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ionospheric refraction

ionospheric polarization In an ionized medium, such as the ionosphere, in the presence of a magnetic field, a plane-polarized radio wave is split into two characteristic waves. In the ionosphere these are circularly polarized waves, each propagating independently. The wave that most closely approximates wave propagation in the absence of the magnetic field is called the ordinary (o) wave and the other is the extraordinary

(x) wave. For the ordinary (extraordinary) wave propagating in the direction of the magnetic field the electric vector rotates in the opposite (same) direction to that of an electron gyrating in the field and for propagation perpendicular to the magnetic field, the electric vector oscillates parallel (perpendicular) to the magnetic field. The partitioning of the energy between the two components depends on the angle the radio wave makes with the Earth’s magnetic field. At low frequencies, the extraordinary wave is heavily attenuated relative to the ordinary wave.

ionospheric propagation mode A single ray path between the transmitter and receiver. An ionospheric propagation path may comprise several different individual paths, all of which can propagate the signal from the transmitter to the receiver. The mode is labeled in terms of the number of reflections it experiences for each of the layers from which it is reflected. The simplest ionospheric mode is a single hop path having one reflection in the ionosphere (e.g., 1E, 1F). More complex modes can have several reflections from a layer (e.g., 2F) and may also have reflections from more than one layer (e.g., 2Es1F). See ionospheric radio propagation path.

ionospheric radio propagation path The radio propagation path may describe any path traversed between two or more terminals by a radio wave carrying a signal from one terminal to the other(s). The terminals of a propagation path are the points from which a signal is transmitted or received. The terms “radio link”, “radio circuit”, and “circuit” are used interchangeably to describe the path along which information from a transmitter is passed to a receiver. Thus, a broadcast to an area may comprise many individual circuits. When the radio propagation path includes the ionosphere, then the properties of the ionosphere can often dom-

inate the design and management of the link. A simple ionospheric link may have a single reflection from the ionosphere and the ionospheric properties at the mid-point of the link are important. Likewise, for an Earth-satellite path it is the location of the sub-ionospheric point, or pierce point, where the Earth-to-satellite path passes through the ionosphere, that is important. Radio waves propagated by the ionosphere are called HF sky waves, or just sky waves. Because of the nature of the antennas used, and the ionosphere, several paths may be possible simultaneously. Each path is called a propagation mode. A mode may contain one or more ionospheric reflections. Thus, a one-hop mode will have one ionospheric reflection. One level of link management uses prediction programs to decide on the state of the ionosphere to determine optimum operational conditions for an HF system. Many of these programs treat the propagation mode in terms of control points and the ionospheric properties at these points are used to determine the basic MUF of the layer. The control point is the point in the ionosphere where the radio path is reflected, or refracted, back to the Earth’s surface. A one-hop mode will have one control point, at the center of the great circle path joining the transmitter and receiver sites. Each mode, and the whole path, may be described in terms of the maximum and minimum frequencies that can be used. Several terms are used to describe this. Among them are the LUF, MUF, OWF, and MOF. See atmospheric noise, basic MUF, diversity reception, frequency of optimum traffic, galactic noise, ionospheric propa- gation mode, Lowest Useable Frequency, Max- imum Observable Frequency, operational MUF, Optimum Working Frequency, radio frequency interference, radio frequency spectrum, skip fading.

ionospheric refraction When an obliquely propagating radio wave enters the ionosphere it encounters a medium where the refractive index decreases as the electron density increases. This causes the wave to bend away from the vertical, refracting it back towards the direction from which it has come. This process continues as the wave penetrates further into the ionosphere until the refracted angle is 90◦ and the wave has penetrated to its deepest point. It is

© 2001 by CRC Press LLC

ionospheric regions

subsequently refracted towards the vertical as it leaves the ionosphere. The amount of refraction depends on the electron density of the ionosphere and the operating frequency. Although oblique radio waves are actually refracted in the ionosphere, it is often referred to as being reflected from the ionosphere. The latter is common in connection with simple mirror models of the ionosphere used for estimating HF system parameters (e.g., ASAPS, IONCAP) or when referring to ionospheric propagation modes. See ionospheric radio propagation path.

ionospheric regions The ionized part of the Earth’s upper atmosphere above about 70 km is known as the ionosphere consisting of overlapping ionized layers. These layers are produced by the action of solar electromagnetic radiation (ultraviolet and X-rays) and cosmic rays on the neutral particles. As these regions are ionized, the electrons affect the radio frequency propagation between any points on Earth or from outer space to the Earth. The number density of positive ions equals the electron density to keep charge neutrality at all heights. The layers have an altitude of maximum, above and below which the ionization drops off. The altitude profiles of ionization are functions of solar activity, time of day, latitude, season, and extra-terrestial events such as magnetospheric storms, solar wind, interplanetary magnetic field and energetic particles, and solar flares to name a few.

