On-line Glossary of Solar - Terrestrial Physics Terms

Taken from the SESC Glossary of Solar-Terrestrial Terms. DOC/NOAA/ERL/SEL

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     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 derived as the average of
               the eight 3-hourly a indices.
     
     ACTIVE.   Geomagnetic levels such that 15 is less than  Ap which is less than 29.
     
     ACTIVE.   Solar activity levels with at least one geophysical event or
               several larger radio events (10cm) per day (Class M Flares)
     
     ACTIVE DARK FILAMENT (ADF).  An ACTIVE PROMINENCE seen on the DISK.
     
     ACTIVE LONGITUDE.  The approximate center of a range of heliographic
               longitudes in which ACTIVE REGIONS are more numerous and more
               FLARE-active than the average.
     
     ACTIVE PROMINENCE.  A PROMINENCE displaying material motion and changes
               in appearance over a few minutes of time.
     
     ACTIVE PROMINENCE REGION (APR).  A portion of the solar LIMB displaying
               ACTIVE PROMINENCEs.
     
     ACTIVE REGION (AR).  A localized, transient volume of the solar atmosphere
               in which PLAGEs, SUNSPOTS, FACULAe, FLAREs, etc. may be observed.
     
     ACTIVE SURGE REGION (ASR). An ACTIVE REGION that exhibits a group or
               series of spike-like surges that rise above the limb.
     
     AFRED.    Abbreviation for the A INDEX for Fredericksburg.
     
     ANGSTROM. A unit of length = 1.0E-08cm.
     
     Ap INDEX. An averaged planetary A INDEX based on data from a set of specific
               stations.
     
     ARCH FILAMENT SYSTEM (AFS).  A bright, compact PLAGE crossed by a system of
               small, arched FILAMENTS, which is often a sign of rapid or contin-
               ued growth in an ACTIVE REGION.
     
     ASTRONOMICAL UNIT (AU).  The mean earth-sun distance, equal to 1.496E+13cm
               or 214.94 solar radii.
     
     AURORA.   A faint visual phenomenon associated with geomagnetic activity,
               which occurs mainly in the high-latitude night sky; typical
               auroras are 100 to 250 km above the ground.
     
     AURORAL OVAL. An oval band around each geomagnetic pole which is the locus
               of structured AURORAe.
     
     AUTUMNAL EQUINOX.  The equinox that occurs in September.
     
     
     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.
     
     BRIGHT SURGE ON THE DISK (BSD).  A bright gaseous stream (SURGE) emanating
               from the CHROMOSPHERE.
     
     BRIGHT SURGE ON THE LIMB (BSL).  A large gaseous stream (SURGE) that moves
               outward more than 0.15 solar radius above the LIMB.
     
     BURST.    A transient enhancement of the solar RADIO EMISSION, usually
               associated with an ACTIVE REGION or FLARE.
     
     CARRINGTON LONGITUDE.  A system of fixed longitudes rotating with the sun.
     
     CENTIMETER BURST.  A solar radio burst in the centimeter wavelength range.
     
     CENTRAL MERIDIAN PASSAGE (CMP).  The passage of an ACTIVE REGION or other
               feature across the longitude meridian that passes through the
               apparent center of the solar DISK.
     
     CHROMOSPHERE.  The layer of the solar atmosphere above the PHOTOSPHERE and
               beneath the transition region and the CORONA.
     
     CHROMOSPHERIC EVENTS.  Some flares are just Chromospheric Events 
                            without Centimetric Bursts or Ionospheric 
                            Effects. (SID) (Class C flare)
     
     COMPREHENSIVE FLARE INDEX (CFI). The 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;
                  (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**-22Watt/m**2/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**-22Watt/m**2/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.
     
     CORONA.   The outermost layer of the solar atmosphere, characterized by low
               densities (<1.0E+09/cc) and high temperatures (>1,0E+06deg.K).
     
     CORONAL HOLE.  An extended region of the CORONA, exceptionally low in density
               and associated with unipolar photospheric regions.
     
     CORONAL RAIN (CRN).  Material condensing in the CORONA and appearing to rain
               down into the CHROMOSPHERE as observed in H-ALPHA at the solar LIMB
               above strong SUNSPOTS.
     
     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.
     
     CROCHET.  A sudden deviation in the sunlit geomagnetic field (H component;
               see GEOMAGNETIC ELEMENTS) 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.
     
     DARK SURGE ON DISK (DSD).  Dark gaseous ejections visible in H-ALPHA.
     
