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70-484: Omanawa Caldera is inferred by an area of magnetic anomaly that exists to the north-west of the Rotorua Caldera . It is also located to the north west of the present boundary of the modern Taupō Volcanic Zone but its existence would be compatible with activity in the area of intersection of Taupo Rift and Hauraki Rift before 1.9 million years ago. The area of the caldera is covered by Mamaku Ignambrite from

140-660: A base station repeatedly or by having another magnetometer that periodically measures the field at a fixed location. Second, since the anomaly is the local contribution to the magnetic field, the main geomagnetic field must be subtracted from it. The International Geomagnetic Reference Field is usually used for this purpose. This is a large-scale, time-averaged mathematical model of the Earth's field based on measurements from satellites, magnetic observatories and other surveys. Some corrections that are needed for gravity anomalies are less important for magnetic anomalies. For example,

210-411: A bit to absorption on frequencies above. However, during intense sporadic E events, the E s layer can reflect frequencies up to 50 MHz and higher. The vertical structure of the E layer is primarily determined by the competing effects of ionization and recombination. At night the E layer weakens because the primary source of ionization is no longer present. After sunset an increase in the height of

280-654: A geomagnetic storm the F₂ layer will become unstable, fragment, and may even disappear completely. In the Northern and Southern polar regions of the Earth aurorae will be observable in the night sky. Lightning can cause ionospheric perturbations in the D-region in one of two ways. The first is through VLF (very low frequency) radio waves launched into the magnetosphere . These so-called "whistler" mode waves can interact with radiation belt particles and cause them to precipitate onto

350-487: A high frequency (3–30 MHz) radio blackout that can persist for many hours after strong flares. During this time very low frequency (3–30 kHz) signals will be reflected by the D layer instead of the E layer, where the increased atmospheric density will usually increase the absorption of the wave and thus dampen it. As soon as the X-rays end, the sudden ionospheric disturbance (SID) or radio black-out steadily declines as

420-417: A radio wave reaches the ionosphere, the electric field in the wave forces the electrons in the ionosphere into oscillation at the same frequency as the radio wave. Some of the radio-frequency energy is given up to this resonant oscillation. The oscillating electrons will then either be lost to recombination or will re-radiate the original wave energy. Total refraction can occur when the collision frequency of

490-455: A regional survey of deeper rocks. In shipborne surveys, a magnetometer is towed a few hundred meters behind a ship in a device called a fish . The sensor is kept at a constant depth of about 15 m. Otherwise, the procedure is similar to that used in aeromagnetic surveys. Sputnik 3 in 1958 was the first spacecraft to carry a magnetometer. In the autumn of 1979, Magsat was launched and jointly operated by NASA and USGS until

560-490: A remanent magnetization or remanence. This remanence can last for millions of years, so it may be in a completely different direction from the present Earth's field. If a remanence is present, it is difficult to separate from the induced magnetization unless samples of the rock are measured. The ratio of the magnitudes, Q = M r / M i , is called the Koenigsberger ratio . Interpretation of magnetic anomalies

630-479: A result of lightning activity. Their subsequent research has focused on the mechanism by which this process can occur. Due to the ability of ionized atmospheric gases to refract high frequency (HF, or shortwave ) radio waves, the ionosphere can reflect radio waves directed into the sky back toward the Earth. Radio waves directed at an angle into the sky can return to Earth beyond the horizon. This technique, called "skip" or " skywave " propagation, has been used since

700-444: A sensitivity of 10 nT or less. There are three main types of magnetometer used to measure magnetic anomalies: In ground-based surveys, measurements are made at a series of stations, typically 15 to 60 m apart. Usually a proton precession magnetometer is used and it is often mounted on a pole. Raising the magnetometer reduces the influence of small ferrous objects that were discarded by humans. To further reduce unwanted signals,

