Jump Quickly to: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z |
a INDEX A 3-hourly "equivalent amplitude" index of local geomagnetic activity; "a" is related to the 3-hourly K INDEX according to the following scale: K 0 1 2 3 4 5 6 7 8 9 a 0 3 7 15 27 48 80 140 240 400 A INDEX A daily index of geomagnetic activity equal to the average of the eight 3-hourly "a" indices. 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. Absorption can further be described as DEVIATIVE or NONDEVIATIVE. ACTIVE For Geomagnetic activity, levels where Ap is greater than 15 but less than 29. For Solar activity, levels with at least one geophysical event or several larger radio events (10cm) per day (Class M Flares). ACTIVE PROMINENCE A PROMINENCE displaying material motion and changes in appearance over a few minutes of time. ACTIVE REGION (AR) A localized, transient volume of the solar atmosphere in which PLAGEs, SUNSPOTS, FACULAe, FLAREs, etc. may be observed. AFRED Abbreviation for the A INDEX for Fredericksburg. ALL SKY CAMERA A camera having a very wide field of view used to observe the overhead sky from horizon to horizon in studies of the visible aurora. ANGSTROM (Å) A unit of length = 10^-8 cm. Ten angstroms = 1 nanometer. Ap INDEX An averaged planetary A INDEX based on data from a set of specific stations. ASTRONOMICAL UNIT (AU) The mean Earth-sun distance, equal to 1.496 X 10^13 cm or 214.94 solar radii. 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. AURORAL OVAL An oval band situated between MAGNETIC LATITUDES of 64 and 70 degrees 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 CAPS. BARTEL'S ROTATION NUMBER The serial number assigned to 27-day rotation periods of solar and geophysical parameters. Rotation 1 in this sequence was assigned arbitrarily by Bartel to begin in January 1833. BURST A transient enhancement of the solar RADIO EMISSION, usually associated with an ACTIVE REGION or FLARE. CENTIMETER BURST A solar radio burst in the centimeter WAVELENGTH range. CHORDAL MODE An ionospheric propagation mode characterized by two successive Earthward reflections from an ionized layer without an intermediate ground reflection, usually caused by an ionospheric tilt. CHROMOSPHERE The layer of the solar atmosphere above the PHOTOSPHERE and beneath the transition region and the CORONA. CHROMOSPHERIC EVENTS A FLARE that is only a Chromospheric Event without Centimetric Bursts or Ionospheric Effects. (SID) (Class C flare) COMPREHENSIVE FLARE INDEX (CFI) An index indicative of solar flare importance given by the sum of the following five components: a) Importance of ionizing radiation as indicated by time-associated Short Wave Fade or Sudden Ionospheric Disturbance (SID); (Scale 0-3) b) Importance of H-Alpha flare; (Scale 0-3) c) Magnitude of 10cm flux; (Characteristic of log of flux in units of 10^-22 Watt/m2/Hz) d) Dynamic spectrum; (Type II = 1, Continuum = 2, Type IV with duration > 10 minutes = 3) e) Magnitude of 200MHz flux; (Characteristic of log of flux in units of 10^-22 Watt/m2/Hz) CONJUGATE POINTS Two points on the Earth's surface, at opposite ends of a geomagnetic field line. CONTINUUM STORM (CTM) General term for solar noise lasting for hours and sometimes days. COORDINATED UNIVERSAL TIME By international agreement, the local time at the prime meridian, which passes through Greenwich, England. Therefore, it is also known as GREENWICH MEAN TIME, or sometimes simply UNIVERSAL TIME (UT or UTC). CORONA The outermost layer of the solar atmosphere, characterized by low densities (<10^9/cc) and high temperatures (>10^6 degK). CORONAL HOLE An extended region of the CORONA, exceptionally low in density and associated with unipolar photospheric regions. CORONAL MASS EJECTION (CME) A major solar event in which a large amount of coronal mass (as much as 10^16 grams) is ejected from the sun at speeds of tens of km/sec up to 1000 km/sec. CMEs are thought to occur with a frequency of one per day. CORONAL TRANSIENTS A general term for short-time-scale changes in the CORONA, but principally used to describe outward-moving PLASMA clouds. COSMIC RAY An extremely energetic (relativistic) charged particle. CRITICAL FREQUENCY The frequency at which a radio wave just penetrates a given ionospheric layer. Higher frequencies pass through the layer; signals at lower frequencies are returned by reflection from the layer. CROCHET A sudden deviation in the H component of the sunlit geomagnetic field associated with large solar FLARE X-ray emission. 