Ionospheric Propagation of Radio Waves: Theories and Applications, Notas de estudo de Física

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PROFESSOR ANGELO ANTONIO LEITHOLD

The Ionosphere

CURITIBA, 1994

Atmospheric electricity abounds in the environment; some traces of it are found less than four feet from the surface of the earth, but on attaining greater height it becomes more apparent. Electric currents created in sunward ionosphere.In outer space, the magnetopause flows along the boundary between the region around an astronomical object (called the "magnetosphere") and surrounding plasma, in which electric phenomena are dominated or organized by this magnetic field. Earth is surrounded by a magnetosphere, as are the magnetized planets Jupiter, Saturn, Uranus and Neptune. Mercury is magnetized, but too weakly to trap plasma. Mars has patchy surface magnetization. The magnetosphere is the location where the outward magnetic pressure of the Earth's magnetic field is counterbalanced by the solar wind, a plasma. Most of solar particles are deflected to either side of the magnetopause, much like water is deflected before the bow of a ship. However, some particles become trapped within the Earth's magnetic field and form radiation belts. The Van Allen radiation belt is a torus of energetic charged particles (i.e. a plasma) around Earth, trapped by Earth's magnetic field.At elevations above the clouds, atmospheric electricity forms a continuous and distinct element (called the electrosphere) in which the Earth is surrounded. The electrosphere layer (tens of kilometers above the surface of the earth to the ionosphere) has a high electrical conductivity and is essentially at a constant electric potential. The ionosphere is the inner edge of the magnetosphere and is the part of the atmosphere that is ionized by solar radiation. Photoionisation is a physical process in which a photon is incident on an atom, ion or molecule, resulting in the ejection of one or more electrons. The ionosphere forms the inner edge of the magnetosphere.

atmosphere's charge polarity was positive in fair weather. H. B. Saussure (1779) recorded data relating to a conductor's induced charge in the atmosphere. Saussure's instrument (which contained two small spheres suspended in parallel with two thin wires) was a precursor to the electrometer. Saussure found that the fair weather condition had an annual variation. Saussure found that there was a variation with height, as well. In 1785, C. A. Coulomb discovered the conductivity of air. His discovery was contrary to the prevailing thought at the time that the atmospheric gases were insulators (which they are to some extent, or at least not very good conductors when not ionized). His research was unfortunately completely ignored. P. Erman (1804) theorized that the Earth was negatively charged. J. C. A. Peltier (1842) tested and confirmed Erman's idea. Lord Kelvin (1860s) proposed that atmospheric positive charges explained the fair weather condition and, later, recognized the existence of atmospheric electric fields. Over the course of the next century, using the ideas of Alessandro Volta and Francis Ronald, several researchers contributed to the growing body of knowledge about atmospheric electrical phenomena. With the invention of the portable electrometer and Lord Kelvin's 19th century water-dropping condenser, a greater level of precision was introduced into observational results. Towards the end of the 19th century came the discovery by W. Linss (1887) that even the most perfectly insulated conductors lose their charge, as Coulomb before him had found, and that this loss depended on atmospheric conditions. H. H. Hoffert (1888) identified individual lightning downward strokes using early camera and would report this in "Intermittent Lightning-Flashes". J. Elster and H. F. Geitel, who also

worked on thermionic emission, proposed a theory to explain thunderstorm's electrical structure (1885) and, later, discovered atmospheric radioactivity (1899). By then it had become clear that freely charged positive and negative ions were always present in the atmosphere, and that radiant emanations could be collected. F. Pockels (1897) estimated lightning current intensity by analyzing lightning flashes in basalt and studying the left-over magnetic fields (basalt, being a ferromagnetic mineral, becomes magnetically polarised when exposed to a large external field such as those generated in a lightning strike). Using a Peltier electrometer, Luigi Palmieri researched atmospheric electricity. Nikola Tesla and Hermann Plauson investigated the production of energy and power via atmospheric electricity. Tesla also proposed to use the atmospheric electrical circuit to transmit energy wirelessly over large distances. The Polish Polar Station, Hornsund, has researched the magnitude of the earth's electric field and recording its vertical component. Discoveries about the electrification of the atmosphere via sensitive electrical instruments and ideas on how the Earth’s negative charge is maintained were developed mainly in the 20th century. Whilst a certain amount of observational work has been done in the branches of atmospheric electricity, the science has not developed to a considerable extent. It is thought that any apparatus which might be used to extract useful energy from atmospheric electricity would be prohibitively costly to build and maintain, which is probably why the field has not attracted much interest. In 1899, Nikola Tesla researched ways to utilize the ionosphere to transmit energy wirelessly over long