770-450: A series of parallel runs at a constant height and with intervals of anywhere from a hundred meters to several kilometers. These are crossed by occasional tie lines, perpendicular to the main survey, to check for errors. The plane is a source of magnetism, so sensors are either mounted on a boom (as in the figure) or towed behind on a cable. Aeromagnetic surveys have a lower spatial resolution than ground surveys, but this can be an advantage for

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840-638: 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. However, Marconi did achieve transatlantic wireless communications in Glace Bay, Nova Scotia , one year later. In 1902, Oliver Heaviside proposed

910-404: Is actually lower in the local summer months. This effect is known as the winter anomaly. The anomaly is always present in the northern hemisphere, but is usually absent in the southern hemisphere during periods of low solar activity. Within approximately ± 20 degrees of the magnetic equator , is the equatorial anomaly. It is the occurrence of a trough in the ionization in the F 2 layer at

980-460: Is also common, sometimes to distances of 15,000 km (9,300 mi) or more. The F layer or region, also known as the Appleton–Barnett layer, extends from about 150 km (93 mi) to more than 500 km (310 mi) above the surface of Earth. It is the layer with the highest electron density, which implies signals penetrating this layer will escape into space. Electron production

1050-677: Is currently used to compensate for ionospheric effects in GPS . This model was developed at the US Air Force Geophysical Research Laboratory circa 1974 by John (Jack) Klobuchar . The Galileo navigation system uses the NeQuick model . GALILEO broadcasts 3 coefficients to compute the effective ionization level, which is then used by the NeQuick model to compute a range delay along the line-of-sight. The open system electrodynamic tether , which uses

1120-448: Is dominated by extreme ultraviolet (UV, 10–100 nm) radiation ionizing atomic oxygen. The F layer consists of one layer (F 2 ) at night, but during the day, a secondary peak (labelled F 1 ) often forms in the electron density profile. Because the F 2 layer remains by day and night, it is responsible for most skywave propagation of radio waves and long distance high frequency (HF, or shortwave ) radio communications. Above

1190-508: Is enough to absorb most (if not all) transpolar HF radio signal transmissions. Such events typically last less than 24 to 48 hours. The E layer is the middle layer, 90 to 150 km (56 to 93 mi) 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 (O 2 ). Normally, at oblique incidence, this layer can only reflect radio waves having frequencies lower than about 10 MHz and may contribute

1260-537: Is higher than the plasma frequency of the ionosphere, then the electrons cannot respond fast enough, and they are not able to re-radiate the signal. It is calculated as shown below: where N = electron density per m and f critical is in Hz. 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 α {\displaystyle \alpha } = angle of arrival ,

1330-488: Is named TMA-2, and one in the Olduvai Gorge is found in 2513 and retroactively named TMA-0 because it was first encountered by primitive humans. Ionosphere The ionosphere ( / aɪ ˈ ɒ n ə ˌ s f ɪər / ) is the ionized part of the upper atmosphere of Earth , from about 48 km (30 mi) to 965 km (600 mi) above sea level , a region that includes the thermosphere and parts of

1400-481: Is no other obvious closer inferred volcanic structure to assign a super volcanic eruption to, so at least some of these eruptions are likely to be associated with the Omanawa Caldera magnetic anomaly. Magnetic anomaly Magnetic anomalies are generally a small fraction of the magnetic field. The total field ranges from 25,000 to 65,000  nanoteslas (nT). To measure anomalies, magnetometers need

1470-405: Is primarily permanent magnetization carried by titanomagnetite minerals in basalt and gabbros . They are magnetized when ocean crust is formed at the ridge. As magma rises to the surface and cools, the rock acquires a thermoremanent magnetization in the direction of the field. Then the rock is carried away from the ridge by the motions of the tectonic plates . Every few hundred thousand years,

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1540-425: Is removing short-term variations in the field from external sources; e.g., temporal variations which include diurnal variations that have a period of 24 hours and magnitudes of up to 30 nT, probably from the action of the solar wind on the ionosphere . In addition, magnetic storms can have peak magnitudes of 1000 nT and can last for several days. Their contribution can be measured by returning to

