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ADC
Analogue to digital converter.

Absorption
The process by which energy in a radio wave passing through the ionosphere is converted to heat through electron collisions with other particles. For applications using frequencies above the VLF range, absorption predominately occurs in the D region. The amount of absorbed energy is normally expressed as a ratio of the expected level to the measured level, and is given in decibels (dB).

Aurora
A faint visual phenomenon associated with geomagnetic activity, occurring mainly in the high-latitude night sky. Typical auroras occur 100 to 250 km above the ground as high speed particles from the solar wind collide with atmospheric gasses at these altitudes. When observed in the northern hemisphere this phenomena is known as the Aurora Borealis (northern lights), and when viewed in the southern hemisphere it is the Aurora Australis.

Auroral Oval
An oval band situated between magnetic latitudes of 64 and 70° where the visible aurora occurs overhead. During disturbed geomagnetic conditions, the auroral oval generally expands equatorward. Areas within the auroral ovals are called the polar cap.

Beam Forming Network
Each port of the BFN corresponds to one lobe of the given MBA. Impressing a signal to one of the n ports produces a beam pointing in a direction unique to that port. MBA systems use both variable and fixed power dividing or combining BFNs. With the latter, the radiation pattern shape is determined during manufacture of the antenna; a variable BFN permits the radiation pattern shape to meet changing operational requirements for which the antenna is designed. IRIS uses a Butler Matrix (Butler Beam Forming Matrix, BBFM) to form 49 pencil shaped lobes.

BFN
Beam Forming Network.

Butler matrix
Beam forming network that produces multiple narrow lobes from an array of antennas.

Crossed-dipole
An antenna consisting of 2 half-wavelength dipoles, intersecting at 90°.

Crossed-dipole photograph
Crossed dipole (click to enlarge).


D Region
A daytime layer of the Earth's ionosphere approximately 50 to 90 km in altitude. The D layer is effective as a reflector only for frequencies below VLF. It is the primary cause of absorption for signals in the HF band.

Directivity for Antenna Arrays
The calculation of the antenna directivity, or voltage reception pattern, for an array of antenna elements is a straightforward superposition of the field amplitudes in the radiation zone. If identical antennas are used, with uniform spacing and orientation over the array, the calculation of the array factors is independent of the antenna type, and the total directivity becomes the product of the directivity of a single antenna, with the factors representing the contributions from the geometrical superposition of the antenna signals.

Distributed Array
A distributed array is an array of antenna elements where each element is connected to its own receiver/transmitter unit.

E Region
A daytime layer of the Earth's ionosphere roughly between the altitudes of 85 and 140 km.

F Region
The upper layer of the ionosphere, approximately 120 to 1500 km in altitude.

Field of View
When talking about Multiple Beam Antennas: The n beams produced by exciting each port of the BFN individually span the field of view.

High Frequency (HF)
The portion of the radio frequency spectrum between 3 and 30 MHz. This frequency range, characterized by wavelengths between 10 and 100  meters, is commonly called the Short Wave band.

Imaging or narrow beams
Beams generated from the 8 × 8 array of crossed dipoles.

Imaging Riometer
A single riometer station which measures the absorption not only in zenith, but in some angular interval. This is done by using an antenna with many narrow lobes. All the measurements together can be displayed in an image showing the current absorption not only in one point of the celestial sphere but in a whole region, for example inside a region of 200 × 200 km.

Ionosphere
The region of the Earth's upper atmosphere containing a small percentage of free electrons and ions produced by photoionization of the constituents of the atmosphere by solar ultraviolet radiation at very short wavelengths (<100 nm). The ionosphere significantly influences radiowave propagation of frequencies less than about 30 MHz. See also our Introduction to the Ionosphere

IRIS
Imaging Riometer for Ionospheric Studies.

Isotropic radiator
An antenna which behaves identically in all directions.

MBA
Multiple Beam Antenna

Multiple Beam Antenna
Multiple antennas whose outputs are combined by a beam-forming network (BFN).