     DIFFERENTIAL ROTATION.  The change in SOLAR ROTATION RATE with latitude.  Low
               latitudes rotate at a faster angular rate  (approx. 14 degrees per
               day) than do high latitudes (approx. 12 degrees per day).
     
     DISAPPEARING SOLAR FILAMENT (DSF).  The sudden (timescale of minutes to
               hours) disappearance of a solar FILAMENT (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. 
     
     E REGION.  A daytime layer of the earth's ionosphere roughly between the
               altitudes of 85 and 140 km.
     
     EMERGING FLUX REGION (EFR).  An area on the sun where new magnetic flux is
               erupting.
     
     ERUPTIVE. Solar activity levels with at least one radio event(10cm) and
               several chromospheric events per day (Class C Flares)
     
     ERUPTIVE PROMINENCE ON LIMB (EPL).  A solar PROMINENCE that becomes activa-
               ted and is seen to ascend from the sun.
     
     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 angstroms.
     
     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 inter-
               planetary space.
     
     F REGION.  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.
     
     FIBRIL.   A linear pattern in the H-ALPHA CHROMOSPHERE of the sun, as seen
               through an H-alpha filter, occurring near strong SUNSPOTS and
               PLAGE or in FILAMENT channels.
     
     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.
     
     FILAMENT CHANNEL.  A broad pattern of FIBRILS in the CHROMOSPHERE, marking
               where a FILAMENT may soon form or where a filament recently
               disappeared.
     
     
     FLARE.    A sudden eruption of energy on the solar DISK lasting minutes to
               hours, from which radiation and particles are emitted.
     
     FLUENCE.  Time integrated flux.  
     
     FLUX.  The rate of flow of a physical quantitiy through a reference surface.
     
     fMIN.     The lowest radiowave frequency that can be reflected from the
               IONOSPHERE.
     
     foEs.     The maximum ORDINARY MODE radiowave frequency capable of reflec-
               tion from the SPORADIC E REGION of the IONOSPHERE.
     
     foF2.     The maximum ORDINARY MODE radiowave frequency capable of reflec-
               tion 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 1 x 10.0E-05 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 sur-
               face of the earth.  In SESC use, the northward and eastward
               components are often called the H and D components, where the
               D component is expressed in gammas and is derived from D (the
               declination angle) using the small angle approximation.
     
     GEOMAGNETIC FIELD.  The magnetic field observed in and around the earth.
               The intensity of the magnetic 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.
     
               Minor Geomagnetic Storm:  A storm for which the Ap index was
                    greater than 29 and less than 50.
     
               Major Geomagnetic Storm:  A storm for which the Ap index was
                    greater than 49 and less than 100.
     
               Severe Geomagnetic 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 hori-
                    zontal 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)
     
     GEOSYNCHRONOUS.  Term applied to any equatorial satellite with an orbital
               velocity equal to the rotational velocity of the earth.  The net
               effect is that the satellite is virtually motionless with respect
               to an observer on the ground.
     
     GMT.      Greenwich Mean Time. (See COORDINATED UNIVERSAL TIME.)
     
     GRADUAL COMMENCEMENT.  The commencement of a geomagnetic storm that has
               no well-defined onset.
     
     GRANULATION.   Cellular structure of the PHOTOSPHERE visible at high spatial
               resolution.
     
     GREEN LINE.  The green line is one of the strongest (and first-recognized)
               visible coronal lines.  It identifies moderate temperature regions
               of the CORONA.
     
     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, occur-
               ring only a few times each SOLAR CYCLE.
     
     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.
     
     H-component of the Geomagnetic Field.  See GEOMAGNETIC ELEMENTS.
     
     HIGH ENERGY EVENT. Flares (class two or more) with outstanding Centimetric
               Bursts and SID. High Energy Protons are reported at the Earth in
               case of most of these events occurring on the western part of solar
               disk. (Class X flares)
      
     HIGH FREQUENCY (HF).  That portion of the radio frequency spectrum between
               between 3 and 30 MHz.
     
     HIGH LATITUDES.  With specific reference to zones of geomagnetic activity,
               quot;high latitudes" refers to 50o to 80o geomagnetic.
     
     HIGH-SPEED STREAM.  A feature of the SOLAR WIND having velocities that are
               about double average solar wind values.
     
     HOMOLOGOUS FLARES.  Solar flares that occur repetitively in the same 
                         ACTIVE REGION, with essentially the same position and with a common
                         pattern of development.
     
