GLOSSARY OF SOLAR-TERRESTRIAL TERMS
A
a index. See ak index.
aa index. A daily and half daily index of geomagnetic
activity determined from the k indexes scaled at two
nearly antipodal stations at invariant magnetic latitude
50 degrees(Hartland, England, and Canberra, Australia). The
aa values are in units of 1 nT. The index is available back to 1868,
and is provided by the Institut de Physique du Globe de Paris, France.
absorption line. In spectroscopy, and in particular
the solar Fraunhofer spectrum, a characteristic wavelength
of emitted radiation that is partially absorbed by the medium between
the source and the observer. (See H alpha.)
active. A descriptive word specifically meaning (1)
a probability of > or = 50% for an M-class x-ray flare
(see x-ray flare class) in a sunspot region; (2)
disturbed geomagnetic levels such that 16 < or = Ak index < 30.
active dark filament (ADF). A filament displaying motion or
changes in shape, location, or absorption characteristics.
active longitude. The approximate center of a range
of heliographic longitudes in either the northern or southern solar
hemisphere (seldom both at the same time) containing one or more large
and complex active regions formed by the frequent,
localized emergence of new magnetic flux. Individual sunspot groups
within the complex can have relatively short lifetimes (a week or
two); the complex may persist for several solar rotations because
additional spot groups form as earlier ones decay.
active prominence. A prominence moving
and changing in appearance over a few minutes of time.
active prominence region. A portion of the solar
limb displaying active prominences; typically associated
with an active region.
active region (AR). A localized, transient volume of
the solar atmosphere in which plages, sunspots, facula,
flares, etc., may be observed. Active regions are the result
of enhanced magnetic fields; they are at least bipolar and may
be complex if the region contains two or more bipolar groups.
active surge region (ASR). An active region that exhibits a group
or series of spike-like surges that rise no higher than 0.15 solar
radii above the limb. (See bright surge on the limb.)
ADF. See active dark filament.
AE index. A geomagnetic index of the auroral electroj, which
characterizes the maximum range of excursion (both positive
and negative) from quiet levels; measured at a given
universal time by using the combined data from a worldwide ring of
high-latitude magnetic observatories. AU (A upper) refers to
the greatest positive deviation from the quiet time reference and
AL (A lower) to the most negative. By definition AE = AU - AL. AO
refers to the mean of AU and AL: AO = 1/2 (AU + AL). The AE and companion
indexes are provided by the Data Analysis Center for Geomagnetism
and Spacemagnetism of Kyoto University, Kyoto, Japan.
AFR. The Ak index observed at Fredericksburg, Virginia.
AFS. See arch filament system.
ak index. A 3-hourly "equivalent amplitude"
index of geomagnetic activity for a specific station
or network of stations (represented generically here by k) expressing
the range of disturbance in the horizontal magnetic field. "ak"
is scaled from the 3-hourly K index according
to the following table:
_______________________________________________________
K 0 1 2 3 4 5 6 7 8 9
ak 0 3 7 15 27 48 80 140 240 400
_______________________________________________________
At SESC these values are used directly for operational purposes. But
to convert the ak values to nanoteslas (nT), a local (station-dependent)
conversion factor must be found by dividing the station's lower limit
for K=9 by 250. For example, at Boulder and Fredericksburg the lower
limit for K=9 is 500 nT so the factor is 2; therefore the ak values
for these stations are in units of 2 nT. (To obtain an equivalent
amplitude in nanoteslas for Boulder or Fredericksburg, the index value
must be doubled).
Ak index. A daily index of geomagnetic activity for a specific
station or network of stations (represented generically here by k)
derived as the average of the eight 3-hourly ak indexes
in a Universal Time day.
Alfven wave. A transverse wave in magnetized plasma
characterized by a change of direction of the magnetic field
(rather than a change of intensity).
am index. A mean, 3-hourly "equivalent amplitude" of geomagnetic
activity based on standardized K index data from a global network
of 23 Northern and Southern Hemisphere stations by the Institut de
Physique du Globe de Paris, France; am values are given in units of 1 nT.
Am index. The daily Ak index determined from the eight daily
am indexes.
An index. The daily Ak index determined from only the Northern
Hemisphere stations of the am index network.
anomaly. In typical SESC use, an unexpected response
of a spacecraft.
ap index. A mean, 3-hourly "equivalent amplitude"
of magnetic activity based on K index data from a planetary
network of 11 Northern and 2 Southern Hemisphere magnetic observatories
between the geomagnetic latitudes of 46 degrees and 63 degrees
by the Institut fur Geophysik at Gottingen, F.R. Germany; ap values
Ap index. Formally the daily Ak index, determined from the eight daily
ap indexes. However, for daily operational uses (since several weeks are
required to collect the data and calculate the index), Air Force Space
Forecast Center estimates the value of the Ap index by measuring the
geomagnetic field in near-real time at several Western Hemisphere
magnetometer stations and statistically weighting the data to represent
the Gottingen Ap. The value of this estimated Ap index is reported in
SESC daily and weekly summaries of geophysical activity.
aphelion. That point on the path of a sun-orbiting
object most distant from the center of the sun. Compare perihelion.
apogee. That point on the path of an earth-orbiting satellite most
distant from the center of the earth. Compare perigee.
APR. See active prominence region.
AR. See active region.
arcade. A series of magnetic loops, overlying a solar inversion line.
arch filament system (AFS). A system of small, arched linear-absorption
features connecting bright, compact plage of opposite polarity.
An AFS is a sign of emerging bipolar magnetic flux and possibly rapid
or continued growth in an active region.
As index. The daily Ak index determined from only the Southern
Hemisphere stations of the am index network.
ASR. See active surge region.
atmospherics. Also known as "sferics," transient
radio waves produced by naturally occurring electric discharges
(e.g., lightning) in the earth's atmosphere.
AU. The mean distance between the earth and sun, equal to
214.94 solar radii or 1.496E+11m.
aurora. A sporadic, faint visual phenomenon associated with
geomagnetic activity that occurs mainly in the high-latitude
night sky. Auroras occur within a band of latitudes known as
the auroral oval, the location of which is dependent on
geomagnetic activity. Auroras are a result of collisions between
atmospheric gases and precipitating charged particles (mostly
electrons) guided by the geomagnetic field from the magnetotail.
Each gas (oxygen and nitrogen molecules and atoms) gives out its
own particular color when bombarded, and atmospheric composition
varies with altitude. Since the faster precipitating particles
penetrate deeper, certain auroral colors originate preferentially
from certain heights in the sky. The auroral altitude range is 80
to 1000 km, but typical auroras are 100 to 250 km above the ground;
the color of the typical aurora is yellow-green, from a specific
transition of atomic oxygen. Auroral light from lower levels in the
atmosphere is dominated by blue and red bands from molecular nitrogen
and molecular oxygen. Above 250 km, auroral light is characterized
by a red spectral line of atomic oxygen. To an observer on the ground,
the combined light of these three fluctuating, primary colors produces
an extraordinary visual display. Auroras in the Northern Hemisphere
are called the aurora borealis or "northern lights." Auroras in
the Southern Hemisphere are called aurora australis. The patterns
and forms of the aurora include quiescent arcs, rapidly moving rays
and curtains, patches, and veils.
auroral electrojet. See electrojet.
auroral oval. An elliptical band around each geomagnetic
pole ranging from about 75 degrees magnetic latitude at local
noon to about 67 degrees magnetic latitude at midnight under
average conditions. It is the locus of those locations of the maximum
occurrence of auroras and widens to both higher and
lower latitudes during the expansion phase of a magnetic substorm.
autumnal equinox. The equinox that occurs in September. Compare
vernal equinox.
C index. A subjective daily character figure (index)
of geomagnetic activity for a single observatory; for
each UTC day the figure is 0 for very quiet magnetic
conditions, 1 for moderately disturbed conditions, and 2
for severely disturbed conditions.
Carrington longitude. A system of fixed solar longitudes
rotating at a uniform synodic period of 27.2753 days
(a sidereal period of 25.38 days). Carrington selected the meridian
that passed through the ascending node of the sun's equator at 1200
UTC on 1 January 1854 as the original prime meridian. The daily
Carrington longitude of the central point of the apparent solar disk
is listed (with other solar coordinates in The Astronomical Almanac
published annually by the U.S. Naval Observatory. Compare Bartels'
rotation number.
Castelli U. See U burst.
celestial equator. The projection of earth's geographic
equator onto the celestial sphere.
celestial sphere. An imaginary spherical shell around
the earth and concentric with it.
centimeter burst. A solar radio burst in the centimeter
wavelength range (1 to 10 cm or 0.01 to 0.1 m), or 30 000 to 3000
MHz in the frequency range.
central meridian passage (CMP). The rotation of an active region
or other feature across the longitude meridian that
passes through the apparent center of the solar disk.
CFI. See comprehensive flare index.
chromosphere. The layer of the solar atmosphere above
the photosphere and beneath the transition
region and the corona. The chromosphere is the
source of the strongest lines in the solar spectrum, including the
Balmer alpha line of hydrogen and the H and K lines of calcium, and
is the source of the red (chromium) color often seen around the rim
of the moon at total solar eclipses.
Ci index. The daily international magnetic character
figure formed by taking the arithmetic mean of the C
index values from all reporting observatories.
cleft. See cusp.
CMD. Central Meridian Distance. (See solar
coordinates).
CME. See coronal mass ejection.
