text:
Intifada

Intifada (انتفاضة) is an Arabic word for a rebellion or uprising, or a resistance movement. It can be used to refer to an uprising against oppression.

The word intifada was first used in modern times in 1952, when Iraqi parties took to the streets to protest their monarchy, which was known as the Iraqi Intifada. Other later examples include the Western Sahara's Zemla Intifada, the First Sahrawi Intifada, and the Second Sahrawi Intifada. In the Israeli-Palestinian conflict context, it refers to uprising by Palestinian people against Israeli occupation or Israel, involving both violent and nonviolent methods of resistance, including the First Intifada (1987–1993) and the Second Intifada (2000–2005).

In English-language usage, the word primarily refers to Palestinian uprisings against Israeli occupation. While in Arabic-language usage, uprisings globally can be referred to as intifada, including for example, the 1916 Easter Rising, the 1943 Warsaw Ghetto Uprising, and the 1949 Jeju uprising.

Etymology
Intifada is an Arabic word literally meaning, as a noun, "tremor", "shivering", "shuddering". It is derived from an Arabic term nafada meaning "to shake", "shake off", "get rid of", as a dog might shrug off water, or as one might shake off sleep, or dirt from one's sandals.

History
In the Palestinian context, the word refers to attempts to "shake off" the Israeli occupation of the West Bank and Gaza Strip in the First and Second Intifadas, where it was originally chosen to connote "aggressive nonviolent resistance", a meaning it bore among Palestinian students in struggles in the 1980s and which they adopted as less confrontational than terms in earlier militant rhetoric since it bore no nuance of violence. The First Intifada was characterized by protests and violent riots, especially stone-throwing, while the Second Intifada was characterized by a period of heightened violence. The suicide bombings carried out by Palestinian assailants became one of the more prominent features of the Second Intifada and mainly targeted Israeli civilians, contrasting with the relatively less violent nature of the First Intifada.

Israeli–Palestinian conflict
The term Intifada, in context of the Israeli–Palestinian conflict refers to major Palestinian uprisings against Israeli control, with the First Intifada occurring between 1987 and 1993 and the Second Intifada between 2000 and 2005, both involving numerous attacks against Israeli civilians and security forces.

The phrase "Globalize the Intifada" is a slogan used to promote worldwide activism in solidarity with the Palestinian resistance. The chant and its associated chants have caused controversy, particularly concerning their impact and connotations. Critics claimed it encourages widespread violence or terrorism.

List of events named Intifada
In Arabic texts, uprisings anywhere can be referred to using the Arabic word "intifada", including, for example, the 1916 Easter Rising (انتفاضة الفصح), the 1943 Warsaw Ghetto Uprising (انتفاضة غيتو وارسو), and the 1949 Jeju uprising (انتفاضة جيجو).

In English, the word may refer to these events, overwhelmingly in the Arabic-speaking world:
 * Iraqi Intifada, a series of strikes and riots in Iraq in 1952, aimed against the Hashemite monarchy rule
 * October Revolution, a series of strikes, riots, and demonstrations in Sudan, that ended with the dissolution of the Abbud military regime and the beginning of second civilian rule in 1964
 * March Intifada, a leftist uprising against the British colonial presence in Bahrain in March 1965
 * Zemla Intifada, against Spanish colonial rule in then Spanish Sahara, in June 1970
 * In Lebanese internal conflicts:
 * February 6 Intifada (1984), during the Lebanese Civil War
 * Cedar Revolution or "Intifada of Independence", the events in Lebanon after Rafic Hariri's 2005 assassination
 * The 1984 Egyptian intifada, (October 1, 1984 - October 2, 1984), a bloody uprising and civil resistance movement
 * In the Israeli–Palestinian conflict:
 * First Intifada, a Palestinian uprising against the Israeli occupation lasting from December 1987 to 1993
 * Second Intifada, also called Al-Aqsa Intifada, a period of intensified Israeli-Palestinian violence, which began in late September 2000 and ended around 2005
 * 2014 Jerusalem unrest, a series of violent acts and attacks in Jerusalem in 2014 sometimes referred to as "Intifada"
 * 2015–2016 wave of violence in the Israeli–Palestinian conflict – 2015 escalation in Israeli–Palestinian conflict, sometimes referred to as "Al-Quds Intifada" or "Jerusalem Intifada" or "Knife Intifada"
 * 2023-present Israel–Hamas war, referred to by some commentators as the Third Intifada
 * The ongoing international 2024 pro-Palestinian protests on university campuses, related to the war and sometimes referred to as the Student Intifada