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. DEVIATIVE Type of ABSORPTION occurring wherever the ray path bends significantly such as near the top of a ray trajectory. Deviative absorption predominately occurs near a layer critical frequency. DIFFERENTIAL ROTATION The change in Solar rotation rate with latitude. Low latitudes rotate at a faster angular rate (approx. 14° per day) than do high latitudes (approx. 12° per day). DISAPPEARING SOLAR FILAMENT (DSF) The sudden (timescale of minutes to hours) disappearance of a solar FILAMENT (Also see PROMINENCE). DISK The visible surface of the sun (or any heavenly body) projected against the sky. Dst INDEX A geomagnetic index describing variations in the equatorial ringcurrent. ELECTROJET A region of sheet current flowing in the E Region of the ionosphere. At high latitudes, the auroral electrojet may flow east-west or west-east in an arc centered on the geomagnetic pole. An equatorial electrojet also exists. ELECTRON An elementary particle carrying a charge of negative electricity equal to 1.602 X 10^-19 coulomb and having a resting mass of about 9.107 X 10^-28 gram. E REGION A daytime layer of the Earth's ionosphere roughly between the altitudes of 85 and 140 km. ERUPTIVE Solar activity levels with at least one radio event (10cm) and several chromospheric events per day (Class C Flares) EXTRAORDINARY MODE (X-MODE)) A radio wave entering the ionosphere with right hand elliptical polarization in the northern magnetic hemisphere (or left hand in the southern hemisphere). Also see O - MODE. EXTREMELY LOW FREQUENCY (ELF) That portion of the radio frequency spectrum from 30 to 3000 hertz. EXTREME ULTRAVIOLET (EUV) A portion of the electromagnetic spectrum from approximately 100 to 1000 Å. F CORONA Of the white-light CORONA (that is, the corona seen by the eye at a total solar eclipse), that portion which is caused by sunlight scattered or reflected by solid particles (dust) in interplanetary space. F REGION (Also F LAYER). The upper layer of the IONOSPHERE, approximately 120 to 1500 km in altitude. The F region is subdivided into the F1 and F2 regions. The F2 region is the most dense and peaks at altitudes between 200 and 600 km. The F1 region is a smaller peak in electron density, which forms at lower altitudes in the daytime. FACULA A bright region of the PHOTOSPHERE seen in white light, seldom visible except near the solar LIMB. FILAMENT A mass of gas suspended over the PHOTOSPHERE by magnetic fields and seen as dark lines threaded over the solar DISK. A filament on the LIMB of the sun seen in emission against the dark sky is called a PROMINENCE. FLARE A sudden eruption of energy on the solar DISK lasting minutes to hours, from which radiation and particles are emitted. (See also X-RAY FLARE.) FLUTTER A common term describing rapid variations in amplitude and sometimes frequency (doppler) on signals that pass through the POLAR CAP. Flutter is thought to be a MULTIPATH effect produced by discrete patches of ionization moving rapidly through the polar cap region. FLUX The rate of flow of a physical quantitiy through a reference surface. Also see SOLAR FLUX. fMIN The lowest radiowave frequency that can be reflected from the IONOSPHERE. foEs The maximum ORDINARY MODE radiowave frequency capable of reflection from the SPORADIC E region of the IONOSPHERE. foF2 The maximum ORDINARY MODE radiowave frequency capable of reflection from the F2 REGION of the IONOSPHERE. FORBUSH DECREASE An abrupt decrease, of at least 10%, of the background galactic COSMIC RAY intensity as observed