CMP. See central meridian passage.
comprehensive flare index (CFI). A method of evaluating
the significance of a complex flare event. The CFI
= A + B + C + D + E. The value of each component is given below; a
value of zero is assigned if the effect did not occur. The CFI values
range from 1 to 17 (non-occurrence gives a zero value); values
>10 indicate flares with unusually strong electromagnetic radiation.
A-Originally the importance of ionizing radiation as indicated
by the importance of associated SID, scale 1-3; but currently scaled
from the x-ray flare class, class C being 1,
class M being 2, and class X being 3.
B-Importance of H alpha flare;
scale 1-3 (3 includes flare importance classes 3 and 4).
C-Log of 10.7-cm peak radio flux in units of 10E-22 W/sq m/Hz.
D-Effects associated with the dynamic radio spectrum: Type II
burst = 1, continuum storm = 2, Type IV burst = 3.
E-Log of 200-MHz flux in same units as C.
The CFI was devised and documented by Helen Dodson Prince and Ruth
Hedeman at the McMath-Hulbert Observatory.
conjugate points. Two points on the earth's surface
at opposite ends of a geomagnetic field line.
continuum. Optical radiation arising from broadband
emission from the photosphere.
continuum storm (CTM). General term for solar noise
lasting for hours and sometimes days, in which the intensity varies
smoothly with frequency over a wide range in the meter and decimeter
wavelengths.
convection. The bulk transport of plasma (or gas) from one place to another,
in response to mechanical forces (for example, viscous interaction with the
solar wind) or electromagnetic forces.
Coordinated Universal Time (UTC). By international
agreement, the local time at the prime meridian, which passes through
Greenwich, England. It was formerly known as Greenwich Mean Time,
or sometimes simply Universal Time. There are 24 time zones around
the world, labeled alphabetically. The time zone centered at Greenwich
has the double designation of A and Z. Especially in the military
community, Coordinated Universal Time is often referenced as Z or
Zulu Time.
corona. The outermost layer of the solar atmosphere,
characterized by low densities (<10E+9 per cubic cm or 10E+15 per cubic m
and high temperatures (>10E+6 K).
coronagraph. An optical device that makes it possible
to observe the corona at times other than during an eclipse.
A simple lens focuses the sun onto an occulting disk that
prevents the light from the solar disk from proceeding
farther along the optical path, effectively providing an artificial
eclipse.
coronal hole. An extended region of the corona,
exceptionally low in density and associated with unipolar photospheric
regions having "open" magnetic field topology. Coronal holes are
largest and most stable at or near the solar poles, and are a source
of high-speed solar wind. Coronal holes are visible
in several wavelengths, most notably solar x-ray s,
but at SESC, coronal holes are determined from solar images in He
1083 nm provided by the Kitt Peak National Solar Observatory.
coronal loops. A typical structure of enhanced corona
observed in EUV lines and soft x-rays. They are sometimes related to
H alpha loops. Coronal loops represent "closed" magnetic topology.
coronal mass ejection (CME). A transient outflow of
plasma from or through the solar corona. CMEs are often
but not always associated with erupting prominences,
disappearing solar filaments, and flares.
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 streamer. A large-scale structure in
the white-light corona often overlying a principal
inversion line in the solar photospheric magnetic fields.
(See helmet streamer ).
coronal transients. A general term for short-time-scale changes in the corona.
corrected geomagnetic coordinates. A nonspherical coordinate
system based on a magnetic dipole axis that is offset from the earth's
center by about 450 km toward a location in the Pacific Ocean
(15.6 N 150.9 E). This "eccentric dipole" axis intersects the surface at
81N 85 W, and 75 S 120 E.
cosmic noise. The broad spectrum of radio noise arriving
at the earth from sources outside the solar system.
cosmic ray. An extremely energetic (relativistic) charged
particle primarily originating outside the earth's magnetosphere.
Cp index. A daily index of geomagnetic activity
analogous to the Ci index, obtained from the sum of
the eight daily values of the ap index. The range of
Cp is 0.0 to 2.5, 2.5 representing the most disturbed.
critical frequency. In ionospheric radio propagation,
that frequency capable of penetration just to the layer of maximum
ionization with vertical propagation. Radiowaves of lower frequencies
are refracted back to the ground; higher frequencies pass through.
CRN. See coronal rain.
crochet. A sudden deviation in the sunlit geomagnetic
field H component (see geomagnetic elements ) associated
with extraordinary solar flare x-ray emission.
The effect can be as much as 50 nT and last up to 30 minutes. The
event is also known as an SFE (solar flare effect).
CTM. See continuum storm.
cusp(s). In the magnetosphere, two regions
near magnetic local noon and approximately 15 degrees of latitude
equatorward of the north and the south magnetic poles. The cusps mark
the division between geomagnetic field lines on the sunward side
(which are approximately dipolar but somewhat compressed
by the solar wind ) and the field lines in the polar
cap that are swept back into the magnetotail by the
solar wind. The term cusp implies conical symmetry around the axis
of the bundle of converging (Northern Hemisphere) or diverging (Southern
Hemisphere) field lines. In practice, "cusp" and "cleft" are
often used interchangeably. However, "cleft" implies greater extension
in longitude (local time) and hence a wedge-shaped structure.
D
D component of the geomagnetic field. See geomagnetic
elements.
D region. A daytime region of the earth's ionosphere
beginning at approximately 40 km, to 90 km altitude.
Radiowave absorption in layers in this region can be significantly
increased in response to increased ionization associated with solar
activity.
dark surge on the disk (DSD). Dark gaseous ejections
on the sun visible in H alpha. They usually originate
from small subflare-like brightenings. Material is usually seen
to be ejected, to decelerate at a gravitational rate, and to flow
back to the point of origin. DSDs can occur intermittently for days
from an active region.
dB (decibel). A unit used to express the ratio between two levels
of power. By definition dB = 10 log (P2/P1). (Doubling the power
ration is approx an increase of 3 dB).
DB. disparition brusque. See disappearing
solar filament.
declination. (1) The angular distance of an astronomical
body north (+) or south (-) of the celestial equator.
(2) In geomagnetic applications, the angle between true north and
the horizontal component of the local geomagnetic field.
differential charging. The charging of different areas
of a spacecraft or satellite to different potentials in response to
sunlight, the charged particle environment, and the design and composition
of the structural materials themselves. Discharge may occur through
arcing and generally is detrimental.
differential particle flux. The differential particle
directional flux j (E,w ) denotes the
number of particles of energy E per unit energy interval,
per unit area, per unit time, per unit solid angle of observation,
passing through an area perpendicular to the viewing direction; the
angle w is the angle between the viewing direction
and the local magnetic field. It is approximately obtained from the
count rate of a physical detector measuring the flux
of particles between energy E and E +dE, geometric factor
G, and solid angle of view dW through the relationship
j(E,w) = C/(G * dE * dW * dt)
where C is the number of detector counts in time dt.
differential rotation. The change in solar rotation rate with latitude.
Low latitudes rotate at a faster angular rate (approx. 14 degrees/day) than
do high latitudes (approx. 12 degrees/day).
dip. The geomagnetic inclination angle. See
geomagnetic elements.
dip equator. An irregular, imaginary line around the earth where the
geomagnetic inclination angle is measured to be zero.
It lies near the geographic equator.
disappearing solar filament (DSF). A solar filament
(prominence) that disappears suddenly (on a time scale of minutes
to hours). The prominence material is often seen to ascend but is
also seen to fall into the sun or just fade. (Historically, DSFs have
been called disparitions brusques because they were first studied
by French astronomers.) DSFs are a possible indicator of coronal
mass ejections.
disk. The visible surface of the sun (or any heavenly
body) projected against the sky.
disparition brusque (DB). See disappearing solar filament.
Doppler shift. A change in the perceived frequency
of a radiated signal caused by motion of the source relative to the
observer.
dose rate. The rate at which radiation energy is absorbed
in living tissue, expressed in centisieverts per unit time.
DSD. See dark surge on the disk.
DSF. See disappearing solar filament.
Dst index. A measure of variation in the geomagnetic
field due to the equatorial ring current. It is
computed from the H-components at approximately four near-equatorial
stations at hourly intervals. At a given time, the Dst index is
the average of variation over all longitudes; the reference level
is set so that Dst is statistically zero on internationally designated
quiet days. An index of -50 or deeper indicates a storm-level
disturbance, and an index of -200 or deeper is associated with middle-
latitude auroras. Dst is determined by the World Data
Center C2 for Geomagnetism, Kyoto University, Kyoto, Japan.
E
E region. A daytime region of the earth's ionosphere
roughly between the altitudes of 90 and 160 km. E region characteristics
(electron density, height, etc.) depend on the solar zenith angle
and solar activity. The ionization in the E layer is caused mainly
by x-ray s in the range 0.8 to 10.4 nm. (See also sporadic E ).
eccentric dipole. See corrected geomagnetic coordinates.
eclipse. The obscuring of one celestial body by another.
(1) A Solar Eclipse occurs when the moon comes between the earth and
the sun. In a total eclipse, the solar disk is completely obscured; in a
partial eclipse the solar disk is only partly obscured. An annular eclipse
occurs when the moon is near its apogee and the apparent diameter of the
moon is less than that of the sun so that the sun is never completely
obscured. "First and last contacts" are defined as the times of
tangency of the solar and lunar disks. A central eclipse
(which can be total or annular) has two additional times of tangency:
"second contact," when maximum eclipse begins, and "third
contact," when it ends. The last glimpses of the sun through the lunar
valleys, just before second contact, are known as Baily's
beads. (2) A lunar eclipse occurs when the moon
enters the shadow cast by the earth. (3) Spacecraft in the earth's
shadow are said to be in eclipse.
ecliptic. The great circle made by the intersection
of the plane of the earth's orbit with the celestial sphere.