     HYDER FLARE.  A FILAMENT-associated TWO-RIBBON FLARE, often occurring in
               spotless regions.  The flare presumably results from the impact
               on the CHROMOSPHERE of infalling FILAMENT material.
     
     INTERPLANETARY MAGNETIC FIELD (IMF).  The magnetic field carried with the

               SOLAR WIND.
     
     
     
     IONOSPHERE.  The region of the earth's upper atmosphere containing a small
               percentage of free electrons and ions produced by photoioniza-
               tion of the constituents of the atmosphere by solar ultraviolet
               radiation at very short wavelengths (l.t.1000 angstroms).  The
               ionosphere significantly influences radiowave propagation of fre-
               quencies less than about 30 MHz.
     
     
     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.
               Range is from 0 to 9.  The K index measures the deviation of the
               most disturbed horizontal 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.
     
     LIGHT BRIDGE.  Observed in white light, a bright tongue or streaks penetra-
               ting or crossing SUNSPOT UMBRAe.  The appearance of a light bridge
               is frequently a sign of impending region division or dissolution.
     
     LIMB.     The edge of the solar DISK.
     
     LIMB FLARE.  A solar FLARE seen at the edge (LIMB) of the sun.
     
     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).
     
     MAGFLARE. A geomagnetic and/or cosmic storm has been associated with this
               flare.
     
     MAGNETIC BAY.  A relatively smooth excursion of the H (horizontal) component
               (see GEOMAGNETIC ELEMENTS) of the geomagnetic field away from and
               returning to quiet levels.
      
     MAGNETOGRAM.  Solar magnetograms are a graphic representation of solar mag-
               netic field strengths and polarity.
     
     MAGNETOPAUSE.  The boundary layer between the SOLAR WIND and the MAGNETO-
               SPHERE.
     
     MAGNETOSPHERE.  The magnetic cavity surrounding the earth, carved out of the
               passing SOLAR WIND by virtue of the GEOMAGNETIC FIELD, which pre-
               vents, or at least impedes, the direct entry of the SOLAR WIND
               PLASMA into the cavity.
     
     MAJORFLARE. This flare is the basis for the forecast of geomagstorm, cosmic
               storm and/or protons in the earth's vicinity.
     
     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 deg. to 50 deg. geomagnetic.
     
     MOUNT WILSON MAGNETIC CLASSIFICATIONS.
     
               Alpha.  Denotes a unipolar SUNSPOT group.
     
               Beta.  A sunspot group having both positive and negative mag-
                    netic 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 nega-
                    tive polarities are so irregularly distributed as to prevent
                    classification as a bipolar group.
     
     NANOTESLA (nT).  A unit of magnetism 10.0E-09 tesla, equivalent to a gamma
               (10.0E-05 gauss).
     
     NEUTRAL LINE.  The line that separates longitudinal magnetic fields of oppo-
               site polarity.
     
     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.
     
     PERSISTENCE.  Continuation of existing conditions.  When a physical parameter
               varies slowly, the best prediction is often persistence.
     
     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.
     
     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.
     
     PLAGE CORRIDOR.  A space in chromospheric (see CHROMOSPHERE) PLAGE lacking
               plage intensity, coinciding with polarity inversion line.
     
     PLAGENIL. Spotless disc free of calcium plage
     
     PLASMA.   Any ionized gas, that is, any gas containing ions and electrons.
     
     POLAR CAP ABSORPTION (PCA).  An anomalous condition of the polar IONOSPHERE
               whereby HF and VHF (3 - 300  MHz) radiowaves are absorbed, and LF
               and VLF (3 - 300 kHz) 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.
     
     POST-FLARE LOOPS.  A LOOP PROMINENCE SYSTEM often seen after a major 
                    TWO-RIBBON FLARE, which bridges the ribbons.
     
     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.   Solar activity levels with at least on high energy event (Class X
               Flares)
     
     PROTON EVENT.  By definition, the measurement of at least 10
               protons/sq.cm/sec/steradian at energies greater than 10 MeV.
     
     PROTON FLARE.  Any FLARE producing significant FLUXes of greater-than-10 MeV
               protons in the vicinity of the earth.
     
     QUIESCENT PROMINENCE (FILAMENT).  Long, sheet-like prominences nearly vertical
               to the solar surface.
     
     QUIET.    A descriptive word specifically meaning geomagnetic levels such that
               Ap < 8 (see Ap INDEX).
     