The D region is situated normally around 85 km and present only during the day. The E region peak is situated around 110 km. The D and E region consist mainly of NO+, O+2 ions and electrons. The F region consists of two regions, F1 and F2. The F1 layer is situated at a height of about 180 to 200 km and is absent at night. The F2 region peaks around 250 to 300 km. The F region consists mainly of O+, NO+, N+ and electrons. The region above F2 is called the topside ionosphere. As we go higher in the topside ionosphere, we encounter the region called “heliosphere” above about 400 km, where helium ions are predominant. The region above about 600 km is called as “protonsphere”, where atomic hydrogen ions are predominant. At higher latitudes, H+ and He+ may escape along field lines as “polar wind” into the magnetosphere. At low and middle latitudes, the

ions are trapped along inclined magnetic lines of force and do not escape. This region is called the “plasmasphere”. The outer edge of the plasmasphere where the magnetic line of force is open to the magnetosphere is called “plasmapause” and is located along the line of force mapping down to about 60◦ magnetic latitude.

ionospheric sounder Usually, a radio transmitter on the ground probing the electron density distribution of ionosphere below the density maximum. It is based on the tendency of a plasma to reflect radio signals below its plasma frequency. The sounder sends out a series of signals of increasing frequency, and measures the delay at which their echoes return: each delay gives the lowest altitude at which the electron density sufficed to reflect the signal.

Based on ionospheric soundings, the socalled D, E, and F layers of the ionosphere were identified long ago, the latter resolved into the F1 and F2 layers. Above the F-layer the ion density slowly decreases again, and its density profile there was first probed in 1962 by the orbiting ionospheric sounder aboard the Canadian satellite Alouette 1. That plasma ends at the plasmapause.

ionospheric sounding The process of obtaining ionospheric information using a radio sounding technique. This usually refers to operating an ionosonde.

ionospheric storm A global disturbance in the F region of the ionosphere, which occurs in connection with geomagnetic activity and especially a geomagnetic storm. The high latitude atmosphere is heated during geomagnetic activity, causing neutral winds to blow towards the equator. These winds substantially alter the chemistry of the atmosphere and thereby alter the global ionosphere. During the first few hours of a storm, the positive phase, the daytime F- region ionization can increase, sometimes significantly. It is generally attributed to the action of winds forcing ionization up the magnetic field lines and is most apparent in the middle latitudes. Generally, this phase ceases after sunset and is often followed by the negative phase, when a decrease in F-region ionization occurs and sometimes lasts a few days. Individ-

© 2001 by CRC Press LLC

IRIS (Incorporated Research Institutions for Seismology)

ual storms can vary, and their behavior depends on geomagnetic latitude, season, and local time. Storms affect higher geomagnetic latitudes to a greater extent and are more prevalent in the summer hemisphere. Severe storms may also affect the lower F region. Occasionally, the upper F-region ionization (foF2) can drop below the lower F-region ionization (foF1). In these circumstances, the lower F1 region will support HF propagation instead of the F2 region. See geomagnetic storm, ionosphere.

ionospheric variability The average ionospheric behavior is estimated by monthly medians of the hourly observations of the key ionospheric layer parameters (e.g., foF2). Diurnal, seasonal, and solar cycle dependencies for locations around the globe have been tabulated and modeled empirically and, possibly with less success, physically. However, there is also a large residual ionospheric variability in the hourly observations about the medians. Ionospheric variability is described by an accumulative statistic, often upper and lower quartiles, and is summed over many different physical processes. As an example, the F-region peak electron density, measured by foF2, may vary because of changes in several processes, some of which are solar ionizing radiation, geomagnetic activity including geomagnetic storms and the resultant ionospheric storms, and the effects of traveling ionospheric disturbances. Each of these sources of variability has different latitudinal and temporal properties. Ionospheric variability can also be described by correlation coefficients that depend on separation, as well as geographic latitude, longitude, season, and time of day. Generally, estimates of variability at a remote location can be improved by 50% using the locally observed ionospheric information, provided separations are less than typically 250 to 500 km in latitude and 500 to 1000 km in longitude. Since these are gross statistics, they should be interpreted with caution. See ionosphere.

ion torus (1.) A thick accretion disk, whose existence has been suggested to explain the collimation of radio jets, and the low bolometric luminosity (compared to the mechanical energy needed to produce radio lobes) of powerful radio sources. An ion torus is a structure supported

by the pressure of the ions of very hot plasma, with kinetic energies as high as 100 MeV. It is expected to surround a supermassive black hole with steep walls creating a narrow funnel which collimates the highest energy radiation produced from inside the funnel and, if a magnetic field is present, is expected to collimate the radio jet itself.

(2.) Donut-shaped cloud of neutral and ionized gases (plasma) along the orbit of the Jovian satellite Io, maintained by volcanic eruptions on Io.

ion trap See Faraday Cup.