(Less properly, the apparent path of the sun around the sky during
the year.)
EFR. See emerging flux region.
EHF. See extremely high frequency.
electrojet. (1) Auroral: A current that flows in the
ionosphere in the auroral zone. (2) Equatorial: A thin electric
current layer in the ionosphere over the dip equator
at about 100 to 115 km altitude.
electrostatic discharge (ESD). An abrupt equalization
of electric potentials. In space, ESD can occur between objects or
portions of a single object (see differential charging );
ESD may occur locally within a dielectric or cable. The consequences
may include material damage, a spacecraft anomaly, phantom
command s, disrupted telemetry, and contaminated data.
ELF. See extremely low frequency.
emerging flux region (EFR). An area on the
sun where new magnetic flux is erupting. An EFR is a bipolar
magnetic region that first produces a small bipolar plage
visible in the chromosphere, which may develop an arch
filament system and the initial spots of a sunspot group.
An EFR may be isolated from other solar activity or may occur
within an active region.
emission line. In spectroscopy, a particular wavelength
of emitted radiation, more intense than the background continuum.
emission measure. The integral of the square of the
electron density over volume; the units are inverse volume (per cubic m).
ephemeris. An astronomical almanac listing solar
coordinates and the positions of the sun and other heavenly bodies
at regular intervals in time.
EPL. See eruptive prominence on limb.
equatorial electrojet. See electrojet.
equinox. One of the two points of intersection of the
celestial equator and the ecliptic. The sun passes
through the vernal equinox on about 21 March and through
the autumnal equinox on about 22 September.
eruptive. With regard to solar flare
predictions, a probability of >50% that an active region will produce
C class x-ray flares. (See x-ray flare class.)
eruptive prominence on limb (EPL). A solar prominence
that becomes activated and is seen to ascend from the sun; sometimes
associated with a coronal mass ejection. (See also disappearing
solar filament).
ESD. See electrostatic discharge.
estimated hemispherical power input. For the earth,
an estimate made from NOAA/TIROS particle measurements of the
instantaneous power dissipated daily in a single auroral zone by
auroral particle precipitation. The power ranges from approximately
5 gigawatts during quiet intervals up to more than 100 in very active
times. The magnitude of this power input corresponds closely to the
level of geomagnetic activity.
EUV. See extreme ultraviolet.
Evershed effect. Horizontal motion of the solar atmosphere
near a sunspot, having velocities of a few kilometers
per second. In the photosphere, matter streams away
from the umbra. In the chromosphere, the direction of
flow is toward the umbra.
exosphere. The earth's atmosphere above 500-600 km.
expert system. A computer program intended to simulate
human logic for analyzing a complex situation on the basis of a sequence
of behavior rules supplied by a human expert. (See Theophrastus).
extraordinary mode. One of the two modes of propagation
of electromagnetic waves in a magnetic plasma. For
propagation along the direction of the magnetic field, it is the mode
in which the electric vector rotates in the same sense that an electron
gyrates freely about the field. For propagation perpendicular to
the magnetic field, the electric vector oscillates perpendicular to
the primary magnetic field. (See also ordinary mode.)
extreme ultraviolet (EUV). A portion of the electromagnetic
spectrum from approximately 10 to 100 nm.
extremely high frequency (EHF). That portion of the
radio frequency spectrum from 30-300 GHz.
extremely low frequency (ELF). That portion of the
radio frequency spectrum from 30 to 3000 Hz.
G
gamma rays. High-energy radiation (energies in
excess of 100 keV) observed during large, extremely energetic solar
flares.
GEOALERT. An IUWDS special message summarizing by code
the current and predicted levels of solar activity
and geomagnetic activity.
geocorona. The outer region of the earth's atmosphere
lying above the thermosphere and composed mostly of hydrogen.
geomagnetic activity. Natural variations in the geomagnetic
field classified quantitatively into quiet,
unsettled, active, and geomagnetic storm levels
according to the observed a index:
Category Range of index
quiet 0 - 7
unsettled 8 - 15
active 16 - 29
minor storm 30 - 49
major storm 50 - 99
severe storm 100 - 400
geomagnetic elements. The components of the geomagnetic
field at the surface of the earth. These elements are usually
denoted thus in the literature:
X-the geographic northward component
Y-the geographic eastward component
Z-the vertical component, reckoned positive downward
H-the horizontal intensity, of magnitude sq rt((X)squared + (Y)squared)
F-the total intensity sq rt((H)squared + (Z)squared)
I-the inclination (or dip) angle, arctan (Z/H)
D-the declination angle, measured from the geographic north
direction to the H component direction, positive in an eastward
direction.
D = arctan (Y/X)
However, in SESC use, the geomagnetic northward and geomagnetic
eastward components are called the H and D components. The H axis
direction is defined by the mean direction of the horizontal component
of the field; the D component is expressed in nanoteslas and is related
to the direction of the horizontal component relative to geomagnetic
north by using the small-angle approximation. Thus the D component
= H (the horizontal intensity) multiplied by delta D (the
declination angle relative to geomagnetic north, expressed in radians).
geomagnetic field. The magnetic field in and around
the earth. The intensity of the magnetic field at the earth's surface
is approximately 32,.000 nT at the equator and 62,000 nT
at the north pole (the place where a compass needle points vertically
downward). The geomagnetic field is dynamic and undergoes continual
slow secular changes as well as short-term disturbances (see geomagnetic
activity ). The geomagnetic field can be approximated
by a centered dipole field, with the axis of the dipole inclined to
the earth's rotational axis by about 11.5 degrees. Geomagnetic
dipole north is near geographic coordinate 78.3 N 69 W (Thule, Greenland),
and dipole south is near 79 S 110 E (near Vostok, Antarctica). The observed
or dip poles, where the magnetic field is vertical to the earth's surface,
are near 76 N 101 W, and 66 S 141 E. The adopted origin of geomagnetic
longitude is the meridian passing through the geomagnetic poles
(dipole model) and the geographic south pole. (See also corrected
geomagnetic coordinates.)
geomagnetic storm. A worldwide disturbance of the earth's
magnetic field, distinct from regular diurnal variations. A storm
is precisely defined as occurring when the daily Ap index exceeds
29. (See geomagnetic activity ).
Initial Phase: Of a geomagnetic storm, that period when there may
be an increase of the middle-latitude horizontal
intensity (H) (see geomagnetic elements ) at the surface
of the earth. The initial phase can last for hours (up to a day),
but some storms proceed directly into the main phase without showing
an initial phase.
Main Phase: Of a geomagnetic storm, that period when the horizontal
magnetic field at middle latitudes is generally decreasing, owing
to the effects of an increasing westward-flowing magnetospheric
ring current. The northward component can be depressed as much
as several hundred nanoteslas in intense storms. The main phase can
last for hours, but typically lasts less than 1 day.
Recovery Phase: Of a geomagnetic storm, that period when the depressed
northward field component returns to normal levels. Recovery is typically
complete in one to two days, but can take longer.
geomagnetic time. See magnetic local time.
geosynchronous. Term applied to any equatorial satellite
with an orbital velocity equal to the rotational velocity of the earth.
The geosynchronous altitude is near 6.6 earth radii (approximately
36 000 km above the earth's surface). To be geostationary as well,
the satellite must satisfy the additional restriction that its orbital
inclination be exactly zero degrees. The net effect is that a geostationary
satellite is virtually motionless with respect to an observer on the
ground.
GLE. See ground-level event.
GMT. Greenwich Mean Time. (See Coordinated
Universal Time.)
GPS. Global Positioning System: a network of earth-orbiting
satellites used for precise position-finding in surveying
and navigation.
gradual commencement. The commencement of a geomagnetic
storm that has no well-defined onset. (See also sudden
commencement.)
granulation. Cellular structure of the photosphere
visible at high spatial resolution. Individual granules, which
represent the tops of small convection cells, are 200 to 2000 km in
diameter and have lifetimes of 8 to 10 minutes.
Greenwich Mean Time (GMT). See Coordinated
Universal Time.
green line. A coronal emission line
at 530.3 nm from Fe XIV (an iron atom from which 13 electrons have
been stripped). The green line is one of the strongest (and first-recognized)
visible coronal lines. It identifies moderate-temperature
regions of the corona ; it is enhanced in coronal
streamers above inversion lines, and diminished
in coronal holes.
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. When they occur, GLEs begin a few
minutes after flare maximum and last for a few tens
of minutes to hours. Intense particle fluxes at lower
energies can be expected to follow this initial burst of relativistic
particles. GLEs are detected by neutron monitors, e.g., the monitor
at Thule, Greenland.
J
K
K (kelvin). A unit of absolute temperature. One kelvin is equal
to 1 degree C, but zero on the kelvin scale corresponds to absolute
zero (-273.15 degrees C).
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. This is the "true" corona. Corona
graphs are specifically constructed to separate the K corona from
the F corona.
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 (see geomagnetic
elements ).
Kelvin-Helmholtz instability. A mechanism often
invoked to explain phenomena at the magnetopause (and
sometimes the plasmapause), especially the observed
magnetic pulsations.
Km index. A 3-hourly planetary index of geomagnetic
activity calculated by the Institut de Physique du Globe
de Paris, France, from the K indexes observed at a
large, symmetrically located network of stations. The
Km indexes are used to determine the am indexes.