 * 1990s uprising in Bahrain, an uprising demanding a return to democratic rule, also known as the "1990s Intifada"
 * 1991 Iraqi uprisings, an armed uprising against Saddam Hussein in Iraq, also known as "Iraqi Intifada of 1991"
 * In the Western Sahara conflict:
 * First Sahrawi Intifada, protests by Sahrawi activists in the Western Sahara, south of Morocco (1999–2004)
 * Second Sahrawi Intifada or Independence Intifada, demonstrations and riots in Western Sahara, south of Morocco, beginning in May 2005
 * Gdeim Izik protest camp, also referred as Third Sahrawi Intifada or simply Third Intifada
 * 2005 French riots referred to be the historian Andrew Hussey as "the French Intifada"
 * Arab Spring, a revolutionary wave which began on 18 December 2010 in Tunisia, sometimes referred to as "Intifada":
 * Tunisian Revolution, or Tunisian Intifada
 * Yemeni Revolution, or Yemeni Intifada
 * 2011 Egyptian revolution, or Egyptian Intifada
 * 2011–2013 Sudanese protests, or Sudanese Intifada
 * Second Arab Spring, a series of anti-government protests in several Arab countries starting in late 2019, also known as "Arab Spring 2.0":
 * 17 October Revolution, nicknamed the Tax Intifada
 * 2019–2021 Iraqi protests, nicknamed Iraqi Intifada

title: Intifada


text:
Ionosphere

The ionosphere is the ionized part of the upper atmosphere of Earth, from about 48 km to 965 km 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. It also affects GPS signals that travel through this layer.

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 152.4 m 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. 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. 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:

"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 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 50 km to more than 1000 km. It exists primarily due to ultraviolet radiation from the Sun.

The lowest part of the Earth's atmosphere, the troposphere, extends from the surface to about 10 km. 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, 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 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 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, 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 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 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 the ionosphere and decrease the ionization.

Sydney Chapman proposed that the region below the ionosphere be called neutrosphere (the neutral atmosphere).

Layers of ionization

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 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.

D layer
The D layer is the innermost layer, 48 to 90 km 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 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 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 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 is enough to absorb most (if not all) transpolar HF radio signal transmissions. Such events typically last less than 24 to 48 hours.

E layer
The E layer is the middle layer, 90 to 150 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 (O$2$). Normally, at oblique incidence, this layer can only reflect radio waves having frequencies lower than about 10 MHz and may contribute 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 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 was detected by Edward V. Appleton and Miles Barnett.

E$s$ layer
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 1640 km. Distances for one hop propagation can be anywhere from 900 to 2500 km. Multi-hop propagation over 3500 km is also common, sometimes to distances of 15000 km or more.

F layer

The F layer or region, also known as the Appleton–Barnett layer, extends from about 150 km to more than 500 km 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 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 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.

Ionospheric model
An ionospheric model is a mathematical description of the ionosphere as a function of location, altitude, day of year, phase of 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 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 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 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).

Persistent anomalies to the idealized model
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.

Winter anomaly
At mid-latitudes, the F2 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 F2 ionization 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.

Equatorial anomaly
Within approximately ± 20 degrees of the magnetic equator, is the equatorial anomaly. It is the occurrence of a trough in the ionization in the F2 layer at 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 the magnetic equator. This phenomenon is known as the equatorial fountain.

Equatorial electrojet
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 altitude). Resulting from this current is an electrostatic field directed west–east (dawn–dusk) in the equatorial day side of 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.

X-rays: sudden ionospheric disturbances (SID)
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 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 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.

Protons: polar cap absorption (PCA)
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 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.

Storms
Geomagnetic storms and ionospheric storms are temporary and intense disturbances of the Earth's magnetosphere and ionosphere.

During 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
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 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 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 (Es) 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 Es layer was indeed enhanced as a result of lightning activity. Their subsequent research has focused on the mechanism by which this process can occur.

Radio communication
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 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 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 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.

Mechanism of refraction
When 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 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 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 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:

 * $$f_{\text{critical}} = 9 \times\sqrt{N}$$

where N = electron density per m3 and fcritical 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.

 * $$f_\text{muf} = \frac{f_\text{critical}}{ \sin \alpha} $$

where $$\alpha$$ = angle of arrival, 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.

GPS/GNSS ionospheric correction
There are a number of models used to understand the effects of the ionosphere on global navigation satellite systems. The Klobuchar model 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.

Other applications
The open system electrodynamic 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.