1610-411: Is sufficient to affect radio propagation . This portion of the atmosphere is partially ionized and contains a plasma which is referred to as the ionosphere. Ultraviolet (UV), X-ray and shorter wavelengths of solar radiation are ionizing, since photons at these frequencies contain sufficient energy to dislodge an electron from a neutral gas atom or molecule upon absorption. In this process

1680-405: Is the main reason for absorption of HF radio waves , particularly at 10 MHz and below, with progressively less absorption at higher frequencies. This effect peaks around noon and is reduced at night due to a decrease in the D layer's thickness; only a small part remains due to cosmic rays . A common example of the D layer in action is the disappearance of distant AM broadcast band stations in

1750-502: Is useful in crossing international boundaries and covering large areas at low cost. Automated services still use shortwave radio frequencies, as do radio amateur hobbyists for private recreational contacts and to assist with emergency communications during natural disasters. Armed forces use shortwave so as to be independent of vulnerable infrastructure, including satellites, and the low latency of shortwave communications make it attractive to stock traders, where milliseconds count. When

1820-434: Is usually done by matching observed and modeled values of the anomalous magnetic field. An algorithm developed by Talwani and Heirtzler(1964) (and further elaborated by Kravchinsky et al., 2019) treats both induced and remnant magnetizations as vectors and allows theoretical estimation of the remnant magnetization from the existing apparent polar wander paths for different tectonic units or continents. Magnetic surveys over

1890-601: The Committee on Space Research (COSPAR) and the International Union of Radio Science (URSI). The major data sources are the worldwide network of ionosondes , the powerful incoherent scatter radars (Jicamarca, Arecibo , Millstone Hill, Malvern, St Santin), the ISIS and Alouette topside sounders , and in situ instruments on several satellites and rockets. IRI is updated yearly. IRI is more accurate in describing

1960-562: 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 . It also affects GPS signals that travel through this layer. As early as 1839, the German mathematician and physicist Carl Friedrich Gauss postulated that an electrically conducting region of

2030-448: The troposphere , extends from the surface to about 10 km (6 mi). Above that is the stratosphere , followed by the mesosphere. In the stratosphere incoming solar radiation creates the ozone layer . At heights of above 80 km (50 mi), 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

2100-399: The 1920s to communicate at international or intercontinental distances. The returning radio waves can reflect off the Earth's surface into the sky again, allowing greater ranges to be achieved with multiple hops . This communication method is variable and unreliable, with reception over a given path depending on time of day or night, the seasons, weather, and the 11-year sunspot cycle . During

2170-443: The D layer, so there are many more neutral air molecules than ions. Medium frequency (MF) and lower high frequency (HF) radio waves are significantly attenuated within the D layer, as the passing radio waves cause electrons to move, which then collide with the neutral molecules, giving up their energy. Lower frequencies experience greater absorption because they move the electrons farther, leading to greater chance of collisions. This

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2240-706: The E layer maximum increases the range to which radio waves can travel by reflection from the layer. This region is also known as the Kennelly–Heaviside layer or simply the Heaviside layer. Its existence was predicted in 1902 independently and almost simultaneously by the American electrical engineer Arthur Edwin Kennelly (1861–1939) and the British physicist Oliver Heaviside (1850–1925). In 1924 its existence

2310-532: The F 1 layer. The F 2 layer persists by day and night and is the main region responsible for the refraction and reflection of radio waves. The D layer is the innermost layer, 48 to 90 km (30 to 56 mi) above the surface of the Earth. Ionization here is due to Lyman series -alpha hydrogen radiation at a wavelength of 121.6 nanometre (nm) ionizing nitric oxide (NO). In addition, solar flares can generate hard X-rays (wavelength < 1 nm ) that ionize N 2 and O 2 . Recombination rates are high in

2380-458: The F 2 layer daytime ion production is higher in the summer, as expected, since the Sun shines more directly on the Earth. However, there are seasonal changes in the molecular-to-atomic ratio of the neutral atmosphere that cause the summer ion loss rate to be even higher. The result is that the increase in the summertime loss overwhelms the increase in summertime production, and total F 2 ionization