Kp index. A 3-hourly planetary index of geomagnetic activity
calculated by the Institut fur Geophysik der Gottingen Universitat,
F.R. Germany, from the K indexes observed at 13 stations primarily
in the Northern Hemisphere. The Kp indexes, which date from 1932,
are used to determine the ap indexes.
L
L. Heliographic longitude of a solar feature. (See solar coordinates.)
latchup. With reference to the effect of energetic
particles on spacecraft microcircuits, a serious type of single
event upset in which the microcircuit is either permanently stuck
or cannot be reset without being turned off and on.
LDE. See long duration (or decay) event.
leader spot. In a magnetically bipolar or multipolar
sunspot group, the main spot in that portion of the group west
of the principal inversion line ; also called the preceding
or p-spot. Leader and follower describe the positions
of spots with respect to apparent motion due to solar rotation. (Compare
follower spot.)
LEO. Among satellite operators, a common abbreviation for Low Earth Orbit.
LET. See linear energy transfer.
LF. See low frequency.
light bridge. Observed in white light, a bright tongue
or streaks penetrating or crossing sunspot umbra e.
Light bridges typically develop slowly and have lifetimes of several
days. The appearance of a light bridge is frequently a sign of impending
active region division or dissolution. The more brilliant
forms occur with overlying bright plage and often occur
during the most active phase of the sunspot group.
light curve. A plot of intensity in a particular wavelength
or band of wavelengths against time, especially with reference to
a solar flare; for example, the time history of the x-ray output of a flare.
limb. The edge of the solar disk, corresponding
to the level at which the solar atmosphere becomes transparent to
visible light.
limb darkening. For certain solar spectral lines, a
lessening of the intensity of the line from the center of the solar
disk to the limb, caused by the existence of a
temperature gradient in the sun and the line-of-sight
through the solar atmosphere.
limb flare. A flare at the edge (limb) of the solar disk ; the elevated
portions of the flare are seen with particular clarity against the dark
sky background.
linear energy transfer (LET). The energy per unit path
length that an ionizing particle loses to the medium through which
it is traveling. The greater the LET, the more damaging the particle.
lobes. In the magnetotail, the two regions (north and south)
separated by the neutral sheet.
long duration ( or decay) event (LDE). With reference to x-ray
events, those events that are not impulsive in appearance. The exact
time threshold separating impulsive from long-duration events is not
well defined, but operationally, any event requiring hours (1 or more)
to return to background levels would probably be regarded as an LDE.
It has been shown that the likelihood of a coronal mass ejection
increases with the duration of an x-ray event, and becomes virtually
certain for durations of 6 hours or more.
longitudinal component. That component of magnetic
field vector parallel to the direction of view, radial from the solar
surface at disk center.
loop prominence system (LPS). A system of prominences
in the form of loops associated with major flares, bridging the
magnetic inversion line. The lifetime of an LPS is a few hours.
Loop prominences observed in H alpha are distinctly brighter than
other prominences, and material typically flows downward along
both legs from condensation "knots" near the top of the loop. LPSs
show a high correlation with proton flares.
low frequency (LF). That portion of the radio frequency
spectrum from 30 to 300 kHz.
lowest usable frequency (LUF). The lowest frequency
that allows reliable long-range HF radio communication by
ionospheric refraction.
LPS. See loop prominence system.
LUF. See lowest usable frequency.
M
M(3000). The ratio of the maximum frequency reflected
once from an ionospheric layer over a 3000-km range to the
critical frequency of the layer.
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. Bays are "positive" if H increases and "negative"
if H decreases.
magnetic cloud. In general, any identifiable parcel of solar wind.
More specifically, a region of about 0.25 AU in radial dimension in
which the magnetic field strength is high and the direction of one
component of the magnetic field changes appreciably by means of
a rotation nearly parallel to a plane. Magnetic clouds may be one
manifestation of coronal mass ejections in the interplanetary medium.
magnetic local time (MLT). On earth, analogous to geographic
local time; MLT at a given location is determined by the angle subtended
at the geomagnetic axis between the geomagnetic midnight meridian
and the meridian that passes through the location. 15 degrees = 1 h.
The geomagnetic meridian containing the sub-solar point
defines geomagnetic local noon, and the opposite meridian defines
geomagnetic midnight. (See geomagnetic field.)
magnetic sunspot classifications. See Mount Wilson magnetic classification.
magnetogram. A plot showing the amplitude of one or
more vector components of a magnetic field versus space or time. Solar
magnetograms are a graphic representation of solar magnetic field
strengths and polarity.
magnetohydrodynamics (MHD). The study of the dynamics of an electrically
conducting fluid in the presence of a magnetic field.
magnetopause. The boundary surface between the solar
wind and the magnetosphere, where the pressure
of the magnetic field of the object effectively equals the dynamic
pressure of the solar wind.
magnetopause current sheet. An electric current sheet
that more or less coincides with the magnetopause.
magnetosheath. The region between the bow
shock and the magnetopause, characterized by very
turbulent plasma. For the earth, along the sun-earth
axis, the magnetosheath is about 2 earth radii thick.
magnetosphere. The magnetic cavity surrounding a magnetized
body, carved out of the passing solar wind by virtue
of the magnetic field, which prevents, or at least impedes, the direct
entry of the solar wind plasma into the cavity.
magnetotail. The extension of the magnetosphere in the
antisunward direction as a result of interaction with
the solar wind. In the inner magnetotail, the field
lines maintain a roughly dipolar configuration. But at greater distances
in the antisunward direction, the field lines are stretched into northern
and southern lobes, separated by a plasmasheet.
There is observational evidence for traces of the earth's magnetotail
as far as 1000 earth radii downstream.
MAGSTORM. A telegraphic abbreviation used to denote
a geomagnetic storm.
main phase. See geomagnetic storm.
Maunder minimum. An approximately 70-year period,
centered near 1670, during which practically no sunspots
were observed.
maximum usable frequency (MUF). The highest frequency
that allows reliable HF radio communication over a given ground range
by ionospheric refraction. Frequencies higher than the MUF penetrate
the ionosphere and become useful for extraterrestrial
communications.
MDP. See mound prominence.
medium frequency (MF). That portion of the radio frequency
spectrum from 0.3 to 3 MHz.
mesosphere. The region of the earth's atmosphere between
the upper limit of the stratosphere (approximately
30 km altitude) and the lower limit of the thermosphere
(approximately 80 km altitude).
MHD. See magnetohydrodynamics.
micropulsation. See pulsation.
microwave burst. A radiowave signal associated with
optical and/or x-ray flare s. Microwave
bursts occur mostly at centimeter wavelengths (6 cm = 4995 MHz) but
are generally broadband, often extending into the millimeter and decimeter
domains. (See also U - burst.)
microwaves. Generically, any radio frequency of 500 MHz or more.
middle latitude. With reference to zones of geomagnetic activity,
20 degrees to 50 degrees geomagnetic latitude. Other zones
are equatorial, polar, and high latitude.
Moreton wave. A wave disturbance (also known as a flare
blast wave) generated by large flares, which is seen
to propagate horizontally across the disk of the sun
at a typical velocity of about 1000 km /s. Its presence
is more visible in wings of the H alpha line. It can cause
filaments to erupt as the wave apparently disturbs supporting
magnetic fields.
mound prominence (MDP). H alpha structure
at the solar limb that is the elevated top of numerous
small surges and/or a dense, low-lying prominence.
Mount Wilson magnetic classification. Classification
of the magnetic character of sunspot s according to
rules set forth by the Mount Wilson Observatory in California.
alpha. A unipolar sunspot group.
beta. A sunspot group having both positive and
negative magnetic polarities (bipolar), with a simple and distinct
division between the polarities.
gamma. A complex active region
in which the positive and negative polarities are so irregularly distributed
as to prevent classification as a bipolar group.
beta-gamma. A sunspot group that
is bipolar but which is sufficiently complex that no single, continuous
line can be drawn between spots of opposite polarities.
delta. A qualifier to magnetic class (see below)
indicating that umbra e separated by less than 2 degrees
within one penumbra have opposite polarity.
beta-delta. A sunspot group of general
beta magnetic classification but containing one (or
more) delta spot(s).
beta-gamma-delta. A sunspot group of beta-gamma magnetic classification
but containing one (or more) delta spot(s).
gamma-delta. A sunspot group of gamma magnetic classification but
containing one (or more) delta spot(s).
multipath. Describing a degraded condition of radio
propagation in which the radio wave splits and arrives at the receiver
via different paths. Because each path will generally have different
lengths, arrival times, and phases, the signal received will suffer fading.
N
network. (1) Chromospheric: a large-scale brightness
pattern in chromospheric (see chromosphere ) and
transition region spectral lines, which is located at the borders
of the photospheric (see photosphere ) supergranulation
and coincides with regions of local magnetic enhancement. These cellular
patterns are typically 3 x 10E+4 km across. (2) Photospheric: a bright pattern
that appears in spectroheliograms in certain Fraunhofer spectrum lines.
It coincides in gross outline with the chromospheric network.
neutral line. The line that separates solar magnetic
fields of opposite polarity, typically determined from solar magnetograms
recording the longitudinal magnetic component. Neutral
lines are, more properly, inversion line s).
neutron monitor. A ground-based detector that
counts secondary neutrons generated by processes originating with
the impact of atmospheric molecules and atoms by very energetic particles
(galactic or solar cosmic rays).
nm (nanometer). A unit of length, 10E-9m.
noise storm. A transient enhancement of solar radio
emission, particularly at 245 MHz, consisting of an elevated background
emission (radiation) and Type I radio bursts.
non-great-circle propagation. Describing
a degraded condition of radio propagation caused by horizontal gradients
in the ionospheric electron density. The radio wave is refracted away
from its normal great-circle path, which is the shortest distance
between two points on the earth. Strong horizontal gradients are associated
with the equatorward boundary of the auroral oval (especially
in the night sector) and the sunrise terminator.
nT (nanotesla ). 10E-9 tesla or 0.000000001 tesla.