Overview
Scientists explore the structure of the ionosphere by a wide variety of methods. They include:
 * passive observations of optical and radio emissions generated in the ionosphere
 * bouncing radio waves of different frequencies from it
 * incoherent scatter radars such as the EISCAT, Sondre Stromfjord, Millstone Hill, Arecibo, Advanced Modular Incoherent Scatter Radar (AMISR) and Jicamarca radars
 * coherent scatter radars such as the Super Dual Auroral Radar Network (SuperDARN) radars
 * special receivers to detect how the reflected waves have changed from the transmitted waves.

A variety of experiments, such as HAARP (High Frequency Active Auroral Research Program), involve high power radio transmitters to modify the properties of the ionosphere. These investigations focus on studying the properties and behavior of ionospheric plasma, with particular emphasis on being able to understand and use it to enhance communications and surveillance systems for both civilian and military purposes. HAARP was started in 1993 as a proposed twenty-year experiment, and is currently active near Gakona, Alaska.

The SuperDARN radar project researches the high- and mid-latitudes using coherent backscatter of radio waves in the 8 to 20 MHz range. Coherent backscatter is similar to Bragg scattering in crystals and involves the constructive interference of scattering from ionospheric density irregularities. The project involves more than 11 countries and multiple radars in both hemispheres.

Scientists are also examining the ionosphere by the changes to radio waves, from satellites and stars, passing through it. The Arecibo Telescope located in Puerto Rico, was originally intended to study Earth's ionosphere.

Ionograms
Ionograms show the virtual heights and critical frequencies of the ionospheric layers and which are measured by an ionosonde. An ionosonde sweeps a range of frequencies, usually from 0.1 to 30 MHz, transmitting at vertical incidence to the ionosphere. As the frequency increases, each wave is refracted less by the ionization in the layer, and so each penetrates further before it is reflected. Eventually, a frequency is reached that enables the wave to penetrate the layer without being reflected. For ordinary mode waves, this occurs when the transmitted frequency just exceeds the peak plasma, or critical, frequency of the layer. Tracings of the reflected high frequency radio pulses are known as ionograms. Reduction rules are given in: "URSI Handbook of Ionogram Interpretation and Reduction", edited by William Roy Piggott and Karl Rawer, Elsevier Amsterdam, 1961 (translations into Chinese, French, Japanese and Russian are available).

Incoherent scatter radars
Incoherent scatter radars operate above the critical frequencies. Therefore, the technique allows probing the ionosphere, unlike ionosondes, also above the electron density peaks. The thermal fluctuations of the electron density scattering the transmitted signals lack coherence, which gave the technique its name. Their power spectrum contains information not only on the density, but also on the ion and electron temperatures, ion masses and drift velocities. Incoherent scatter radars can also measure neutral atmosphere movements, such as atmospheric tides, after making assumptions about ion-neutral collision frequency across the ionospheric dynamo region.

GNSS radio occultation
Radio occultation is a remote sensing technique where a GNSS signal tangentially scrapes the Earth, passing through the atmosphere, and is received by a Low Earth Orbit (LEO) satellite. As the signal passes through the atmosphere, it is refracted, curved and delayed. An LEO satellite samples the total electron content and bending angle of many such signal paths as it watches the GNSS satellite rise or set behind the Earth. Using an Inverse Abel's transform, a radial profile of refractivity at that tangent point on earth can be reconstructed.

Major GNSS radio occultation missions include the GRACE, CHAMP, and COSMIC.

Indices of the ionosphere
In empirical models of the ionosphere such as Nequick, the following indices are used as indirect indicators of the state of the ionosphere.

Solar intensity
F10.7 and R12 are two indices commonly used in ionospheric modelling. Both are valuable for their long historical records covering multiple solar cycles. F10.7 is a measurement of the intensity of solar radio emissions at a frequency of 2800 MHz made using a ground radio telescope. R12 is a 12 months average of daily sunspot numbers. The two indices have been shown to be correlated with each other.

However, both indices are only indirect indicators of solar ultraviolet and X-ray emissions, which are primarily responsible for causing ionization in the Earth's upper atmosphere. We now have data from the GOES spacecraft that measures the background X-ray flux from the Sun, a parameter more closely related to the ionization levels in the ionosphere.

Geomagnetic disturbances

 * The A- and K-indices are a measurement of the behavior of the horizontal component of the geomagnetic field. The K-index uses a semi-logarithmic scale from 0 to 9 to measure the strength of the horizontal component of the geomagnetic field. The Boulder K-index is measured at the Boulder Geomagnetic Observatory.
 * The geomagnetic activity levels of the Earth are measured by the fluctuation of the Earth's magnetic field in SI units called teslas (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).

