Glossary of Solar-Terrestrial Terms

Glossary of Solar-Terrestrial Terms

Glossary of Solar Terrestrial Terms
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
References: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