947 Nobel Prize in Physics . Physicists resisted

180-448: The Earth, thereby limiting how far it can reflect radio waves. Conversely, on the night ( lee ) side of the Earth, the solar wind drags the ionosphere further away, thereby greatly increasing the range which radio waves can travel by reflection. The extent of the effect is further influenced by the season , and the amount of sunspot activity. Existence of a reflective layer was predicted in 1902 independently and almost simultaneously by

200-646: The O atomic ions that make up the F2 layer. The F1 layer has approximately 5 × 10 e/cm (free electrons per cubic centimeter) at noontime and minimum sunspot activity, and increases to roughly 2 × 10 e/cm during maximum sunspot activity. The density falls off to below 10 e/cm at night. Critical F 2 layer frequencies are the frequencies that will not go through the F 2 layer. Under rare atmospheric conditions, F2 propagation can occur, resulting in VHF television and FM radio signals being received over great distances, well beyond

220-650: The idea of the reflecting layer for one very good reason; it would require total internal reflection , which in turn would require that the speed of light in the ionosphere would be greater than in the atmosphere below it. Since the latter speed is essentially the same as the speed of light in vacuum ( c ), scientists were unwilling to believe the speed in the ionosphere could be higher. Nevertheless, Marconi had received signals in Newfoundland that were broadcast in England, so clearly there must be some mechanism allowing

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240-533: The ionosphere can reflect radio waves. The geometric mean of the phase velocity and the group velocity cannot exceed c , so when the phase velocity goes above c , the group velocity must go below it. In 1925, Americans Gregory Breit and Merle A. Tuve first mapped the Heaviside layer's variations in altitude. The ITU standard model of absorption and reflection of radio waves by the Heaviside Layer

260-526: The normal 40–100 miles (64–161  km ) reception area. E region The Heaviside layer , sometimes called the Kennelly–Heaviside layer , named after Arthur E. Kennelly and Oliver Heaviside , is a layer of ionised gas occurring roughly between 90km and 150 km (56 and 93 mi) above the ground — one of several layers in the Earth 's ionosphere . It is also known as

280-419: The phase of the first crest. This implies kx = ω t , and so v = x / t = ω / k . Formally, we let the phase φ = kx - ωt and see immediately that ω = -dφ / d t and k = dφ / d x . So, it immediately follows that As a result, we observe an inverse relation between the angular frequency and wavevector . If the wave has higher frequency oscillations, the wavelength must be shortened for

300-491: The phase velocity to remain constant. Additionally, the phase velocity of electromagnetic radiation may – under certain circumstances (for example anomalous dispersion ) – exceed the speed of light in vacuum, but this does not indicate any superluminal information or energy transfer. It was theoretically described by physicists such as Arnold Sommerfeld and Léon Brillouin . The previous definition of phase velocity has been demonstrated for an isolated wave. However, such

320-434: The resultant superposition of the waves can result in an "envelope" wave as well as a "carrier" wave that lies inside the envelope. This commonly appears in wireless communication when modulation (a change in amplitude and/or phase) is employed to send data. To gain some intuition for this definition, we consider a superposition of (cosine) waves f(x, t) with their respective angular frequencies and wavevectors. So, we have

340-481: The transmission to reach that far. The paradox was resolved by the discovery that there were two velocities of light, the phase velocity and the group velocity . The phase velocity can in fact be greater than c , but the group velocity, being capable of transmitting information, cannot, by special relativity , be greater than c . The phase velocity for radio waves in the ionosphere is indeed greater than c , and that makes total internal reflection possible, and so

360-403: The wave number, so in general, the phase velocity and the group velocity depend on specific medium and frequency. The ratio between the speed of light c and the phase velocity v p is known as the refractive index , n = c / v p = ck / ω . In this way, we can obtain another form for group velocity for electromagnetics. Writing n = n (ω) , a quick way to derive this form

380-409: The wave's angular frequency ω , which specifies angular change per unit of time, and wavenumber (or angular wave number) k , which represent the angular change per unit of space, To gain some basic intuition for this equation, we consider a propagating (cosine) wave A cos( kx − ωt ) . We want to see how fast a particular phase of the wave travels. For example, we can choose kx - ωt = 0 ,

400-544: Was developed by the British Ionospheric physicist Louis Muggleton in the 1970s. Around 1910, William Eccles proposed the name "Heaviside Layer" for the radio-wave reflecting layer in the upper atmosphere, and the name has subsequently been widely adopted. The name Kennelly–Heaviside layer was proposed in 1925 to give credit to the work of Kennelly, which predated the proposal by Heaviside by several months. Phase velocity The phase velocity of

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