2450-488: The F layer, the number of oxygen ions decreases and lighter ions such as hydrogen and helium become dominant. This region above the F layer peak and below the plasmasphere is called the topside ionosphere. From 1972 to 1975 NASA launched the AEROS and AEROS B satellites to study the F region. An ionospheric model is a mathematical description of the ionosphere as a function of location, altitude, day of year, phase of

2520-934: The Mamaku eruption of 240,000 years ago that formed the Rotorua Caldera. Eruptions from the Omanawa Caldera would explain formations such as the Waiteariki ignimbrite which covers much of the Bay of Plenty and forms the bulk of the Whakamarama Plateau. This would date the major caldera formation to 2.1 million years ago. However, there are at least eight large eruptions that occurred in the Tauranga Volcanic Centre between 2.4 and 1.9 million years ago and at this time which ones relate to this caldera can not be definite. However to date there

2590-453: The Sun at any one time. Sunspot active regions are the source of increased coronal heating and accompanying increases in EUV and X-ray irradiance, particularly during episodic magnetic eruptions that include solar flares that increase ionization on the sunlit side of the Earth and solar energetic particle events that can increase ionization in the polar regions. Thus the degree of ionization in

2660-441: 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 refraction from the layer. There are a number of models used to understand the effects of the ionosphere on global navigation satellite systems. The Klobuchar model

2730-551: The atmosphere could account for observed variations of Earth's magnetic field. 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 152.4 m (500 ft) kite-supported antenna for reception. The transmitting station in Poldhu , Cornwall, used a spark-gap transmitter to produce

2800-449: The atmosphere near the magnetic poles increasing the ionization of the D and E layers. PCA's typically last anywhere from about an hour to several days, with an average of around 24 to 36 hours. Coronal mass ejections can also release energetic protons that enhance D-region absorption in the polar regions. Geomagnetic storms and ionospheric storms are temporary and intense disturbances of the Earth's magnetosphere and ionosphere. During

2870-412: The daytime. During solar proton events , ionization can reach unusually high levels in the D-region over high and polar latitudes. Such very rare events are known as Polar Cap Absorption (or PCA) events, because the increased ionization significantly enhances the absorption of radio signals passing through the region. In fact, absorption levels can increase by many tens of dB during intense events, which

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2940-509: The direction of the magnetic field reverses . Thus, the pattern of stripes is a global phenomenon and can be used to calculate the velocity of seafloor spreading . In the Space Odyssey series by Arthur C. Clarke , a series of monoliths are left by extraterrestrials for humans to find. One near the crater Tycho is found by its unnaturally powerful magnetic field and named Tycho Magnetic Anomaly 1 (TMA-1). One orbiting Jupiter

3010-539: The discovery of HF radio propagation via the ionosphere in 1923. 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

3080-423: The electrons in the D-region recombine rapidly and propagation gradually returns to pre-flare conditions over minutes to hours depending on the solar flare strength and frequency. Associated with solar flares is a release of high-energy protons. These particles can hit the Earth within 15 minutes to 2 hours of the solar flare. The protons spiral around and down the magnetic field lines of the Earth and penetrate into

3150-431: The equator and crests at about 17 degrees in magnetic latitude. 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

3220-609: The existence of the Kennelly–Heaviside layer of the ionosphere which bears his name. 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

3290-415: The first half of the 20th century it was widely used for transoceanic telephone and telegraph service, and business and diplomatic communication. Due to its relative unreliability, shortwave radio communication has been mostly abandoned by the telecommunications industry, though it remains important for high-latitude communication where satellite-based radio communication is not possible. Shortwave broadcasting

3360-457: 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

3430-508: The interactions of the ions and electrons with the neutral atmosphere and sunlight, or it may be a statistical description based on a large number of observations or a combination of physics and observations. One of the most widely used models is the International Reference Ionosphere (IRI), which is based on data and specifies the four parameters just mentioned. The IRI is an international project sponsored by