O
ordinary mode. One of the two modes of propagation
of electromagnetic waves in a magnetic plasma. For
propagation along the direction of the magnetic field, it is the mode
in which the electric vector rotates opposite to the direction of
an electron gyrating freely about the field. For propagation perpendicular
to the magnetic field, the electric vector oscillates parallel to
the primary magnetic field. (See also extraordinary mode.)
P
P-angle. See solar coordinates.
p-spot. See leader spot.
PCA. See polar cap absorption.
particle flux unit (p.f.u.). 1 p/sq cm/s/sr(steradian).
penumbra. The sunspot area that may
surround the darker umbra or umbrae. In its mature
form it consists of linear bright and dark elements radial from the
sunspot umbra.
perigee. That point on the orbit of an earth-orbiting
satellite nearest to the earth. Compare apogee.
perihelion. That point on the orbit of a sun-orbiting
body nearest to the sun. Compare aphelion.
persistence. Continuation of existing conditions. When
a physical parameter varies slowly, the best prediction is often persistence.
p.f.u. See particle flux unit.
phantom command. An apparent (but unintended) spacecraft
command caused by the natural environment. (See single
event upset or electrostatic discharge.)
photosphere. The lowest visible layer of the solar
atmosphere; corresponds to the solar surface viewed in white
light. Sunspots and faculae are observed in the photosphere.
pitch angle. In a plasma, the angle
between the velocity vector of a charged particle and the direction
of the ambient magnetic field.
plage. On the sun, an extended emission feature of
an active region that is seen from the time of emergence
of the first magnetic flux until the widely scattered remnant magnetic
fields merge with the background. Magnetic fields are more intense
in plage, and temperatures are higher than in surrounding, quiescent
regions.
plage corridor. A low-intensity division in chromospheric
(see chromosphere ) plage coinciding
with a polarity inversion line and marked by narrow
filament segments and/or fibrils spanning the corridor.
plasma. A gas that is sufficiently ionized so as to
affect its dynamical behavior.
plasma frequency. The characteristic frequency
of free plasma oscillations, determined by the balance
between electron kinetic energy and ion Coulomb attraction.
plasmapause. The outer surface of the plasmasphere.
plasmasheet. In the magnetosphere, the
core of the magnetotail in which the plasma
is hotter and denser than in the tail lobes north and south of it.
The plasmasheet is thought to be separated from the tail lobes by
the sheet of the "last closed field lines" and it typically lies
beyond geosynchronous orbit.
plasmasphere. In the magnetosphere,
a region of relatively cool (low energy) and dense plasma
that may be considered an outer extension of the ionosphere
with which it is coupled. Like the ionosphere, the plasmasphere tends
to co-rotate with the earth.
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. PCAs generally originate
with major solar flares, beginning within a few hours
of the event and maximizing within a day or two of onset. As measured
by a riometer, the PCA event threshold is 2 dB of absorption
at 30 MHz for daytime and 0.5 dB at night. In practice, the absorption
is inferred from the proton flux at energies greater than 10 MeV,
so that PCAs and proton event s are simultaneous. However,
the transpolar radio paths may still be disturbed for days, up to
weeks, following the end of a proton event, and there is some ambiguity
about the operational use of the term PCA.
polar crown. A nearly continuous ring of filaments occasionally
encircling either polar region of the sun (latitudes higher than
50 degrees).
polar plumes. Fine, ray-like structures of the
solar corona, best observed in the solar polar regions
during solar minimum.
polar rain. In the earth's upper atmosphere, a weak,
structureless, near-isotropic flux of electrons precipitating
into the polar caps.
pore. A feature in the photosphere,
1 to 3 arc seconds in extent, usually not much darker than the dark
spaces between photospheric granules. It is distinguished from a sun
spot by its short lifetime, 10 to 100 minutes.
post-flare loops. A loop prominence
system often seen after a major two-ribbon flare,
which bridges the ribbons. Lifetimes are several hours.
preheating. A slow brightening of an
active region, both optically and in x-rays,
that sometimes precedes moderate and larger solar flare
events by some tens of minutes.
PRESTO. An alert issued by a Regional Warning Center
to give rapid notification of significant solar or geophysical activity
in progress or just concluded.
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. Prominences are further classified by their shape (for example,
mound prominence, coronal rain ) and activity. They are most clearly
and most often observed in H alpha.
proton event. The measurement of proton flux reaching and sustaining
> = 10 p.f.u. for at least 15 min at energies > 10 MeV
by the primary SESC geosynchronous satellite. (See polar cap absorption.)
The start time of the event is defined as the earliest time at which event
thresholds have been reached. There are two event thresholds, namely p10 and p100.
(p10, a proton event reaching 10 p.f.u. at > 10 MeV and p100 reaching 100 p.f.u. at > 100 MeV).
proton flare. Any flare producing significant counts of protons with energies
exceeding 10 MeV in the vicinity of the earth.
pulsation. A rapid fluctuation of the geomagnetic
field having periods from a fraction of a second to tens of minutes
and lasting from minutes to hours. There are two main patterns: Pc
(a continuous, almost sinusoidal pattern), and Pi (an irregular pattern).
Pulsations occur at magnetically quiet as well as disturbed
times. Pc's are grouped, according to their physical and morphological
properties, into five categories:
Pc1 - periods 0.2-5 s. May occur in bursts ("pearls"), or in consecutive
groups of pulsations with sharply decreasing frequency.
Pc2 - periods 5-10 s. Do not seem to be physically related to Pc1 or Pc3.
Pc3 - periods 10-45 s. Are observed over a wide range of latitudes.
Pc4 - periods 45-150 s. Are also known as Pc II or Pc.
Pc5 - periods 150-600 s. Are sometimes called giant micropulsations.
Q
Q index. A 15-minute index of geomagnetic
activity intended for high-latitude (auroral)
stations. After quiet diurnal variations are removed, Q is the largest
deviation scaled from the undisturbed level for the two horizontal
components. (This differs from the K index,
which is scaled from the largest relative deviation.) The 15-minute
periods are centered on the hour and at 15, 30, and 45 minutes past
each hour. The range of Q is from 0 to 11; the upper limit, in nanoteslas,
for each index value is given below.
Q = 0 1 2 3 4 5 6 7 8 9 10 11
10 20 40 80 140 240 400 660 1000 1500 2200 Unlimited
QDC. See quiet day curve.
quiescent prominence. A long, sheet-like prominence
nearly vertical to the solar surface. Except in an occasional activated
phase, shows little large-scale motion, develops very slowly,
and has a lifetime of several solar rotations. Quiescent prominences
form within the remnants of decayed active regions,
in quiet areas of the sun between active regions, or at high solar
latitudes where active regions seldom or never form. (See filament).
quiet. A descriptive word specifically meaning (1)
a probability of less than 50% for a C-class flare
(see x-ray flare class ) in a sunspot region;
(2) geomagnetic activity levels such that Ak < 8.
quiet day curve (QDC). Especially in connection with
the components of the geomagnetic field (see geomagnetic
elements ), the trace expected in the absence of activity.
The K index and Q index are measured from deviations
relative to a QDC. Riometer and neutron monitor deviations
are also measured relative to a QDC.
R
R-number. See sunspot number.
radar aurora. Radar returns from electron density irregularities
in auroral regions. The strength of radar auroral returns is aspect
dependent.
radiation belts . Regions of the magnetosphere
roughly 1.2 to 6 earth radii above the equator in which charged particles
are stably trapped by closed geomagnetic field lines.
There are two belts. The inner belt is part of the plasmasphere
and corotates with the earth; its maximum proton density lies
near 5000 km. Inner belt protons are mostly high energy (MeV range)
and originate from the decay of secondary neutrons created during
collisions between cosmic ray s and upper atmospheric
particles. The outer belt extends on to the magnetopause
on the sunward side (10 earth radii under normal quiet conditions)
and to about 6 earth radii on the nightside. The altitude of maximum
proton density is near 16 000-20 000 km. Outer belt protons are lower
energy (about 200 eV to 1 MeV) and come from the solar
wind. The outer belt is also characterized by highly variable
fluxes of energetic electrons. The radiation belts are often called
the "Van Allen radiation belts" because they were discovered in
1968 by a research group at the University of Iowa led by Professor
J. A. Van Allen.
radio burst. See radio emission.
radio emission. Emission of the sun in radio wavelengths
from centimeters to dekameters, under both quiet and disturbed conditions.
Some patterns, known variously as noise storms, bursts, and sweeps,
are identified as described below. These types of emission are subjectively
rated on an importance scale of 1 to 3, 3 representing the most intense.
Type I. A noise storm composed of many short, narrow-band bursts
in the meter wavelength range (300-50 MHz), of extremely variable
intensity. The storm may last from several hours to several days.
Type II. Narrow-band emission (sweep) 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 meter range (300-30 MHz). These bursts occur with some major
flare events; they begin 10 to 20 minutes after the flare maximum
and can last for hours.
Type V. Short-duration (a few minutes) continuum noise in the
dekameter range usually associated with Type III bursts.