     QUIET.    Solar activity 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.
     
               Type I.  A noise storm composed of many short, narrow-band bursts
                    in the metric range (300 - 50 MHz).
     
               Type II.  Narrow-band emission that begins in the meter range
                    (300 MHz) and sweeps slowly (tens of minutes) toward deka-
                    meter 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
                    meter range (300 - 30 MHz).  These bursts are associated with
                    some major flare events beginning 10 to 20 minutes after the
                    flare maximum, and can last for hours.
     
     RADIO EVENT. Flares with Centimetric Bursts and/or definite Ionospheric Event
               (SID).
     
     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.
     
     SECTOR BOUNDARY.  In the SOLAR WIND, the area of demarcation between sec-
               tors, which are large-scale features distinguished by the
               predominant direction of the interplanetary magnetic field,
               toward or away from the sun.
     
     SHORT WAVE FADE (SWF).  A particular ionospheric solar flare effect under
               the broad category of sudden ionospheric disturbances (SIDs)
               whereby short-wavelength radio transmissions, VLF, through
               HF, are absorbed for a period of minutes to hours.
     
     SMOOTHED SUNSPOT NUMBER.  An average of 13 monthly RI numbers, centered on
               the month of concern.
     
     SOLAR COORDINATES.
     
               Central Meridian Distance (CMD).  The angular distance in solar
                    longitude measured from the central meridian.
     
     SOLAR CYCLE.  The approximately 11-year quasi-periodic variation in frequency
               or number of solar active events.
     
     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 most recent minimum occurred in September 1986.
     
     SOLAR SECTOR BOUNDARY (SSB).  The apparent solar origin, or base, of the
               interplanetary SECTOR BOUNDARY marked by the larger-scale polarity
               inversion lines.
     
     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.
     
     SPOTNIL.  Spotless disk
     
     SUDDEN COMMENCEMENT(SC, or SSC for Storm Sudden Commencement).  An abrupt
               increase or decrease in the northward component of the geomagnetic
               field, which marks the beginning of a GEOMAGNETIC STORM.
     
     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).  HF propagation anomalies 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 deg. but
                    not 15 deg.
     
               F -  An elongated bipolar sunspot group with penumbra on both
                    ends.  Longitudinal extent of penumbra exceeds 15 deg.
     
               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.
     
     SURGE.    A jet of material from ACTIVE REGIONs that reaches coronal heights
               and then either fades or returns into the CHROMOSPHERE along the
               trajectory of ascent.
     
     TWO-RIBBON FLARE.  A FLARE that has developed as a pair of bright strands
               (ribbons) on both sides of the main inversion ("neutral") line
               of the magnetic field of the ACTIVE REGION.
     
     TYPE I, II, III, IV.  See RADIO EMISSION
     
     
     U BURST.  A fast radio burst spectrum of a FLARE.  It has a U-shaped appear-
               ance in an intensity-vs.-frequency plot.
     
     ULTRA HIGH FREQUENCY (UHF).  Those radio frequencies exceeding 300 MHz.
     
     UMBRA.    The dark core or cores (umbrae) in a SUNSPOT with PENUMBRA, or a
               sunspot lacking penumbra.
     
     UNIVERSAL TIME (UT).  See COORDINATED UNIVERSAL TIME.
     
     UNSETTLED.  With regard to geomagnetic levels, a descriptive word speci-
               fically meaning that 8 is less than or equal to the Ap INDEX which is less than or equal to15.
     
     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.
     
     WHITE LIGHT (WL).  Sunlight integrated over the visible portion of the spec-
               trum (4000 - 7000 angstroms) so that all colors are blended to
               appear white to the eye.
     
     WHITE LIGHT FLARE.  A major FLARE in which small parts become visible in
               white light.  Such flares are usually strong X-ray, radio, and
               particle emitters.
     
     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
               angstrom range.  It is a midday minimum designed to reduce the
               effects of FLAREs.
     
     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-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 inten-
               sity (I) measured at the earth in the 1 to 8 angstrom band as
               follows:
     
                  Class          (in Watt/sq. Meter)
     
                    B              I less than (l.t.) 10.0E-06
     
                    C              10.0E-06 l.e.= I  l.t.= 10.0E-05
     
                    M              10.0E-05 l.e.= I  l.t.= 10.0E-04
     
                    X              I g.e.= 10.0E-04
     
     
     
     ZURICH SUNSPOT CLASSIFICATION.  A sunspot classification system that has been
               modified for SESC use.
     

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