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. Edward V. Appleton was awarded in 1947 a Nobel Prize for his confirmation of the existence of the ionosphere in 1927. 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, all for measuring the ionosphere.

The ionosphere The ionosphere is the part of the atmosphere that is ionized by solar radiation. It plays an important part 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 the Earth. The lowest part of the Earth's atmosphere is called the troposphere and it extends from the surface up to about 10 km. The atmosphere above 10 km is called the stratosphere, followed by the mesosphere. It is in the stratosphere that incoming solar radiation creates the ozone layer. At heights of above 80 km, in the thermosphere, the atmosphere is so thin that free electrons can exist for short periods of time before they are captured by a nearby positive ion. The number of these free electrons is sufficient to affect radio propagation. This portion of the atmosphere is ionized and contains a plasma which is referred to as the ionosphere. In a plasma, the negative free electron and the positive ions are attracted to each other by the electromagnetic force, but they

Ionospheric Layers D Layer The D layer is the innermost layer, 50 km to 90 km above the surface of the Earth. Ionization here is due to Lyman series-alpha hydrogen radiation at a wavelength of 121.5 nanometre (nm) ionizing nitric oxide (NO). In addition, when the sun is active with 50 or more sunspots, hard X-rays (wavelength < 1 nm) ionize the air (N2, O2). During the night cosmic rays produce a residual amount of ionization. Recombination is high in this layer, thus the net ionization effect is very low and as a result high- frequency (HF) radio waves aren't reflected by the D layer. E Layer The E layer is the middle layer, 90 km to 120 km above the surface of the Earth. Ionization is due to soft X-ray (1-10 nm) and far ultraviolet (UV) solar radiation ionization of molecular oxygen (O2). This layer can only reflect radio waves having frequencies less than about 10 MHz. It has a negative effect on frequencies above 10 MHz due to its partial absorption of these waves. The vertical structure of the E layer is primarily determined by the competing effects of ionization and recombination. At night the E layer begins to disappear because the primary source of ionization is no

longer present. This results in an increase in the height where the layer maximizes because recombination is faster in the lower layers. Diurnal changes in the high altitude neutral winds also plays a role. The increase in the height of the E layer maximum increases the range to which radio waves can travel by reflection from the layer. F Layer The F layer or region, also known as the Appleton layer, is 120 km to 400 km above the surface of the Earth. Here extreme ultraviolet (UV) (10-100 nm) solar radiation ionizes atomic oxygen (O). The F region is the most important part of the ionosphere in terms of HF communications. The F layer combines into one layer at night, and in the presence of sunlight (during daytime), it divides into two layers, the F1 and F2. The F layers are responsible for most skywave propagation of radio waves, and are thickest and most reflective of radio on the side of the Earth facing the sun. Ionospheric Model The atmospheric physics community contributes to the definition and maintenance of an ionospheric model: the International Reference Ionosphere, through a series of

Equatorial Anomaly Electric currents created in sunward ionosphere. Within approximately ± 20 degrees of the magnetic equator, is the Equatorial Anomaly. It is the occurrence of a trough of concentrated ionization in the F2 layer. The Earth's magnetic field lines are horizontal at the magnetic equator. Solar heating and tidal oscillations in the lower ionosphere move plasma up and across the magnetic field lines. This sets up a sheet of electric current in the E region which, with the horizontal magnetic field, forces ionization up into the F layer, concentrating at ± 20 degrees from the magnetic equator. This phenomenon is known as the equatorial fountain.