Rayleigh-Taylor instability. A fluted or ripple-like
instability that can develop on a fluid or plasma boundary surface
and propagate along it. This instability is often invoked to explain
phenomena in the ionosphere and magnetosphere.
reconnection. A process by which differently directed
field lines link up, allowing topological changes of the magnetic
field to occur, determining patterns of plasma flow, and resulting
in conversion of magnetic energy to kinetic and thermal energy of
the plasma. Reconnection is invoked to explain the energization and
acceleration of the plasma s that are observed in solar flares, magnetic
substorms, and elsewhere in the solar system.
recurrence. Used especially to express a tendency of
some solar and geophysical parameters to repeat a trend and sometimes
the actual value of the parameter itself every 27 days (the approximate
rotation period of the sun).
red line. An intense coronal emission line at 637.4 nm from Fe X
(an iron atom from which nine electrons have been stripped).
It identifies relatively cooler regions of the corona.
region number. A number assigned by SESC to a plage
region or sunspot group if one of the following
conditions exists: (1) the region is a group of at least sunspot
classification C; (2) two or more separated optical reports confirm
the presence of smaller spots; (3) the region produces a solar flare;
(4) the region is clearly evident in H alpha and exceeds 5 heliographic
degrees in either latitude or longitude. (See also active region.)
regression. A functional relationship between two or
more correlated variables that is often empirically determined from
data and is used especially to predict values of one variable when
values of the others are given.
RI. The international standard relative sunspot number.
right ascension. The angular distance measured eastward
along the celestial equator from the vernal
equinox. It is expressed in hours, minutes, and seconds
(the circumference of the celestial equator is defined as 24 hours).
rigidity. A measure of how easily a particle
is deflected by a magnetic field, expressed in megavolts (MV) per
nucleon. It is the momentum per unit charge. The integral proton spectrum
of a flare can be expressed as an exponential function
of rigidity rather than a power function of energy.
ring current. In the magnetosphere,
a region of current that flows in a disk-shaped region near
the geomagnetic equator in the outer of the Van Allen radiation
belts. The current is produced by the gradient and curvature drift
of the trapped charged particles. The ring current is greatly augmented
during magnetic storms because of the hot plasma injected
from the magnetotail. The ring current causes a worldwide
depression of the horizontal geomagnetic field during
a magnetic storm.
riometer (Relative Ionospheric Opacity meter). A specially
designed ground-level 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. Absorption events are known as PCA s
(polar cap absorption) and are primarily associated with major
solar flares.
rudimentary. A type of sunspot penumbra
characterized by granular (rather than filamentary) structure, brighter
intensity than the umbra, and narrow extent, and possibly
only partially surrounding the umbra. Penumbrae are typically rudimentary
during the sunspot formative and decay phases.
S
Satellite Anomaly. The usually undesirable response of spacecraft systems to
variations in the space environment. High energy particles cause detector
noise and/or physical damage to solar cells, electronics, and memory devices
(single event upsets or "bitflips"). Large and varying low-to-medium energy
particle fluxes can result in a charge buildup between spacecraft components,
especially during the eclipse season and during spacecraft maneuvers.
Atmospheric drag on spacecraft below approximately 1,000 km can increase
during geomagnetic storms, resulting in cross-track and in-track orbit errors
and orientation problems. Various communication interference problems
result during solar radio bursts from flares when the Sun is within the field of
view of the ground tracking dish. Ionospheric irregularities during
geomagnetic storms can cause radio telemetry scintillation and fading
S-band. Radio frequencies between 1.55 and 5.20
GHz. For satellite communication, the term usually refers to frequencies
used for earth-space communication near 2.2 GHz.
S component. The slowly varying (weeks or longer) fluctuation
observed in solar radio emission at microwave frequencies
(wavelengths from 3 to 100 cm).
SC. See sudden commencement.
scintillation. Describing a degraded condition of radio
propagation characterized by a rapid variation in amplitude and/or
phase of a radio signal (usually on a satellite communication link)
caused by abrupt variations in electron density anywhere along the
signal path. It is positively correlated with spread F and
to a lesser degree, sporadic E. Scintillation effects
are the most severe at low latitudes, but can also be a problem at
high latitudes, especially in the auroral oval and over the polar caps.
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 the sun (a negative sector), or away from the sun
(a positive sector). The sector boundary separating fields
of opposite polarity is normally narrow, passing the earth within
minutes to hours as opposed to the week or so needed for passage of
a typical sector. The solar wind velocities in the boundary region
are typically among the lowest observed.
SEU. See single event upset.
SFE. Solar flare effect. (See crochet.)
s.f.u. See solar flux unit. 10E-22 W/sq m/Hz = 10 000 jansky.
SHF. See super high frequency.
shock. A discontinuity in pressure, density, and particle
velocity, propagating through a compressible fluid or plasma.
short wave fade (SWF). An abrupt decrease of HF radio
signal strength, lasting from minutes to hours, caused by increased
day-side ionization from some solar flares. An SWF is one effect
under the broad category of sudden ionospheric disturbances (SIDs).
SI. See sudden impulse.
SID. See sudden ionospheric disturbance.
sidereal. Referring to a coordinate system fixed with
respect to the distant stars.
simultaneous flares. Unrelated solar flares
that occur at nearly the same time. Compare sympathetic
flares.
single event upset (SEU). With reference to the effects
of energetic particles on spacecraft microcircuits, an unexpected
change in the logic state of a single digital bit. SEUs can be either
"soft" (the microcircuit is not damaged and can be rewritten to
either state), or a latchup, which cannot easily be reset.
smoothed sunspot number. An average of 13 monthly RI
numbers, centered on the month of concern. The 1st and 13th months
are given a weight of 0.5.
solar activity. Transient perturbations of the solar
atmosphere as measured by enhanced x-ray emission
(see x-ray flare class ), typically associated
with flares. Five standard terms are used to describe
the activity observed or expected within a 24-h period:
Very low - x-ray events less than C-class.
Low - C-class x-ray events.
Moderate - isolated (one to 4) M-class x-ray events.
High - several (5 or more) M-class x-ray events,
or isolated (one to 4) M5 or greater x-ray events.
Very high - several (5 or more) M5 or greater x-ray events.
solar constant. The total radiant energy received vertically
from the sun, per unit area per unit of time, at a position just outside
the earth's atmosphere when the earth is at its average distance from
the sun. Radiation at all wavelengths from all parts of the solar disk is
included. Its value is approximately 2.00 cal/sq cm/min = 1.37 kW/sq m
and it varies slightly (by approximately 0.l%) from day to day in response
to overall solar features.
solar coordinates. Specifications for a location on
the solar surface. The location of a specific feature on the sun (for
example, a sunspot ) is complicated by the fact that
there is a tilt of 7.25 degrees between the ecliptic plane and the solar
equatorial plane as well as a true wobble of the
solar rotational axis. (Only twice a year are the solar north pole
and the celestial north pole aligned.) Consequently, to specify a
location on the solar surface, three coordinates (P, B, L) are necessary
to define a grid. Daily values for the coordinates
in Coordinated Universal Time (UTC) are listed in The Astronomical
Almanac published annually by the U.S. Naval Observatory.
The terms used to refer to the coordinates are defined as follows:
P-angle (or P): The position angle between the geocentric
north pole and the solar rotational north pole measured eastward from
geocentric north. The range in P is +/- 26.3l degrees.
Bo: Heliographic latitude of the central point of the solar disk;
also called the B-angle. The range of Bo is +/- 7.23 degrees,
correcting for the tilt of the ecliptic with respect to the solar
equatorial plane.
Example: If (P,Bo) = (-26.21 degrees, -6.54 degrees), the heliographic
latitude of the central point on the solar disk is -6.54 degrees
(the north rotational pole is not visible), and the angle between
the projection onto the disk of the geocentric north pole and the
solar north rotational pole is 26.21 degrees to the west.
Lo: Heliographic longitude of the central point of the solar disk. The
longitude value is determined with reference to a system of fixed
longitudes rotating on the sun at a rate of 13.2 degrees /day
(the mean rate of rotation observed from central meridian transits
of sunspots). The standard meridian on the sun is defined
to be the meridian that passed through the ascending node of the sun's
equator on 1 January 1854 at 1200 UTC and is calculated for the present
day by assuming a uniform sidereal period of rotation
of 25.38 days.
Once P, Bo, and Lo are known, the latitude, central meridian distance,
and longitude of a specific solar feature can be determined as follows:
Latitude. The angular distance from the solar equator, measured north
or south along the meridian.
Central meridian distance (CMD). The angular distance in solar longitude
measured from the central meridian. This position is relative to the
view from earth and will change as the sun rotates; therefore, this
coordinate should not be confused with heliographic positions that
are fixed with respect to the solar surface.