3500-465: The ionosphere and decrease the ionization. Sydney Chapman proposed that the region below the ionosphere be called neutrosphere (the neutral atmosphere ). At night the F layer is the only layer of significant ionization present, while the ionization in the E and D layers is extremely low. During the day, the D and E layers become much more heavily ionized, as does the F layer, which develops an additional, weaker region of ionisation known as

3570-421: The ionosphere follows both a diurnal (time of day) cycle and the 11-year solar cycle . There is also a seasonal dependence in ionization degree since the local winter hemisphere is tipped away from the Sun, thus there is less received solar radiation. Radiation received also varies with geographical location (polar, auroral zones, mid-latitudes , and equatorial regions). There are also mechanisms that disturb

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3640-404: The ionosphere is less than the radio frequency, and if the electron density in the ionosphere is great enough. A qualitative understanding of how an electromagnetic wave propagates through the ionosphere can be obtained by recalling geometric optics . Since the ionosphere is a plasma, it can be shown that the refractive index is less than unity. Hence, the electromagnetic "ray" is bent away from

3710-444: The ionosphere, adding ionization to the D-region. These disturbances are called "lightning-induced electron precipitation " (LEP) events. Additional ionization can also occur from direct heating/ionization as a result of huge motions of charge in lightning strikes. These events are called early/fast. In 1925, C. T. R. Wilson proposed a mechanism by which electrical discharge from lightning storms could propagate upwards from clouds to

3780-474: 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

3850-678: The ionosphere. Around the same time, Robert Watson-Watt, working at the Radio Research Station in Slough, UK, suggested that the ionospheric sporadic E layer (E s ) appeared to be enhanced as a result of lightning but that more work was needed. In 2005, C. Davis and C. Johnson, working at the Rutherford Appleton Laboratory in Oxfordshire, UK, demonstrated that the E s layer was indeed enhanced as

3920-445: The ionosphere. At the magnetic dip equator, where the geomagnetic field is horizontal, this electric field results in an enhanced eastward current flow within ± 3 degrees of the magnetic equator, known as the equatorial electrojet . When the Sun is active, strong solar flares can occur that hit the sunlit side of Earth with hard X-rays. The X-rays penetrate to the D-region, releasing electrons that rapidly increase absorption, causing

3990-462: The light electron obtains a high velocity so that the temperature of the created electronic gas is much higher (of the order of thousand K) than the one of ions and neutrals. The reverse process to ionization is recombination , in which a free electron is "captured" by a positive ion. Recombination occurs spontaneously, and causes the emission of a photon carrying away the energy produced upon recombination. As gas density increases at lower altitudes,

4060-400: The magnetic equator. This phenomenon is known as the equatorial fountain . The worldwide solar-driven wind results in the so-called Sq (solar quiet) current system in the E region of the Earth's ionosphere ( ionospheric dynamo region ) (100–130 km (60–80 mi) altitude). Resulting from this current is an electrostatic field directed west–east (dawn–dusk) in the equatorial day side of

4130-424: The normal rather than toward the normal as would be indicated when the refractive index is greater than unity. It can also be shown that the refractive index of a plasma, and hence the ionosphere, is frequency-dependent, see Dispersion (optics) . The critical frequency is the limiting frequency at or below which a radio wave is reflected by an ionospheric layer at vertical incidence . If the transmitted frequency

4200-415: The oceans have revealed a characteristic pattern of anomalies around mid-ocean ridges. They involve a series of positive and negative anomalies in the intensity of the magnetic field, forming stripes running parallel to each ridge. They are often symmetric about the axis of the ridge. The stripes are generally tens of kilometers wide, and the anomalies are a few hundred nanoteslas. The source of these anomalies