Ionospheres of other planets and natural satellites
Objects in the Solar System that have appreciable atmospheres (i.e., all of the major planets and many of the larger natural satellites) generally produce ionospheres. Planets known to have ionospheres include Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

The atmosphere of Titan includes an ionosphere that ranges from about 880 to 1300 km in altitude and contains carbon compounds. Ionospheres have also been observed at Io, Europa, Ganymede, and Triton.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Ionosphere


text:
Interlingua

Interlingua is an international auxiliary language (IAL) developed between 1937 and 1951 by the American International Auxiliary Language Association (IALA). It is a constructed language of the "naturalistic" variety, whose vocabulary, grammar, and other characteristics are derived from natural languages. Interlingua literature maintains that (written) Interlingua is comprehensible to the hundreds of millions of people who speak Romance languages, though it is actively spoken by only a few hundred.

Interlingua was developed to combine a simple, mostly regular grammar with a vocabulary common to a wide range of western European languages, making it easy to learn for those whose native languages were sources of Interlingua's vocabulary and grammar.

The name Interlingua comes from the Latin words ', meaning 'between', and ', meaning 'tongue' or 'language'. These morphemes are the same in Interlingua; thus, Interlingua would mean 'between language'.

Overview
Interlingua focuses on common vocabulary shared by Western European languages, which are often descended from or heavily influenced by the Latin language (such as the Romance languages) and Greek language. Interlingua organizers have four "primary control languages" where, by default, a word (or variant thereof) is expected to appear in at least three of them to qualify for inclusion in Interlingua. These are English; French; Italian; and a combination of Spanish and Portuguese which are treated as a single mega-language for Interlingua purposes, as both are west Iberian languages. Additionally, German and Russian have been dubbed "secondary control languages". While the result is often akin to Neo-Latin as the most frequent source of commonality, Interlingua words can have origins in any language, as long as they have drifted into the primary control languages as loanwords. For example, the Japanese words geisha and samurai are used as-is in most Western European languages, and are in Interlingua as well; the same with the likes of Guugu Yimithirr gangurru (Interlingua: kanguru, English: kangaroo) or the Finnish sauna.

The maintainers of Interlingua attempt to keep the grammar simple and word formation regular, and use only a small number of roots and affixes. This is intended to make the language quicker to learn.

History
The American heiress Alice Vanderbilt Morris (1874–1950) became interested in linguistics and the international auxiliary language movement in the early 1920s. In 1924, Morris and her husband, Dave Hennen Morris, established the non-profit International Auxiliary Language Association (IALA) in New York City. Their aim was to place the study of IALs on a more complex and scientific basis. Morris developed the research program of IALA in consultation with Edward Sapir, William Edward Collinson, and Otto Jespersen.

Investigations of the auxiliary language problem were in progress at the International Research Council, the American Council on Education, the American Council of Learned Societies, the British, French, Italian, and American Associations for the advancement of science, and other groups of specialists. Morris created IALA as a continuation of this work.

International Auxiliary Language Association
The IALA became a major supporter of mainstream American linguistics. Numerous studies by Sapir, Collinson, and Morris Swadesh in the 1930s and 1940s, for example, were funded by IALA. Alice Morris edited several of these studies and provided much of IALA's financial support. For example, Morris herself edited Sapir and Morris Swadesh's 1932 cross-linguistic study of ending-point phenomena, and Collinson's 1937 study of indication. IALA also received support from groups such as the Carnegie Corporation, the Ford Foundation, the Research Corporation, and the Rockefeller Foundation.

In its early years, IALA concerned itself with three tasks: finding other organizations around the world with similar goals; building a library of books about languages and interlinguistics; and comparing extant IALs, including Esperanto, Esperanto II, Ido, Peano's Interlingua (Latino sine flexione), Novial, and Interlingue (Occidental). In pursuit of the last goal, it conducted parallel studies of these languages, with comparative studies of national languages.

At the Second International Interlanguage Congress, held in Geneva in 1931, IALA began to break new ground; 27 recognized linguists signed a testimonial of support for IALA's research program. An additional eight added their signatures at the third congress, convened in Rome in 1933. That same year, Herbert N. Shenton and Edward Thorndike became influential in IALA's work by authoring studies in the interlinguistic field.

The first steps towards the finalization of Interlingua were taken in 1937, when a committee of 24 linguists from 19 universities published Some Criteria for an International Language and Commentary. However, the outbreak of World War II in 1939 cut short the intended biannual meetings of the committee.

Development of a new language
Originally, the association had not intended to create its own language. Its goal was to identify which auxiliary language already available was best suited for international communication, and how to promote it more effectively. However, after ten years of research, many members of IALA concluded that none of the existing interlanguages were up to the task. By 1937, the members had made the decision to create a new language, to the surprise of the world's interlanguage community.