Longitude. The angular distance from a standard meridian (0 degrees
heliographic longitude), measured from east to west (0 degrees
to 360 degrees) along the sun's equator. It is computed by
combining CMD with the longitude of the central meridian at the time
of the observation, interpolating between ephemeris
values (for 0000 UT) by using the synodic rate of solar
rotation (27.2753 days, 13.2 degrees per day).
solar cycle. See sunspot cycle.
solar flare effect (SFE). See crochet.
solar flux unit (s.f.u.). See s.f.u.
solar maximum. The month(s) during the sunspot cycle
when the smoothed sunspot number reaches
a maximum. A recent solar maximum occurred in December 1979.
solar minimum. The month(s) during the sunspot
cycle when the smoothed sunspot number reaches
a minimum. A recent solar minimum occurred in September 1986.
solar radio emission. See radio emission.
solar rotation rate. (1) synodic: l3.39 degrees -2.7 degrees sin squared
(solar latitude)/day. (2) sidereal: 14.38 degrees -2.7 sin sq.(solar
latitude)/day. The difference between sidereal and synodic
rates is the earth orbital motion of 0.985 degrees/day.
solar sector boundary (SSB). The boundary between large-scale
unipolar magnetic regions on the sun's surface, as determined
from inversion lines mapped using filaments and filament
channels, or large-scale magnetograms. The supposed solar
signature of an interplanetary sector boundary.
solar wind. The outward flow of solar particles and
magnetic fields from the sun. Typically at 1 AU, solar wind velocities
are near 375 km/s and proton and electron densities are near 5 per cubic
centimeter. The total intensity of the interplanetary magnetic field is
nominally 5 nT.
solstice. A point on the ecliptic where
the sun reaches its greatest absolute declination.
There are two of these points, halfway between the equinoxes;
they mark the beginning of summer and winter.
South Atlantic anomaly (SAA). A region of the earth
centered near 25 degrees S 50 degrees W (geographic
coordinates, near the Atlantic coast of Brazil) of low geomagnetic
field intensity owing to the fact that the geomagnetic field axis
is offset from the center of the earth (see corrected geomagnetic
coordinates.) One consequence of the SAA is that trapped particles
in the plasmasphere drift closer to the earth's surface
and can more easily be lost into the atmosphere. The result is that
the F region (see ionosphere ) is highly variable in this region, and
satellites in low earth orbits suffer greater radiation doses when
they pass through the SAA. There is a corresponding location of
maximum geomagnetic field intensity in Southeast Asia.
spacecraft charging. A term that encompasses all the
charging effects on a spacecraft due to the environment in space.
Occasionally this term is used in a more limited sense to mean surface
charging.
spicules. Rapidly changing, predominantly vertical,
spike-like structures in the solar chromosphere
observed above the limb. Spicules appear to be ejected
from the low chromosphere at velocities of 20 to 30 km/s
reaching a height of about 9000 km and then falling back or fading.
The total lifetime is 5 to 10 minutes.
sporadic E (Es). Transient, localized patches of relatively high
electron density in the E region of the ionosphere, which
significantly affect radiowave propagation. Sporadic E
can occur during daytime or nighttime, and it varies markedly with
latitude. Es can be associated with thunderstorms, meteor showers, solar
activity and geomagnetic activity.
spray (SPY). Luminous material ejected from a solar flare with sufficient
velocity to escape the sun (675 km/s). Sprays are usually seen in H alpha
with complex and rapidly changing form. There is little evidence that
sprays are focused by magnetic fields. Compare surge.
spread F. A condition of the F region
of the ionosphere caused by patches of ionization
that scatter or duct radio signals, characterized on ionograms
by a wide range of heights of reflected pulses. In equatorial latitudes
spread F is most commonly observed at night and may be negatively
correlated with geomagnetic activity; at high latitudes
spread F occurs throughout the daytime and is positively correlated
with magnetic activity. The latitude of minimum occurrence of spread
F is near 30 degrees magnetic latitude.
SPY. See spray.
Sq. The diurnal variation of the geomagnetic
field. The Sq variation is explained in terms of solar tidal motions
of the ionosphere and thermally driven ionospheric
winds.
SSB. See solar sector boundary.
SSC. See sudden commencement.
storm. See geomagnetic storm.
stratosphere. That region of the earth's atmosphere
between the troposphere and the mesosphere.
It begins at an altitude of temperature minimum at approximately 13
km and defines a layer of increasing temperature up to about 50 km.
STRATWARM. A code word designating a major
disturbance of the winter, polar, middle atmosphere from the tropopause
to the ionosphere, lasting for several days at a time
and characterized by a warming of the stratospheric temperature by
some tens of degrees. There is no evidence that stratwarms are caused
by solar events, or that they affect the lower atmosphere. (In fact,
the disturbance may be generated by tropospheric conditions).
subflare. See flare.
substorm. A geomagnetic perturbation lasting 1 to 2
hours, which tends to occur during local post-midnight nighttime.
The magnitude of the substorm is largest in the auroral zone, potentially
reaching several thousand nanoteslas. A substorm corresponds to an
injection of charged particles from the magnetotail
into the auroral oval.
sudden commencement ( SC, or SSC for Storm Sudden Commencement).
An abrupt increase or decrease in the northward component
(see geomagnetic elements) of the geomagnetic field, which marks the
beginning of a geomagnetic storm. SCs occur almost simultaneously
worldwide but with locally varying magnitudes.
sudden impulse (SI + or SI - ). A sudden perturbation,
positive or negative, of several nanoteslas in the northward component
(see geomagnetic elements ) 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). Any of several
radio propagation anomalies due to ionospheric changes resulting from
solar flares. Anomalies include short wave
fades, enhancements of atmospherics, phase shifts,
cosmic noise absorptions, and signal enhancements.
sudden ionospheric disturbance (SID). Any of several
radio propagation anomalies due to ionospheric changes resulting from
solar flares. Anomalies include short wave fades, enhancements
of atmospherics, phase shifts, cosmic noise
absorptions, and signal enhancements.
sunspot. An area seen as a dark spot, in contrast with
its surroundings, 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. Larger and darker sunspots sometimes
are surrounded (completely or partially) by penumbrae.
The dark centers are umbrae. The smallest, immature
spots are sometimes called pores.
sunspot classification (Modified Zurich Sunspot
Classification). As devised by McIntosh, a 3-letter designation
of the optical, white-ligh t characteristics of
a sunspot group. The general form of the designation
is Zpc. One letter is chosen from each of the following three categories.
The categories are illustrated in Appendix C.
Z (the modified Zurich class of the group):
A - A small single sunspot or very small group
of spots with the same magnetic polarity, without penumbra.
B - Bipolar sunspot group with no penumbra.
C - An elongated bipolar sunspot group. One sunspot must
have penumbra, and penumbra does not exceed 5 degrees in
longitudinal extent.
D - An elongated bipolar sunspot group with penumbra on both ends of
the group; longitudinal extent of penumbra is more than 5
degrees, but does not exceed 10 degrees.
E - An elongated bipolar sunspot group with penumbra on both ends.
Longitudinal extent of penumbra exceeds 10 degrees but not
15 degrees
F - An elongated bipolar sunspot group with penumbra on both ends.
Longitudinal extent of penumbra exceeds 15 degrees.
H - A unipolar sunspot group with penumbra. Class H sunspot groups
become compact Class D or larger when the penumbra exceeds 5
degrees in longitudinal extent.
p (the penumbra type of the largest spot in the group):
x - no penumbra
r - rudimentary
s - small (< = 2.5 degrees north-south diameter), symmetric
a - small, asymmetric
h - large (> 2.5 degrees north-south diameter), symmetric
k - large, asymmetric
c (the compactness of the group):
x - a single spot
o - open
i - intermediate
c - compact
sunspot cycle. The approximately 11-year quasi-periodic
variation in the sunspot number. The polarity pattern
of the magnetic field reverses with each cycle. Other solar phenomena,
such as the 10.7-cm solar radio emission, exhibit
similar cyclical behavior.
sunspot number. A daily index of sunspot
activity (R), defined as R = k (10g +s ) where s = number of
individual spots, g = number of sunspot groups, and k is an
observatory factor (equal to 1 for the Zurich Observatory and
adjusted for all other observatories to obtain approximately the
same R number). The standard number, RI, once derived at Zurich (see Wolf
number), is now being derived at Brussels and is denoted by RI. Often,
the term "sunspot number" is used in reference to the widely distributed
smoothed sunspot number.
super high frequency (SHF). That portion of the radio
frequency spectrum from 3 GHz to 30 GHz.
supergranulationstrong>. A system of large-scale velocity
cells that does not vary significantly over the quiet solar surface
or with phase of the solar cycle. The cells are presumably convective
in origin with weak upward motions in the center, downward motions
at the borders, and horizontal motions of typically 0.3 to 0.4 km/s.
Magnetic flux is more intense along the borders of the cells.
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. Surges
typically last 10 to 20 minutes and tend to recur at a rate of approximately
1 per hour. Surges are linear and collimated in form, as if highly
directed by magnetic fields. Compare spray.
SWF. See short wave fade.
sympathetic flares.. Solar flares in different active regions that apparently
occur as the common result of activation of a coronal connection between the
regions. Compare simultaneous flares.
synodic. Referring to a coordinate system fixed on the earth.
synoptic chart. A map of the whole sun in absolute
heliographic coordinates, displaying an integrated view of solar features
observed during a Carrington rotation.
T
TEC. See total electron content.
TED. Total (particle) Energy Deposition. The TIROS/NOAA
instrument used to estimate the hemispherical power input. (See
estimated hemispherical power input.)
tenflare. A solar flare accompanied by a 10-cm radio noise burst
of intensity greater than 100% of the pre-event 10-cm flux value.
Theophrastus (Theo). The name of the rule-based
expert system used to assist SESC solar region
analysis and solar flare prediction.
thermosphere. That region of the earth's atmosphere
where the neutral temperature increases with height. It begins above
the mesosphere at about 80-85 km and extends to the exosphere.
total electron content (TEC). The number of electrons
along a ray path between a transmitter and a receiver. Units are electrons
per square meter. This number is significant in determining ionospheric
effects such as refraction, dispersion, and group delay on radio waves,
and can be used to estimate critical frequencies. The TEC is strongly
affected by solar activity and geomagnetic activity.
transition region>. That region of the solar atmosphere
lying between the chromosphere and the corona
where the temperature rises from 10000 K to 1000000 K.