by neutron monitors. GAMMA A unit of magnetic field intensity equal to 10^-5 GAUSS, also equal to 1 NANOTESLA. GAMMA RAYS High energy radiation (energies in excess of 100 keV) observed during large, extremely energetic solar FLARES. GAUSS The unit of magnetic induction in the cgs (centimeter-gram-second) system. GEOMAGNETIC ELEMENTS The components of the geomagnetic field at the surface of the Earth expressed in gammas. The northward, eastward and vertical components are referred to as H, D and Z respectively. GEOMAGNETIC FIELD The magnetic field observed in and around the Earth. The intensity of the field at the Earth's surface is approximately 0.32 gauss at the equator and 0.62 gauss at the north pole. GEOMAGNETIC STORM A worldwide disturbance of the Earth's magnetic field, distinct from regular diurnal variations. Storms are described as follows: Minor Storm: A storm for which the Ap index was greater than 29 and less than 50. Major Storm: A storm for which the Ap index was greater than 49 and less than 100. Severe Storm: A storm for which the Ap index was 100 or more. Initial Phase: Of a geomagnetic storm, that period when there may be an increase of the MIDDLE-LATITUDE horizontal intensity (H). Main Phase: Of a geomagnetic storm, that period when the horizontal magnetic field at middle latitudes is generally decreasing. Recovery Phase: Of a geomagnetic storm, that period when the depressed northward field component returns to normal levels. GEOPHYSICAL EVENTS Flares (Importance two or larger) with Centimetric Outbursts (maximum of the flux higher than the Quiet Sun flux, duration longer 10 minutes) and/or strong SID. Sometimes these flares are followed by Geomagnetic Storms or small PCA. (Class M Flares) GMT Greenwich Mean Time. (See COORDINATED UNIVERSAL TIME.) GRADUAL COMMENCEMENT The commencement of a geomagnetic storm that has no well-defined onset. GREAT CIRCLE The intersection of the Earth's surface with a plane containing the center of the Earth and two points on its surface. A great circle is the shortest distance between those two points. Radio waves usually (but not always) follow great circle paths from transmitter to receiver. GROUND-LEVEL EVENT (GLE) A sharp increase in ground-level COSMIC RAY count to at least 10% above background, associated with solar protons of energies greater than 500 MeV. GLEs are relatively rare, occurring only a few times each SOLAR CYCLE. HAARP HAARP is an acronym for the "High frequency Active Auroral Research Program." H-ALPHA This ABSORPTION LINE of neutral hydrogen falls in the red part of the visible spectrum and is convenient for solar observations. The H-alpha line is universally used for patrol observations of solar flares. HIGH FREQUENCY (HF) That 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. HIGH LATITUDES With specific reference to zones of geomagnetic activity, "high latitudes" refers to 50 - 80 degrees geomagnetic. HIGH-SPEED STREAM A feature of the SOLAR WIND having velocities about double average solar wind values. HIPAS An acronym for High Power Auroral Stimulation. An active ionospheric research facility located near Fairbanks, AK and operated by UCLA. INCOHERENT SCATTER RADAR (ISR) A large, high power RADAR used to study the small scale structure of the overhead ionosphere by analyzing the backscatterred return signals. INTERPLANETARY MAGNETIC FIELD (IMF) The magnetic field carried with the SOLAR WIND. ION An atom that carries a positive or negative electric charge as a result of losing or gaining an electron, respectively. Also, a free ELECTRON. IONOGRAM A chart of ionospheric layer virtual height versus frequency obtained from an ionosonde. IONOSONDE A device used to measure the virtual height of ionospheric layers using a sweep-frequency, pulsed RADAR technique. 