Ionospheric Perturbations Radio Application DX communication, popular among amateur radio enthusiasts, is a term given to communication over great distances. When using High-Frequency bands, the ionosphere is utilized to reflect the transmitted radio beam. The beam returns to the Earth's surface, and may then be reflected back into the ionosphere for a second bounce. Radio waves "hop" from the Earth to the ionosphere and back to the Earth. When a radio wave reaches the ionosphere, the electric field in the wave forces the

where N = electron density per cm3 and fcritical is in MHz. The Maximum Usable Frequency (MUF) is defined as the upper frequency limit that can be used for transmission between two points at a specified time. Where a = angle of attack, the angle of the wave relative to the horizon, and sin is the sine function.

The cutoff frequency is the frequency below which a radio wave fails to penetrate a layer of the ionosphere at the incidence angle required for transmission between two specified points by reflection from the layer. Other Applications The open system space tether, which uses the ionosphere, is being researched. The space tether uses plasma contactors and the ionosphere as parts of a circuit to extract energy from the Earth's magnetic field by electromagnetic induction. The A and K indices are a measurement of the behavior of the horizontal component of the geomagnetic field. The K index uses a scale from 0 to 9 to measure the change in the horizontal component of the geomagnetic field. A new K index is determined at the Table Mountain Observatory, north of Boulder, Colorado. The geomagnetic activity levels of the earth are measured by the fluctuation of the Earth's magnetic field in SI units called tesla (unit)s (or in non-SI gauss, especially in older literature). The Earth's magnetic field is measured around the planet by many observatories. The data retrieved is processed and turned into measurement indices. Daily measurements for the entire planet are made available through an estimate of the ap index, called the planetary A-index (PAI).

ionosphere again. As a result, like a rock "skipping" across water, the wave may actually "bounce" or "skip" between the earth and ionosphere several times. This phenomenon is known as "skip" or multihop propagation. Signals of only a few watts can sometimes be received thousands of miles away as a result. Signals with frequencies above about 30 MHz (VHF and UHF for example) are progressively not returned to the Earth's surface, because they penetrate the ionosphere. (This includes most communications with spacecraft and satellites.) Exceptions include rare occasions of E-skip, when FM and TV signals are reflected. Skywave may be disrupted during geomagnetic storms. Low to mid frequencies below approximately 10 MHz (longer than 30 meters), including broadcasts in the mediumwave and shortwave bands (and to some extent longwave), travel most efficiently by skywave at night. Frequencies above 10 MHz (shorter than 30 meters) travel better during the day. Frequencies lower than 3 kHz have a wavelength longer than the distance between the Earth and the ionosphere. The Maximum usable frequency for skywave propagation is strongly influenced by sunspot number. Because the lower-altitude layers (the E-layer in particular) of the ionosphere largely disappear at night, the refractive layer of the ionosphere is much higher above the surface at night. This leads to an increase in the "skip" or "hop" distance of the skywave at night. Near Vertical Incidence Skywave , NVIS, is a radio antenna configuration that provides usable signals in the range between groundwave and skywave distances (usually 30 to 400 miles, or 50 to 650 km). The usable frequencies are between 1.8 MHz and 15 MHz. with the most common use being

between 3.5 MHz and 7.3 Mhz. NVIS configuration is a horizontally polarized (parallel with the surface of the earth) radiating element that is from 1/20th wavelength to 1/8th wavelength above the ground. That proximity to the ground forces the majority of the radiation to go straight up. Overall efficiency of the antenna can be increased by placing a ground wire of the same length as the antenna, parallel to and directly underneath the antenna. While the ground wire is not necessary under good to excellent propagation conditions antenna, gain in the 3 dB to 6 dB range are common when the ground wire is used. Significant increases in communication will be realized when both the transmitting station and the receiving station use NVIS configuration for their antennas. NVIS is most useful in mountainous areas where line of sight propagation at VHF or UHF frequencies is ineffective or when the communication distance is beyond ground wave (less than 100 Km) and less than sky-wave (450 to 2500 Km Approx.). More simply stated, NVIS communication is most effective for regional use. This may be used to handle emergency communication or simply for fun.