To that point, much of the debate had been equivocal on the decision to use naturalistic (e.g., Peano's Interlingua, Novial and Occidental) or systematic (e.g., Esperanto and Ido) words. During the war years, proponents of a naturalistic interlanguage won out. The first support was Thorndike's paper; the second was a concession by proponents of the systematic languages that thousands of words were already present in many, or even a majority, of the European languages. Their argument was that systematic derivation of words was a Procrustean bed, forcing the learner to unlearn and re-memorize a new derivation scheme when a usable vocabulary was already available. IALA from that point assumed the position that a naturalistic language would be best.

IALA's research activities were based in Liverpool, before relocating to New York due to the outbreak of World War II, where E. Clark Stillman established a new research staff. Stillman, with the assistance of Alexander Gode, constructed the methodology for selecting Interlingua vocabulary based on a comparison of control languages.

In 1943 Stillman left for war work and Gode became Acting Director of Research. IALA began to develop models of the proposed language, the first of which were presented in Morris's General Report in 1945.

The four models
From 1946 to 1948, French linguist André Martinet was Director of Research. During this period IALA continued to develop models and conducted polling to determine the optimal form of the final language. In 1946, IALA sent an extensive survey to more than 3,000 language teachers and related professionals on three continents.

Model P was unchanged from 1945; Model M was relatively modern in comparison to more classical P. Model K was slightly modified in the direction of Ido. The resulting four models that were canvassed were:

An example sentence:

The vote total ended up as follows: P 26.6%, M 37.5%, C 20%, and K 15%. The two more schematic models, C and K, were rejected. Of the two naturalistic models, M attracted somewhat more support than P. Taking national biases into account (for example, the French who were polled disproportionately favored Model M), IALA decided on a compromise between models M and P, with certain elements of C.

Finalization
The German-American Gode and the French Martinet did not get along. Martinet resigned and took up a position at Columbia University in 1948, and Gode took on the last phase of Interlingua's development. His task was to combine elements of Model M and Model P; take the flaws seen in both by the polled community and repair them with elements of Model C as needed; and develop a vocabulary. Alice Vanderbilt Morris died in 1950, and the funding that had sustained IALA ceased, but sufficient funds remained to publish a dictionary and grammar. The vocabulary and grammar of Interlingua were first presented in 1951, when IALA published the finalized Interlingua Grammar and the Interlingua–English Dictionary (IED). In 1954, IALA published an introductory manual entitled Interlingua a Prime Vista ("Interlingua at First Sight").

Interlingua as presented by the IALA is very close to Peano's Interlingua (Latino sine flexione), both in its grammar and especially in its vocabulary. A distinct abbreviation was adopted: IA instead of IL.

Interlingua's first decades
An early practical application of Interlingua was the scientific newsletter Spectroscopia Molecular, published from 1952 to 1980. In 1954, the Second World Cardiological Congress in Washington, D.C. released summaries of its talks in both English and Interlingua. Within a few years, it found similar use at nine further medical congresses. Between the mid-1950s and the late 1970s, some thirty scientific and medical journals provided article summaries in Interlingua. Gode wrote a monthly column in Interlingua in the Science Newsletter published by the Science Service from the early 1950s until his death in 1970.

IALA closed its doors in 1953 but was not formally dissolved until 1956 or later. Its role in promoting Interlingua was largely taken on by Science Service, which hired Gode as head of its newly formed Interlingua Division. Hugh E. Blair, Gode's close friend and colleague, became his assistant. A successor organization, the Interlingua Institute, was founded in 1970 to promote Interlingua in the US and Canada. The new institute supported the work of other linguistic organizations, made considerable scholarly contributions and produced Interlingua summaries for scholarly and medical publications. One of its largest achievements was two immense volumes on phytopathology produced by the American Phytopathological Society in 1976 and 1977.

Beginning in the 1980s, UMI has held international conferences every two years (typical attendance at the earlier meetings was 50 to 100) and launched a publishing programme that eventually produced over 100 volumes. Several Scandinavian schools undertook projects that used Interlingua as a means of teaching the international scientific and intellectual vocabulary.

In 2000, the Interlingua Institute was dissolved amid funding disputes with the UMI; the American Interlingua Society, established the following year, succeeded the institute.

Interlingua today
The original goal of an interlanguage meant for global events has faced competition from English as a lingua franca and International English in the 21st century. The scientific community frequently uses English in international conferences and publications, for example, rather than Interlingua. However, the rise of the Internet has made it easier for the general public with an interest in constructed languages to learn Interlingua. Interlingua is promoted internationally by the Union Mundial pro Interlingua. Periodicals and books are produced by national organizations, such as the Societate American pro Interlingua, the Svenska Sällskapet för Interlingua, and the Union Brazilian pro Interlingua.