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 (<1000 Å). The ionosphere significantly influences radiowave propagation of frequencies less than about 30 MHz. (Also see the Ionosphere pages.) IONOSPHERIC IRREGULARITIES Small volumes within the ionosphere where the electron density deviates slightly from the average background level, producing a small scale non-homogeneous refractive index that can produce scattering of incident electromagnetic energy. IONOSPHERIC STORM A disturbance in the F REGION of the IONOSPHERE, which occurs in connection with geomagnetic activity. K CORONA Of the white-light CORONA (that is, the corona seen by the eye at a total solar eclipse), that portion which is caused by sunlight scattered by electrons in the hot outer atmosphere of the sun. K INDEX A 3-hourly quasi-logarithmic local index of geomagnetic activity relative to an assumed quiet-day curve for the recording site. Ranging from 0 to 9, the K index measures the deviation of the most disturbed horizontal magnetic field component. KELVIN A unit of absolute temperature. Kp INDEX A 3-hourly planetary geomagnetic index of activity generated in Gottingen, Germany, based on the K INDEX from 12 or 13 stations distributed around the world. LEADER SPOT In a magnetically bipolar or multipolar SUNSPOT group, the western part precedes and the main spot in that part is called the leader. LIDAR A LIDAR (LIght Detection And Ranging) instrument uses a low power Laser and sensitive optical detectors to study the chemical composition of a vertical column of the atmosphere. LIMB The edge of the solar DISK. LOOP PROMINENCE SYSTEM (LPS) A system of loop prominences associated with major FLARES. LOW FREQUENCY (LF) That portion of the radio frequency spectrum from 30 to 300 kHz. M 3000 The optimum HIGH FREQUENCY radio wave with a 3000 km range, which reflects only once from the IONOSPHERE (single hop transmission). MAGNETIC BAY A relatively smooth excursion of the H (horizontal) component of the geomagnetic field away from and returning to quiet levels. MAGNETIC DIP The angle between the horizontal plane and the magnetic field vector. By convention, downward angles are positive. Also called magnetic inclination. MAGNETIC EQUATOR The locus of geographic points where the magnetic dip is zero. At the magnetic equator, there is no vertical component of magnetic field strength. MAGNETIC LATITUDE The locus of points having identical magnetic dip angles. Analogous to geographic latitude except referenced to the geomagnetic equator. The magnetic dip (MD) is approximately related to the magnetic latitude (ML) by the equation tan(MD) = 2 * tan(ML) MAGNETIC TIME The time of day reckoned with respect to the Earth's magnetic (not geographic) pole. Magnetic noon is that time when the sun is highest on the GREAT CIRCLE containing the observer and the geomagnetic pole. MAGNETOGRAM Magnetograms are a graphic representation of magnetic field strengths and polarity. MAGNETOMETER A device that responds to changes in the Earth's magnetic field. Magnetometers are used to monitor variations in the field along three mutually orthogonal axes, magnetic north-south, east-west and vertically. MAGNETOPAUSE The boundary layer between the SOLAR WIND and the MAGNETOSPHERE. MAGNETOSPHERE The magnetic cavity surrounding the Earth, carved out of the passing SOLAR WIND by virtue of the GEOMAGNETIC FIELD, which prevents, or at least impedes, the direct entry of the SOLAR WIND PLASMA into the cavity. MAJORFLARE A FLARE significant enough to issue a forecast of geomagnetic storm, cosmic storm and/or protons in the Earth's vicinity. MAXIMUM USABLE FREQUENCY The highest frequency that can be used for ionospheric radio communication between two points on the Earth. MUF is a function of the time of day, season, solar conditions and geometry. MeV Mega (million) electronvolt. A unit of energy used to describe the total energy carried by a particle or photon. MEDIUM FREQUENCY (MF) That portion of the radio frequency spectrum from 0.3 to 3 MHz. MICROWAVE BURST A radiowave signal associated with optical and/or X-ray FLAREs. MIDDLE LATITUDES With specific reference to zones of geomagnetic activity, "middle latitudes" refers to 20 - 50 degrees geomagnetic. MIDLATITUDE The ionospheric region lying