HAARP

Glossary of Solar and Geophysical Terms

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.
    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 trajectory 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 occu 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.

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.

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.

SHORT WAVE FADE (SWF)
  A particular ionospheric solar FLARE effect under the broad category of 
sudden ionosphericdisturbances (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 solar maximum for sunspot 
cycle 23 occurred in April 2000.

SOLAR MINIMUM
  The month(s) during the SOLAR CYCLE when the 12-month mean of monthly
average SUNSPOT numbers reaches a minimum. The solar minimum between 
Cycles 22 and 23 was believed to have occurred in May 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.

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.

TYPE I, II, III, IV. See RADIO EMISSION

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