NaN
The IEEE Standard 754 defines a class of numbers known as NaN, or Not a Number. This value is used by the IRIS processing software to indicate missing data.

Obliquity factor
The increase in path length through the ionosphere that an oblique ray takes relative to a vertical ray.

Planar Array Antenna
N antennas distributed evenly to form an array with N elements. The antennas are combined by a Beam Forming Network to produce N beams which span the field of view (FOV). All elements of the array are identical, and each element covers the entire FOV. Hence the N beams span that space approximately defined by the half-power beamwidth of the radiation pattern of an element in the array. With a phased array, it is possible to produce multiple, truly simultaneous beams.

Polar Cap
The area of the ionosphere located within the auroral oval, either north or south. Plasma is convected across the polar cap from dayside to nightside by electric fields in the magnetosphere.

Polar Cap Absorption (PCA)
An anomalous condition of the polar ionosphere whereby HF and VHF radiowaves are absorbed, and LF and VLF radiowaves are reflected at lower altitudes than normal. In practice, the absorption is inferred from the proton flux at energies greater than 10 MeV, so that PCAs and proton events are simultaneous. Transpolar radio paths may still be disturbed for days, up to weeks, following the end of a proton event.

Quiet-day curve (QDC)
The quiet-day curve shows the background cosmic noise level on a day without absorption, solar radio emissions etc. (i.e., a quiet day). The QDC varies with sidereal time as the background cosmic noise is not constant in all directions. An empirical method is used to generate the QDCs.

RADAR
A Radio Detection And Ranging system used to detect the presence and location of objects by the transmission and return of an electromagnetic signal.

Riometer
Relative ionospheric opacity meter. Measures the intensity of cosmic radio noise at the surface of the Earth. Usually operates at about 20 to 50 MHz. IRIS operates at 38.2 MHz. Radiation of that wavelength is absorbed in the Earth's D region of the ionosphere. The frequency of 38.2 MHz is reserved for research.

S4 index
A method of quantifying the amount of scintillation present.

Scintillation
Signal variation due to naturally-ocurring (or sometimes man-made) irregularities in the ionopshere. The effect is the same as the visible twinkling of stars due to variations in the atmosphere. Scintillation is readily identified in the data. Scintillation is most easily explained by analogy to a diffraction grating. Ionospheric irregularities are the equivalent of the diffraction grating. The received signal is then the sum of signal from multiple paths. As the path lengths change the signal varies due to constructive and destructive interference, hence the star twinkles. In other words, scintillation is variations in amplitude, phase, polarization and angle of arrival of a radio wave upon passing through the ionosphere, such as would occur with a satellite signal. Scintillations can be severe (especially near the equator) and present problems to communication services.

Sensitivity
When talking about riometer experiments, sensitivity refers to the minimum change in sky temperature (measured in Kelvin (K)) that can be detected by the given riometer system. It is also important how big the area is that is affected by the temperature change. Generally, the requirements the antenna system has to fulfil in order to achieve the required spatial resolution are harder to fulfil than the requirements for a reasonable sensitivity. This means that, in general, one need not worry about the sensitivity being too low.

Sidereal time
Time measured with respect to the stars as opposed to the sun. 1 mean sidereal day = 23h 56m 4.09053s of mean solar time. (The Astronomical Almanac 1996 p. B6)

Universal Time (UT)
Universal time is approximately the same as Greenwich Mean Time (GMT). Like GMT UT does not include daylight saving changes. The US Naval Observatory has more information about universal time.

Very High Frequency (VHF)
The portion of the radio frequency spectrum from 30 to 300 MHz.

Very Low Frequency (VLF)
The portion of the radio frequency spectrum from 3 to 30 kHz.

Widebeam
The widebeam antenna is a single crossed-dipole (or other broad beam) antenna. It is not part of the imaging antenna array so it provides no information about the location of any absorption. It is useful for providing an overview of any ionospheric activity.


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