between the AURORAL OVAL and the equatorial band. MOUNT WILSON MAGNETIC CLASSIFICATIONS. Alpha. Denotes a unipolar SUNSPOT group. Beta. A sunspot group having both positive and negative magnetic polarities, with a simple and distinct division between the polarities. Beta-Gamma A sunspot group that is bipolar but in which no continuous line can be drawn separating spots of opposite polarities. Delta. A complex magnetic configuration of a solar sunspot group consisting of opposite polarity UMBRAe within the same PENUMBRA. Gamma. A complex ACTIVE REGION in which the positive and negative polarities are so irregularly distributed as to prevent classification as a bipolar group. MULTIPATH Term applied to propagation conditions where a signal may arrive at a receiving location through more than one geometric path. NANOTESLA (nT) A unit of magnetism 10^-9 tesla, equivalent to a gamma (10^-5 gauss). NEUTRAL GAS That portion of an IONOSPHERIC volume consisting only of non-ionized atoms and gas molecules. NONDEVIATIVE Absorption occurring along non-bending radio wave ray trajectories. This type of absorption is inversely proportional to the square of the radio frequency and (for HF and VHF waves) occurs in the D region. ORDINARY MODE The polarization of a radio wave such that the rotation of the electric field vector opposes the trajjectory of electrons about magnetic field lines. A radio wave entering the ionosphere with left hand elliptical polarization in the northern magnetic hemisphere. (Also see X-MODE) OZONE A molecule consisting of three Oxygen atoms. Ozone is created by ultraviolet radiation from the sun. PENUMBRA The SUNSPOT area that may surround the darker UMBRA or umbrae. It consists of linear bright and dark elements radial from the sunspot umbra. PHOTOSPHERE The lowest layer of the solar atmosphere; corresponds to the solar surface viewed in WHITE LIGHT. SUNSPOTs and FACULAe are observed in the photosphere. PICOTESLA (pT) A unit of magnetism 10^-12 tesla, equivalent to 0.001 gamma. PLAGE An extended emission feature of an ACTIVE REGION that exists from the emergence of the first magnetic flux until the widely scattered remnant magnetic fields merge with the background. PLASMA Any ionized gas, that is, any gas containing IONs and ELECTRONS. POLAR CAP The area of the ionosphere located within the AURORAL OVALS, both north and south. Plasma is convected across the polar cap from day to darkness 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. PROMINENCE A term identifying cloud-like features in the solar atmosphere. The features appear as bright structures in the CORONA above the solar LIMB and as dark FILAMENTs when seen projected against the solar DISK. PROTON An elementary particle with a mass equal to 1.672 X 10^-24 gram and carrying a charge of positive electricity equal to 1.602 X 10^-19 coulomb. A Hydrogen atom consists of one PROTON and one ELECTRON. PROTON EVENT By definition, the measurement of at least 10 protons/cm2/sec/steradian at energies greater than 10 MeV in the vicinity of the Earth. Protons having energies greater than 10 - 30 MeV can produce increased ionization in the D REGION resulting in a PCA event. PROTON FLARE Any FLARE producing significant FLUXes of greater than 10 MeV protons in the vicinity of the Earth. QUIET When applied to the geomagnetic field, a descriptive word specifically meaning geomagnetic levels such that Ap is less than 8. When applied to Solar activity, a descriptive word indicating levels with less than one CHROMOSPHERIC EVENT per day. RADIO EMISSION Emissions of the sun in radio WAVELENGTHS from centimeters to dekameters, under both quiet and disturbed conditions. Radio Emissions are described as follows: Type I A noise storm composed of many short, narrow-band bursts in the metric range (50 - 300 MHz). Type II Narrow-band emission that begins in the meter range (300 MHz) and sweeps slowly (tens of minutes) toward dekameter WAVELENGTHS (10 MHz). Type II emissions occur in loose association with major FLAREs and are indicative of a shock wave moving through the solar atmosphere. Type III Narrow-band bursts that sweep rapidly (seconds) from decimeter to dekameter wavelengths (500 - 0.5 MHz). They often occur in groups and are an occasional feature of complex solar ACTIVE REGIONs. Type IV A smooth continuum of broad-band bursts primarily in the range 30 - 300 MHz. These bursts are associated with some major FLARE events beginning 10 to 20 minutes after the flare maximum, and can last for hours. 