IRAF (Image Reduction and Analysis Facility) A freely available general-use suite of software that is extensively used in many areas of study in observational astronomy. It is distributed by the IRAF project based at the National Optical Astronomical Observatories (NOAO) in Tucson, Arizona. The software and information can be accessed through their web page at: http://iraf.noao.edu/.

IRAF is organized into “packages”, which contain collections of “tasks” (procedures). The packages tend to be organized according to projects (such as Hubble Space Telescope data) and/or types of analyses. Various packages include tasks for CCD reductions, astrometry, photometry, plotting, data management, etc.

Iris Seventh asteroid to be discovered, in 1847. Orbit: semimajor axis 2.3854 AU, eccentricity 0.2304, inclination to the ecliptic 5◦.52412, period 3.68 years.

IRIS (Incorporated Research Institutions for Seismology) A name of a research project propelled by Incorporated Research Institutions for Seismology. The project started in the 1980s, and globally deployed highly efficient longperiod seismographs such as STS. The purpose of the project is to propel research on Earth’s internal structure and source processes of large earthquakes, aiming at building up global seismic networks of next generation seismological observations. For more information, refer to http://www.iris.washington.edu/.

© 2001 by CRC Press LLC

iron Kα line

iron Kα line A spectral line at energy of approximately 6.4 keV from the transition between the L and K shells (i.e., the second innermost and the innermost atomic shells, corresponding to quantum number n = 2 and n = 1, respectively) of an iron atom. The iron fluorescence Kα line is a strong emission feature in the X-ray spectra of active galactic nuclei and of cataclysmic variables. It can be produced by recombination following photoionization in gas irradiated by an intense X-ray source, as in systems powered by accretion. An electron of the K-shell may also be removed by Compton scattering due to an X-ray photon. The line is excited collisionally in the hot gas of stellar flares, supernova remnants and in the intra-cluster medium in clusters of galaxies. Observation of the line has been made possible by X-ray space-borne observatories. The instruments on board the orbiting Japanese observatory ASCA have revealed, in the Kα line profile of several Seyfert galaxies, characteristic effects predicted for radiation coming from a gaseous, rotating disk at a few gravitational radii from a black hole. See accretion disk, Seyfert galaxies.

iron meteorite A meteorite consisting of solid nickel-iron alloy. Such an object must have formed in the core of a substantial planet which was then disrupted, presumably in a collision or close encounter with another object. Iron (or iron-nickel) meteorites are classified as Hexahedrites (less than 6.5% nickel); Octahedrites (6.5% to 13% nickel); and Ataxites with greater amounts of nickel.

irradiance The radiant power per unit area per unit wavelength interval [W m−2 nm−1].

1.[downward (upward) plane irradiance] The downward (upward) directed radiant power

per unit area onto an upward (downward) facing horizontal surface [W m−2 nm−1].

2.[downward (upward) scalar irradiance] The downward (upward) directed radiant power

per unit area onto a spherical collecting surface [W m−2 nm−1].

4.[irradiance ratio] The ratio of the upward plane irradiance to the downward plane irradiance.

5.[reflectance] irradiance ratio.

6.[scalar irradiance] The power per unit

area incident from all directions onto a spherical collecting surface [W m−2 nm−1]; it equals the downward scalar irradiance plus the upward scalar irradiance.

irregular galaxies Galaxies that lack a central bulge, azimuthal symmetry, and that most often show a rather patchy appearance. They are of blue color, have high neutral gas content, and show evidence of ongoing star formation. Irregular galaxies are often dwarf galaxies, on average smaller and less massive than spiral galaxies, with typical masses 108 solar masses. They also have lower rotational velocity, if any, and lower luminosity than spirals. Although a small fraction in large catalogs of galaxies, such as the Revised New General Catalog, irregular galaxies are thought to account for 1/2 to 1/3 of all galaxies in the local universe. They have been further subdivided into the Magellanic type (from the prototype galaxy, the Large Magellanic Cloud), and the amorphous type (or M82-type) galaxies. Magellanic irregulars have a patchy appearance due to clusters of hot stars in star-forming regions spread over the whole galaxy, while amorphous irregulars are smoother in appearance, with a single supergiant star-forming region at the center of the galaxy. This subdivision is now considered mainly of historical importance.

irregular waves Waves without a single, clearly defined period. Also referred to as random waves.

irribarren number A dimensionless param-

eter used to predict wave breaker type. Equal

√

to S/ Hb/Lo, where S is beach slope, Hb is a breaking wave height, and Lo is deepwater wavelength.

irrotational The property of a vector field

3.[net plane irradiance] The downward v (e.g., in fluid dynamics a velocity field) that

plane irradiance minus the upward plane irra-

its curl vanishes: × v = 0. Hence, v is the

diance [W m−2 nm−1].

gradient of a scalar function: v = φ.

© 2001 by CRC Press LLC

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