Panorama In Interlingua is the most prominent of several Interlingua periodicals. It is a 28-page magazine published bimonthly that covers current events, science, editorials, and Interlingua.

Community
It is not certain how many people have an active knowledge of Interlingua. Most constructed languages other than Esperanto have very few speakers. The Hungarian census of 2001, which collected information about languages spoken, found just two people in the entire country who claimed to speak Interlingua.

Advocates say that Interlingua's greatest advantage is that it is the most widely understood international auxiliary language besides Interlingua (IL) de A.p.I. by virtue of its naturalistic (as opposed to schematic) grammar and vocabulary, allowing those familiar with a Romance language, and educated speakers of English, to read and understand it without prior study.

Interlingua web pages include editions of Wikipedia and Wiktionary, and a number of periodicals, including Panorama in Interlingua from the Union Mundial pro Interlingua (UMI).

Every two years, the UMI organizes an international conference in a different country. In the year between, the Scandinavian Interlingua societies co-organize a conference in Sweden, as a number of Interlingua speakers are in Scandinavia. National organizations such as the Union Brazilian pro Interlingua also organize regular conferences.

Interlingua is taught in some high schools and universities, sometimes as a means of teaching other languages quickly, presenting interlinguistics, or introducing an international vocabulary. A two-week course was taught at the University of Granada in Spain in 2007, for example.

, Google Keyboard supports Interlingua.

Orthography
Interlingua has a largely phonemic orthography.

Interlingua alphabet
Interlingua uses the 26 letters of the ISO basic Latin alphabet with no diacritics. The alphabet, pronunciation in IPA and letter names in Interlingua are:

Collateral orthography
The book Grammar of Interlingua defines in §15 a "collateral orthography" that defines how a word is spelt in Interlingua once assimilated regardless of etymology.

Phonology

Interlingua is primarily a written language, and the pronunciation is not entirely settled. The sounds in parentheses are not used by all speakers.

Pronunciation
For the most part, consonants are pronounced as in English, while the vowels are like Spanish. Written double consonants may be geminated as in Italian for extra clarity or pronounced as single as in English or French. Interlingua has five falling diphthongs,, and , although and  are rare.

Stress
The general rule is that stress falls on the vowel before the last consonant (e.g., lingua, 'language', esser, 'to be', requirimento, 'requirement') ignoring the final plural -(e)s (e.g. linguas, the plural of lingua, still has the same stress as the singular), and where that is not possible, on the first vowel (via, 'way', io crea, 'I create'). There are a few exceptions, and the following rules account for most of them:
 * Adjectives and nouns ending in a vowel followed by -le, -ne, or -re are stressed on the third-last syllable (fragile, margine, altere 'other', but illa impone 'she imposes').
 * Words ending in -ica/-ico, -ide/-ido and -ula/-ulo, are stressed on the third-last syllable (politica, scientifico, rapide, stupido, capitula, seculo 'century').
 * Words ending in -ic are stressed on the second-last syllable (cubic).

Speakers may pronounce all words according to the general rule mentioned above. For example, kilometro is acceptable, although kilometro is more common.

Phonotactics
Interlingua has no explicitly defined phonotactics. However, the prototyping procedure for determining Interlingua words, which strives for internationality, should in general lead naturally to words that are easy for most learners to pronounce. In the process of forming new words, an ending cannot always be added without a modification of some kind in between. A good example is the plural -s, which is always preceded by a vowel to prevent the occurrence of a hard-to-pronounce consonant cluster at the end. If the singular does not end in a vowel, the final -s becomes -es.

Loanwords
Unassimilated foreign loanwords, or borrowed words, are spelled as in their language of origin. Their spelling may contain diacritics, or accent marks. If the diacritics do not affect pronunciation, they are removed.

Vocabulary
Words in Interlingua may be taken from any language, as long as their internationality is verified by their presence in seven control languages: Spanish, Portuguese, Italian, French, and English, with German and Russian acting as secondary controls. These are the most widely spoken Romance, Germanic, and Slavic languages, respectively. Because of their close relationship, Spanish and Portuguese are treated as one unit. The largest number of Interlingua words are of Latin origin, with the Greek and Germanic languages providing the second and third largest number. The remainder of the vocabulary originates in Slavic and non-Indo-European languages.

