Active Region (solar) -- A localized, transient volume of the solar atmosphere characterized by complex magnetic fields, often associated with sunspots, flares, coronal mass ejections, plages, faculae, and other solar phenomena.
-- A unit of length = 1.0E-08cm.
mean Sun-Earth distance, a unit of measure widely used in expressing distances
in the solar system. 1 AU = 149,600,000 km = 92,957,000 miles.
oval--the region in which aurora appears at the same time, corresponding
to a "ring of fire" around the magnetic pole, often observed by satellites.
It resembles a circle centered a few hundred kilometers nightward of the
magnetic pole, and its size varies with magnetic activity. During large
magnetic storms it expands greatly, making auroras visible at regions
far from the pole, where they are a rare occurrence.
region on Earth where auroras are common, essentially a smeared-out average
(over time and distance from the magnetic pole) of the auroral oval. Typical
magnetic latitude is 63-65 degrees.
Monitor -- Atmospheric Weather Electromagnetic
System for Observation Modeling and Education
instruments which monitor changes in the Earth’s ionosphere by tracking
VLF signals as they bounce through the Earth’s waveguide. Developed by
the Space, Telecommunications, and Radioscience (STAR) Laboratory, a research
group within the Department of Electrical
Engineering of Stanford University
in association with the
Rotation Number --
The serial number assigned to 27-day recurrence periods of solar and geophysical
parameters. The equatorial rotation rate of the Sun is very close to 27
days. Rotation 1 Day 1 in this sequence was assigned arbitrarily by J.
Bartels to February 8, 1832.
layer of the solar atmosphere above the photosphere
and beneath the transition region and the corona.
It is seen during eclipses as a bright red ring around the Sun, with the
term burning prairie used to describe it.
Coronal Mass Ejection
– see Universal Time
Hole -- an extended region of the corona,
exceptionally low in density (essentially a large open gap), and associated
with photospheric regions. Coronal holes are closely associated with those
regions on the Sun that have an "open" magnetic geometry, that is, the
magnetic field lines associated with them extend far outward into interplanetary
space, rather than looping back to the photosphere.
Ionized material can flow easily along such a path, and this in turn aids
the mechanism that causes high
speed solar wind streams to develop.
mass ejection (CME)--a huge cloud of hot plasma occasionally
expelled from the Sun. It may accelerate ions and electrons and travel
through interplanetary space as far as the Earth’s orbit and beyond. The
leading edges of fast-moving CMEs drive giant shock waves before them
through the solar
wind at speeds up to 1200 km per second. When the shock reaches Earth,
a magnetic storm may result. CMEs occurs on a time scale
between a few minutes and several hours, and involve the appearance of
a new discrete, bright, white
light feature in a coronagraph field of view that displays
a predominantly outward motion. The solar corona material is massive in
size (they can occupy up to a quarter of the solar limb),
and frequently accompanied by the remnants of an eruptive prominence,
and less often by a strong solar flare.
CMEs are the crucial link between a solar disturbance, its propagation
through the heliosphere,
and the effects on the Earth.
rays/radiation -- A steady drizzle of high energy ions arriving
at the solar system from the distant universe. Their energies are enormous,
ranging from 1-2 billion electron volts to perhaps 100,000,000 that much,
though the higher energies are rare. Their total energy flow is comparable
to that of starlight. The origin of their huge energies is thought to
come from expanding shock fronts created by huge cosmic explosions such
compact source of magnetic force, with two magnetic poles. A bar magnet,
coil or current loop, if their size is small, create a dipole field. The
Earth’s field, as a crude approximation, also resembles that of a dipole,
located near the Earth’s center.
magnetically trapped ion or electron moves as if it were attached to a
magnetic field line. Drift is one of the features of such motion, namely
its slow shift from one guiding field line to its neighbor. In the Earth’s
magnetic field, such drifts gradually move particles all the way around
Earth. Viewed from far above the north magnetic pole, ions drift around
the Earth clockwise, electrons counter-clockwise, resulting in an electric
current circling the Earth, the ring current.
radius (RE) -- the average radius of the Earth, a convenient
unit of distance in describing phenomena and orbits in the Earth?s neighborhood
in space. 1 RE = 6371 km = 3960 miles, approximately.
(solar) -- occurs when the Moon
passes between Earth
and the Sun,
thereby totally or partially obscuring Earth's view of the Sun. This configuration
can only happen during a new
moon, when the Sun and Moon are exactly lined up as seen from
the Earth. Total solar eclipses are very rare events for
any given place because totality is only seen where the Moon's shadow
touches the Earth's surface, and only last for a few minutes.