The transition region is only a few thousand kilometers thick.
transverse. Component of magnetic field vector perpendicular
to direction of view, parallel to solar surface at disk center.
troposphere. The lowest layer of the earth's atmosphere,
extending from the ground to the stratosphere at approximately
13 km of altitude.
two-ribbon flare. A flare that has developed as a pair of bright strands
(ribbons) on both sides of an inversion line of the solar magnetic field.
Type I, II, III, IV, V. See radio emission.
U
U-burst. A radio noise burst associated with
some flares. It has a U-shaped appearance in
an intensity-vs.-frequency plot. The minimum intensity
falls roughly between 500 and 2000 MHz. A U-burst is sometimes
called a Castelli U.
UHF. See ultrahigh frequency.
ultrahigh frequency (UHF). That portion of the radio
frequency spectrum from 300 MHz to 3 GHz.
ultraviolet (UV). That part of the electromagnetic
spectrum between 5 and 400 nm.
umbra. The dark core or cores (umbrae) in a sunspot
with penumbra, or a sunspot lacking penumbra.
UMR. See unipolar magnetic region.
unipolar magnetic region (UMR). A large-scale
photospheric region where the magnetic elements are predominantly
of one polarity (for example, the solar polar regions).
Universal Time (UT). A shortened form of the more correct
Coordinated Universal Time (UTC).
unsettled. With regard to geomagnetic activity,
a descriptive word between quiet and active specifically
meaning that the Ak index is between 8 and 16.
>upsets. See single event upsets.
UT or UTC. See Coordinated Universal Time.
UV. See ultraviolet.
V
Van Allen radiation belts. See radiation belts.
vernal equinox. The equinox that occurs in March.
Compare autumnal equinox.
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.
VHF. See very high frequency.
VLF. See very low frequency.
W
.white light (WL). The sum of all visible wavelengths
of light (400-700 nm) so that all colors are blended to appear white
to the eye. No pronounced contribution from any one spectral line
(or light-emitting element) is implied.
white-light flare. A major flare in which small parts become visible
in white light. This rare continuum emission is caused by energetic
particle beams bombarding the lower solar atmosphere. Such flares
are usually strong x-ray, radio, and particle emitters.
wing. Portion of a spectroscopic absorption (or emission) line
between the core of the line and the continuum adjacent to the line.
WL. See white light.
Wolf number. An historic term for sunspot number. In 1849,
R. Wolf of Zurich originated the general procedure
for computing the sunspot number. The record of sunspot numbers that
he began has continued to this day.
WWV. Call letters of the radio station over which National
Institute of Standards and Technology broadcasts time-standard
signals at 2.5, 5, 10, 15, and 20 MHz. Solar-terrestrial conditions
and forecasts are broadcast at 18 minutes past the hour.
X
X-band. Designates those radio frequencies between 5.2 and 10.9 GHz.
x-ray. Radiation of extremely short wavelength (generally less than 1 nm).
x-ray background. A daily average background x-ray flux in the
0.1 to 0.8 nm range. It is a midday minimum given in terms of x-ray flare class.
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. Soft x-rays are those of energies less than
20 keV, or wavelengths longer than 0.05 nm.
x-ray flare class. Rank of a flare
based on its x-ray energy output. Flares are classified by the
Space Environment Services Center according to the order of magnitude
of the peak burst intensity (I) measured at the earth in the 0.1 to
0.8 nm band as follows:
Class Peak, 0.1 to 0.8 nm band
W/square m ergs/square cm/s
B I < 10.0E-06 I < 10.0E-03
C 10.0E-06 < = I < 10.0E-05 10.0E-03 < = I < 10.0E-02
M 10.0E-05 < = I < 10.0E-04 10.0E -02< = I < 10.0E-01
X I > = 10.0E-04 I > = 10.0E-01
x-ray flare termination. The end time is defined as the time the flux has
decayed to 1/2 the peak flux of the event.
Y
yellow line. A coronal emission line at 569.4 nm from
Ca XV (a calcium atom from which 14 electrons have been stripped).
It identifies the hottest regions of the corona.
Z.
Zulu Time. (See Coordinated Universal Time.)
Z component of the geomagnetic field. See geomagnetic elements.
Zeeman effect. The splitting of spectral emission lines
due to the presence of a strong magnetic field. Briefly, the lines
split into three or more components of characteristic polarization;
the components are circular if the local magnetic
field is parallel to the line of sight, and linear
if the field is perpendicular to the line of sight. The amount of
splitting is proportional to the strength of the field.
Zurich sunspot classification. See sunspot classification.
Zurich sunspot number. See sunspot number.
APPENDIX A: ACRONYMS AND INITIALISMS
AFB - Air Force Base
AFGL - formerly the Air Force Geophysics Laboratory at Hanscom
AFB, MA. Recently renamed Phillips Laboratory (See PL)
AFSFC - Air Force Space Forecast Center (at Colorado Springs, CO)
ALMEDS - ALaskan MEteorological Data System
ASCII - American Standard Code for Information Interchange
AUTODIN - AUTOmatic DIgital Network
AWS - Air Weather Service (USAF)
COMEDS - CONUS MEteorological Data System
CONUS - CONtinental United States
DALAS - Disk And Limb Activity Summary
DMS - data management system
DMSP - Defense Meteorological Satellite Program
DOC - Department Of Commerce
DOD - Department Of Defense
EOS - Earth Observing System
ERL - Environmental Research Laboratories
ESA - European Space Agency
GMS - Geostationary Meteorological Satellite (Japan)
GOES - Geostationary Operational Environmental Satellite
(Also called SMS/GOES)
GSFC - Goddard Space Flight Center (Greenbelt, MD)
HAO - High Altitude Observatory
HEPAD - High Energy Proton and Alpha Detector (on GOES and TIROS)
HLMS - High Latitude Monitoring Station
HST - Hubble Space Telescope
IAG - International Association of Geomagnetism and Aeronomy
ICE - International Cometary Explorer (formerly ISEE-3)
IGY - International Geophysical Year
IMP - Interplanetary Monitoring Platform
IMS - International Magnetospheric Study
INTERMAGNET - An International Consortium of Magnetic Observatories
ISEE-3 - International Sun Earth Explorer-3.
ISTP - International Solar-Terrestrial Program
IUGG - International Union of Geodesy and Geophysics
IUWDS - International Ursigram and World Days Service
JPL - Jet Propulsion Laboratory
JSC - Johnson Space Center (Houston, TX)
KPNO - Kitt Peak National Observatory
MEPED - Medium Energy Proton and Electron Detector (on GOES and
TIROS)
MSFC - Marshall Space Flight Center (Huntsville, AL)
NAG - Naval Astronautics Group
NASA - National Aeronautics and Space Administration
NBS - National Bureau of Standards
NCAR - National Center for Atmospheric Research
NESDIS - National Environmental Satellite, Data, and Information
Service
NESS - National Environmental Satellite Service
NGDC - National Geophysical Data Center
NGSDC - National Geophysical and Solar-Terrestrial Data Center
(Boulder, CO)
NIST - National Institute of Standards and Tchnology
NOAA - National Oceanic and Atmospheric Administration
NOAO - National Optical Astronomy Observatories
NORMEDS - NOrthern MEteorological Data System
NOSC - Naval Ocean Systems Center
NRL - Naval Research Laboratory
NSF - National Science Foundation
NSO - National Solar Observatories (combines Sacramento Peak
Observatory and the Solar Section of Kitt Peak Observatory)
NSSDC - National Space Science Data Center (Greenbelt, MD)
OLDS - On-Line Data Systems
PL - Phillips Laboratory (Air Force)
RGON - Remote Geophysical Observing Network
RSTN - Radio Solar Telescope Network (USAF)
RWC - Regional Warning Center
ScI - Science Institute (Space Telescope)
SEL - Space Environment Laboratory (ERL)
SELDADS- Space Environment Laboratory Data Acquisition and
Display System
SELSIS - Space Environment Laboratory Solar Imaging System
SEM - Space Environment Monitor (on GOES and TIROS)
SEON - Solar Electro-Optical Network (USAF)
SESC - Space Environment Services Center
SFC - Space Forecast Center (at Falcon AFB, Colorado)
SGAS - Solar Geophysical Activity Summary
SMM - Solar Maximum Mission
SMS - Synchronous Meteorological Satellite
SOON - Solar Observing Optical Network (USAF)
SPAN - Space Physics Analysis Network
SXI - Solar X-ray Imager
TDRS - Tracking and Data Relay Satellite (NASA)
TED - Total (particle) Energy Detector (on TIROS)
TIROS - Television and Infrared Radiation Observation Satellite
TMO - Table Mountain Observatory
URSI - Union Radio Scientifique Internationale (thus, URSIgram:
message from URSI).
USAF - United States Air Force
USGS - United States Geological Survey
USSFC - United States Space Forecast Center
WDC - World Data Center
WMO - World Meteorological Organization
WWA - World Warning Agency
WWV - call letters of the standard time and frequency radio station
APPENDIX B: UNITS
The preferred system of physical units for publications of the U.S.
Department of Commerce is the International System of Units (SI).
In this system, the base units of length, mass, and time are the meter,
the kilogram, and the second, respectively. In this appendix, various
other common and historical measurement units are listed with appropriate
conversion factors.