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. RADIO EVENT Flares with Centimetric Bursts and/or definite Ionospheric Event (SID). RECOMBINATION The process by which free IONs and ELECTRONs recombine to form a NEUTRAL GAS. RECURRENCE Used especially in reference to the recurrence of physical parameters every 27 days (the rotation period of the sun). RIOMETER (Relative Ionospheric Opacity meter). A specially designed radio receiver for continuous monitoring of cosmic noise. The absorption of cosmic noise in the polar regions is very sensitive to the solar low-energy cosmic ray flux. Also see the HAARP Riometer page for a detailed description. SECTOR BOUNDARY In the SOLAR WIND, the area of demarcation between sectors, which are large-scale features distinguished by the predominant direction of the interplanetary magnetic field, toward or away from the sun. SCINTILLATION 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. SHORT WAVE FADE (SWF) A particular ionospheric solar FLARE effect under the broad category of sudden ionospheric disturbances (SIDs). It consists of a rapid decrease in signal strength for an HF radio signal that may last for several hours. The onset is usually abrupt. SMOOTHED SUNSPOT NUMBER An average of 13 monthly RI numbers, centered on the month of interest. SOLAR CYCLE The approximately 11-year quasi-periodic variation in frequency or number of solar active events such as sunspot number. SOLAR FLUX The daily solar radio noise flux measured at the frequency 2800 MHz (Wavelength=10.7 cm). The solar flux is one of the most commonly used indicators of the general level of solar activity because it is highly correlated with X-ray, EUV and UV fluxes and with many ionospheric and upper atmospheric parameters. Solar flux (SF) is empirically related to smoothed sunspot number (SSN) by the formula SF = 63.74 + 0.727*(SSN) + 0.000895*(SSN)^2 SOLAR MAXIMUM The month(s) during the SOLAR CYCLE when the 12-month mean of monthly average SUNSPOT numbers reaches a maximum. The most recent solar maximum occurred in July 1989. SOLAR MINIMUM The month(s) during the SOLAR CYCLE when the 12-month mean of monthly average SUNSPOT numbers reaches a minimum. The last solar minimum is believed to have occurred in October 1996. SOLAR WIND The outward flux of solar particles and magnetic fields from the sun. Typically, solar wind velocities are near 350 km/s. SPORADIC E A phenomenon occurring in the E REGION of the IONOSPHERE, which significantly affects HF radiowave propagation. Sporadic E can occur during daytime or nighttime and it varies markedly with latitude. SPREAD F A phenomenon in which pulses returned from the ionosphere are of much greater duration than the transmitted pulse. Spread F is seen on an IONOGRAM as a spreading or blurring (in virtual height) of the normal F LAYER return. It is believed to result from scattering from multiple IRREGULARITIES at differing ranges and zenith angles. SUDDEN COMMENCEMENT (SC) An abrupt increase or decrease in the northward component of the geomagnetic field, which marks the beginning of a GEOMAGNETIC STORM. (May also be called Sudden Storm Commencement or SSC.) SUDDEN IMPULSE (SI+ or SI-) A sudden perturbation of several gammas in the northward component of the low-latitude geomagnetic field, not associated with a following GEOMAGNETIC STORM. (An SI becomes an SC if a storm follows.) SUDDEN IONOSPHERIC DISTURBANCE (SID). Propagation anomalies affecting HF signals due to ionospheric changes resulting from solar FLAREs, PROTON EVENTs and GEOMAGNETIC STORMs. SUNSPOT