Eligibility
A word, that is a form with meaning, is eligible for the Interlingua vocabulary if it is verified by at least three of the four primary control languages. Either secondary control language can substitute for a primary language. Any word of Indo-European origin found in a control language can contribute to the eligibility of an international word. In some cases, the archaic or potential presence of a word can contribute to its eligibility.

A word can be potentially present in a language when a derivative is present, but the word itself is not. English proximity, for example, gives support to Interlingua proxime, meaning 'near, close'. This counts as long as one or more control languages actually have this basic root word, which the Romance languages all do. Potentiality also occurs when a concept is represented as a compound or derivative in a control language, the morphemes that make it up are themselves international, and the combination adequately conveys the meaning of the larger word. An example is Italian fiammifero (lit. 'flamebearer'), meaning 'match, lucifer', which leads to Interlingua flammifero, or 'match'. This word is thus said to be potentially present in the other languages although they may represent the meaning with a single morpheme.

Words do not enter the Interlingua vocabulary solely because cognates exist in a sufficient number of languages. If their meanings have become different over time, they are considered different words for the purpose of Interlingua eligibility. If they still have one or more meanings in common, however, the word can enter Interlingua with this smaller set of meanings.

If this procedure did not produce an international word, the word for a concept was originally taken from Latin (see below). This only occurred with a few grammatical particles.

Form
The form of an Interlingua word is considered an international prototype with respect to the other words. On the one hand, it should be neutral, free from characteristics peculiar to one language. On the other hand, it should maximally capture the characteristics common to all contributing languages. As a result, it can be transformed into any of the contributing variants using only these language-specific characteristics. If the word has any derivatives that occur in the source languages with appropriate parallel meanings, then their morphological connection must remain intact; for example, the Interlingua word for 'time' is spelled tempore and not *tempus or *tempo in order to match it with its derived adjectives, such as temporal.

The language-specific characteristics are closely related to the sound laws of the individual languages; the resulting words are often close or even identical to the most recent form common to the contributing words. This sometimes corresponds with that of Vulgar Latin. At other times, it is much more recent or even contemporary. It is never older than the classical period.

An illustration
The French œil, Italian occhio, Spanish ojo, and Portuguese olho appear quite different, but they descend from a historical form oculus. German Auge, Dutch oog and English eye (cf. Czech and Polish oko, Russian and Ukrainian око ) are related to this form in that all three descend from Proto-Indo-European . In addition, international derivatives like ocular and oculista occur in all of Interlingua's control languages. Each of these forms contributes to the eligibility of the Interlingua word. German and English base words do not influence the form of the Interlingua word, because their Indo-European connection is considered too remote. Instead, the remaining base words and especially the derivatives determine the form oculo found in Interlingua.

Free word-building
Words can also be included in Interlingua by deriving them using Interlingua words and affixes; a method called free word-building. Thus, in the Interlingua–English Dictionary (IED), Alexander Gode followed the principle that every word listed is accompanied by all of its clear compounds and derivatives, along with the word or words it is derived from. A reader skimming through the IED notices many entries followed by large groups of derived and compound words. A good example is the Interlingua word nation, which is followed by national, nationalismo, nationalista, nationalitate, nationalisar, international, internationalitate, and many other words.

Other words in the IED do not have derivatives listed. Gode saw these words as potential word families. Although all derived words in the IED are found in at least one control language, speakers may make free use of Interlingua roots and affixes. For example, jada ('jade') can be used to form jadificar, ('to jadify, make into jade, make look like jade'), jadification, and so on. These word forms would be impermissible in English but would be good Interlingua.

Word-building by analogy
Gode and Hugh E. Blair explained in the Interlingua Grammar that the basic principle of practical word-building is analogical. If a pattern can be found in the existing international vocabulary, new words can be formed according to that pattern. A meaning of the suffix -ista is 'person who practices the art or science of....' This suffix allows the derivation of biologista from biologia, physicista from physica, and so on. An Interlingua speaker can freely form saxophonista from saxophone and radiographista from radiographia by following the same pattern.

Usefulness and clarity
As noted above, the only limits to free word-building in Interlingua are clarity and usefulness. These concepts are touched upon here:

Any number of words could be formed by stringing roots and affixes together, but some would be more useful than others. For example, the English word rainer means 'a person who rains', but most people would be surprised that it is included in English dictionaries. The corresponding Interlingua word pluviator is unlikely to appear in a dictionary because of its lack of utility. Interlingua, like any traditional language, could build up large numbers of these words, but this would be undesirable.

Gode stressed the principle of clarity in free word-building. As Gode noted, the noun marinero ('mariner') can be formed from the adjective marin, because its meaning is clear. The noun marina meaning 'navy' cannot be formed, because its meaning would not be clear from the adjective and suffix that gave rise to it.