4270-406: The recombination process prevails, since the gas molecules and ions are closer together. The balance between these two processes determines the quantity of ionization present. Ionization depends primarily on the Sun and its Extreme Ultraviolet (EUV) and X-ray irradiance which varies strongly with solar activity . The more magnetically active the Sun is, the more sunspot active regions there are on

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4340-687: The spring of 1980. It had a caesium vapor scalar magnetometer and a fluxgate vector magnetometer. CHAMP , a German satellite, made precise gravity and magnetic measurements from 2001 to 2010. A Danish satellite, Ørsted , was launched in 1999 and is still in operation, while the Swarm mission of the European Space Agency involves a "constellation" of three satellites that were launched in November, 2013. There are two main corrections that are needed for magnetic measurements. The first

4410-407: The sunspot cycle and geomagnetic activity. Geophysically, the state of the ionospheric plasma may be described by four parameters: electron density, electron and ion temperature and, since several species of ions are present, ionic composition . Radio propagation depends uniquely on electron density. Models are usually expressed as computer programs. The model may be based on basic physics of

4480-635: The surveyors do not carry metallic objects such as keys, knives or compasses, and objects such as motor vehicles, railway lines, and barbed wire fences are avoided. If some such contaminant is overlooked, it may show up as a sharp spike in the anomaly, so such features are treated with suspicion. The main application for ground-based surveys is the detailed search for minerals. Airborne magnetic surveys are often used in oil surveys to provide preliminary information for seismic surveys. In some countries such as Canada, government agencies have made systematic surveys of large areas. The survey generally involves making

4550-537: The term ionosphere in a letter published only in 1969 in Nature : We have in quite recent years seen the universal adoption of the term 'stratosphere'..and..the companion term 'troposphere'... The term 'ionosphere', for the region in which the main characteristic is large scale ionisation with considerable mean free paths, appears appropriate as an addition to this series. In the early 1930s, test transmissions of Radio Luxembourg inadvertently provided evidence of

4620-483: 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

4690-537: The variation of the electron density from bottom of the ionosphere to the altitude of maximum density than in describing the total electron content (TEC). Since 1999 this model is "International Standard" for the terrestrial ionosphere (standard TS16457). Ionograms allow deducing, via computation, the true shape of the different layers. Nonhomogeneous structure of the electron / ion - plasma produces rough echo traces, seen predominantly at night and at higher latitudes, and during disturbed conditions. At mid-latitudes,

4760-556: The vertical gradient of the magnetic field is 0.03 nT/m or less, so an elevation correction is generally not needed. The magnetization in the surveyed rock is the vector sum of induced and remanent magnetization : The induced magnetization of many minerals is the product of the ambient magnetic field and their magnetic susceptibility χ : Some susceptibilities are given in the table. Minerals that are diamagnetic or paramagnetic only have an induced magnetization. Ferromagnetic minerals such as magnetite also can carry

4830-960: Was detected by Edward V. Appleton and Miles Barnett . The E s layer ( sporadic E-layer) is characterized by small, thin clouds of intense ionization, which can support reflection of radio waves, frequently up to 50 MHz and rarely up to 450 MHz. Sporadic-E events may last for just a few minutes to many hours. Sporadic E propagation makes VHF-operating by radio amateurs very exciting when long-distance propagation paths that are generally unreachable "open up" to two-way communication. There are multiple causes of sporadic-E that are still being pursued by researchers. This propagation occurs every day during June and July in northern hemisphere mid-latitudes when high signal levels are often reached. The skip distances are generally around 1,640 km (1,020 mi). Distances for one hop propagation can be anywhere from 900 to 2,500 km (560 to 1,550 mi). Multi-hop propagation over 3,500 km (2,200 mi)

4900-515: Was using this technique to monitor the atmosphere above Australia and Antarctica. The ionosphere is a shell of electrons and electrically charged atoms and molecules that surrounds the Earth, stretching from a height of about 50 km (30 mi) to more than 1,000 km (600 mi). It exists primarily due to ultraviolet radiation from the Sun . The lowest part of the Earth's atmosphere ,

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