An area seen as a dark spot on the PHOTOSPHERE of the sun. Sunspots are concentrations of magnetic flux, typically occurring in bipolar clusters or groups. They appear dark because they are cooler than the surrounding photosphere. SUNSPOT GROUP CLASSIFICATION (Modified Zurich Sunspot Classification) A - A small single unipolar SUNSPOT or very small group of spots without PENUMBRA. B - Bipolar sunspot group with no penumbra. C - An elongated bipolar sunspot group. One sunspot must have penumbra. D - An elongated bipolar sunspot group with penumbra on both ends of the group. E - An elongated bipolar sunspot group with penumbra on both ends. Longitudinal extent of penumbra exceeds 10° but not 15°. F - An elongated bipolar sunspot group with penumbra on both ends. Longitudinal extent of penumbra exceeds 15° H - A unipolar sunspot group with penumbra. SUNSPOT NUMBER A daily index of SUNSPOT activity (R), defined as R = k (10 g + s) where S = number of individual spots, g = number of sunspot groups, and k is an observatory factor. TYPE I, II, III, IV. See RADIO EMISSION TROUGH (Sub-auroral trough) A region of the ionosphere characterized by a depletion of ELECTRONS. The term is commonly applied to the subauroral trough, a region of the F layer lying between 50 - 70 degrees MAGNETIC LATITUDE with a north-south extent of 500 - 1000 km. The most common occurrence is between 1800 - 0600 MAGNETIC TIME. U BURST A fast radio burst spectrum of a FLARE. It has a U-shaped appearance in an intensity versus frequency plot. ULTRA HIGH FREQUENCY (UHF) Those radio frequencies between 300 MHz and 3 Ghz. UMBRA The dark core or cores (umbrae) in a SUNSPOT with PENUMBRA, or a sunspot lacking penumbra. UNIVERSAL TIME (UT or UTC). See COORDINATED UNIVERSAL TIME. UNSETTLED With regard to geomagnetic levels, a descriptive word specifically meaning that the Ap INDEX is greater than 8 but less than or equal to 15. VERY HIGH FREQUENCY (VHF) That portion of the radio frequency spectrum from 30 to 300 MHz. VERY LOW FREQUENCY (VLF) That portion of the radio frequency spectrum from 3 to 30 kHz. WATT The power required to do work at the rate of 1 joule per second. 1000 Watts = 1 kW 1 million Watts = 1 MW. One Horsepower = 746 Watts. WAVELENGTH For a propagating electromagnetic wave, the distance between successive peaks of electric or magnetic field strength. Wavelength (W) can be found from the equation: W = c / F where c is the speed of light and F is the frequency. WHITE LIGHT (WL) Sunlight integrated over the visible portion of the spectrum (4000 - 7000 Å) so that all colors are blended to appear white to the eye. WOLF NUMBER An historic term for SUNSPOT NUMBER. In 1849, R. Wolf of Zurich originated the general procedure for computing the sunspot number. X-RAY BACKGROUND A daily average background X-ray flux in the 1 to 8 Å range. X-RAY BURST A temporary enhancement of the X-ray emission of the sun. The time-intensity profile of soft X-ray bursts is similar to that of the H-ALPHA profile of an associated FLARE. X-MODE See EXTRAORDINARY MODE. X-RAY FLARE A solar FLARE having increased emissions in the WAVELENGTHS range 1 - 8 Å. Because these wavelengths ionize the D REGION, X_RAY flares can have a serious effect on ionospheric radio communications, often producing short wave fadeouts (SWF). X-RAY FLARE CLASS Rank of a FLARE based on its X-ray energy output. Flares are classified by the order of magnitude of the peak burst intensity (I) measured at the Earth in the 1 to 8 Å band as follows: Class (in Watt/sq. Meter) B I less than 10^-6 C I between 10^-6 and 10^-5 M I between 10^-5 and 10^-4 X I greater than 10^-4
1. SESC Glossary of Solar-Terrestrial Terms DOC/NOAA/ERL Space Environment Lab. 2. Kelley, M. C., The Earth's Ionosphere, Academic Press, Inc:San Diego 1989 3. Davies, Kenneth, Ionospheric Radio, Peter Peregrinus Ltd.:London, 1990 4. Van Valkenburg, M. E.(editor), Reference Data for Engineers, Prentice Hall:Carmel,IN, 1993
Home
General Information
Technical Details
Data
Photos
Contents
Glossary
Search