Grammar
Interlingua has been developed to omit any grammatical feature that is absent from any one primary control language. Thus, Interlingua has no noun–adjective agreement by gender, case, or number (cf. Spanish and Portuguese gatas negras or Italian gatte nere, 'black female cats'), because this is absent from English, and it has no progressive verb tenses (English I am reading), because they are absent from French. Conversely, Interlingua distinguishes singular nouns from plural nouns because all the control languages do. With respect to the secondary control languages, Interlingua has articles, unlike Russian.

The definite article le is invariable, as in English ("the"). Nouns have no grammatical gender. Plurals are formed by adding -s, or -es after a final consonant. Personal pronouns take one form for the subject and one for the direct object and reflexive. In the third person, the reflexive is always se. Most adverbs are derived regularly from adjectives by adding -mente, or -amente after a -c. An adverb can be formed from any adjective in this way.

Verbs take the same form for all persons (io vive, tu vive, illa vive, 'I live', 'you live', 'she lives'). The indicative (pare, 'appear', 'appears') is the same as the imperative (pare! 'appear!'), and there is no subjunctive. Three common verbs usually take short forms in the present tense: es for 'is', 'am', 'are;' ha for 'has', 'have;' and va for 'go', 'goes'. A few irregular verb forms are available, but rarely used.

There are four simple tenses (present, past, future, and conditional), three compound tenses (past, future, and conditional), and the passive voice. The compound structures employ an auxiliary plus the infinitive or the past participle (e.g., Ille ha arrivate, 'He has arrived'). Simple and compound tenses can be combined in various ways to express more complex tenses (e.g., Nos haberea morite, 'We would have died').

Word order is subject–verb–object, except that a direct object pronoun or reflexive pronoun comes before the verb (io les vide, 'I see them'). Adjectives may precede or follow the nouns they modify, but they most often follow it. The position of adverbs is flexible, though constrained by common sense.

The grammar of Interlingua has been described as similar to that of the Romance languages, but simplified, primarily under the influence of English. A 1991 paper argued that Interlingua's grammar was similar to the simple grammars of Japanese and particularly Chinese.

F. P. Gopsill has written that Interlingua has no irregularities, although Gode's Interlingua Grammar suggests that Interlingua has a small number of irregularities.

Reception
One criticism that applies to naturalistic constructed languages in general is that if an educated traveler is willing to learn a naturalistic conlang, they may find it even more useful to learn a natural language outright, such as International English. Planned conlangs at least hold out the promise of "fixing" or standardizing certain irregular aspects of natural languages and providing unique advantages, despite the lack of speakers, but naturalistic conlangs have to compete with the natural languages they are based on. In practice, conferences with international attendance tend to be held in a natural language popular among the attendees rather than an international auxiliary language.

Samples
From an essay by Alexander Gode:

 * Interlingua se ha distacate ab le movimento pro le disveloppamento e le introduction de un lingua universal pro tote le humanitate. Si on non crede que un lingua pro tote le humanitate es possibile, si on non crede que le interlingua va devenir un tal lingua, es totalmente indifferente ab le puncto de vista de interlingua mesme. Le sol facto que importa (ab le puncto de vista del interlingua ipse) es que le interlingua, gratias a su ambition de reflecter le homogeneitate cultural e ergo linguistic del occidente, es capace de render servicios tangibile a iste precise momento del historia del mundo. Il es per su contributiones actual e non per le promissas de su adherentes que le interlingua vole esser judicate.

 * Interlingua has detached itself from the movement for the development and introduction of a universal language for all humanity. Whether or not one believes that a language for all humanity is possible, whether or not one believes that Interlingua will become such a language is totally irrelevant from the point of view of Interlingua itself. The only fact that matters (from the point of view of Interlingua itself) is that Interlingua, thanks to its ambition of reflecting the cultural and thus linguistic homogeneity of the West, is capable of rendering tangible services at this precise moment in the history of the world. It is by its present contributions and not by the promises of its adherents that Interlingua wishes to be judged.

Flags and symbols
As with Esperanto, there have been proposals for a flag of Interlingua; the proposal by Czech translator Karel Podrazil is recognized by multilingual sites. It consists of a white four-pointed star extending to the edges of the flag and dividing it into an upper blue and lower red half. The star is symbolic of the four cardinal directions, and the two halves symbolize Romance and non-Romance speakers of Interlingua who understand each other.

Another symbol of Interlingua is the Blue Marble surrounded by twelve stars on a black or blue background, echoing the twelve stars of the Flag of Europe (because the source languages of Interlingua are purely European).

title: Interlingua