-- the plane in which the Earth orbits the Sun
Electric charge -- that which causes electrons and ions to attract each other, and to repel particles of the same kind. The electric charge of electrons is called "negative" (-) and that of ions "positive" (+). Materials such as glass, fur and cloth acquire an electric charge by rubbing against each other, a process which tears electrons off one substance and attaches it to the other. Electric charges (+) and (-) may also be separated by a chemical process, as in an electric battery.
current--a continuous flow of electrons and/or ions through
a material which conducts electricity. A current usually flows in a closed
circuit, without beginning or end. In daily life currents are generally
driven through wires by voltages produced by batteries or generators.
In space plasmas, some currents may be produced this way, but many are
inherent to the way ions and electrons move through magnetic fields, e.g.
field -- the region in which electric forces can be observed,
e.g. near an electric charge. As a field, it may also be viewed as a region
of space modified by the presence of electric charges.
interference – see interference
field/wave -- a combination of oscillating magnetic and electric
fields, spreading in wavelike fashion through space at a speed of about
300 000 km/sec. Such waves include all forms of light--infra-red and ultra-violet,
visible light, as well as radio waves, microwaves, X-rays and gamma rays.
-- a lightweight particle, carrying a negative electric charge and found
in all atoms. Electrons can be energized or even torn from atoms by light
and by collisions, and they are responsible for many electric phenomena
in solid matter and in plasmas.
volt (ev)--a convenient unit of energy applied to ions and
electrons, equal to the energy gains when such particles "fall" across
a voltage difference of 1 volt. Gas molecules at room temperature have
about 0.03 ev, on the Sun’s face about 0.6 ev, typical electrons of the
aurora 5000 ev, typical protons in the inner radiation belt 20,000,000
ev, typical cosmic ray protons near Earth 10,000,000,000 ev, and the highest
energies of cosmic rays may reach up to 10,000,000,000 times more.
particles--charged atomic particles moving rapidly, often at
a significant fraction of the speed of light. They can penetrate matter,
ionize the material which they traverse, and emit energetic photons (e.g.
-- loosely, anything that can cause a machine to move. For example, energy
is contained in moving water, water raised to a high place, heat or magnetic
fields. The energy of fast ions and electrons (measured in "electron volts")
is a measure of their speed, and it enables them (for instance) to penetrate
Low Frequency (ELF) -- that portion of the radio frequency
spectrum from 30 to 3000 hertz.
Farside (solar) – The half of the Sun not seen from Earth at any given time is colloquially referred to as the “farside”. It is somewhat akin to the back side of the Moon, although the Sun does not keep one side locked towards the Earth as the Moon does.
-- the number of back-and-forth cycles per second in a wave
or wave-like process. Expressed this way, the frequency is said to be
given in units of Hertz (Hz), named after the scientist who first produced
and observed radio waves in the lab. Alternating current in homes in the
Gamma ray bursts (GRB) -- short-lived sudden explosions of gamma-ray photons, the most energetic form of light. Lasting anywhere from a few milliseconds to several minutes, gamma-ray bursts shine hundreds of times brighter than a typical supernova and about a million trillion times as bright as the Sun, making them briefly the brightest source of cosmic gamma-ray photons in the observable universe. They can be associated with supernovae, pulsars, convergence of neutron stars or Black Holes, and other energetic cosmic cataclysms.
rays -- electromagnetic waves of the smallest wavelengths and
highest frequencies known; the most energetic of waves in the electromagnetic
spectrum. Gamma rays are generated by transitions within atomic
nuclei such as those in radioactive atoms and nuclear explosions.
This high energy radiation (energies in excess of 100 keV) is observed
during large, extremely energetic solar flares.
-- The unit of magnetic induction in the cgs (centimeter-gram-second)
Field -- The magnetic field observed in and around the
Earth. The intensity of the magnetic field at the Earth's surface is approximately
0.32 gauss at the equator and 0.62 gauss at the North Pole.
Storm -- A worldwide disturbance of the Earth's magnetic
field, distinct from regular diurnal variations.
Geosynchronous – The term applied to any equatorial satellite with an orbital velocity equal to the rotational velocity of the Earth. The net effect is that the satellite is virtually motionless with respect to an observer on the ground.
-- Greenwich Mean Time. See Universal Time
satellites – A collection of Geostationary
Operational Environmental Satellites circling
the Earth in a geosynchronous orbit, allowing them to hover continuously
over one position on the surface. The geosynchronous plane is about 35,800
km (22,300 miles) above the Earth, high enough to allow the satellites
a full-disc view. They provide a constant vigil for the atmospheric triggers
for severe weather conditions such as tornadoes, flash floods, hail storms,
and hurricanes. They also monitor solar flares and activity
on the Sun.
Heliopause -- The region in space where the Sun's atmosphere merges with interstellar space. The position of the heliopause depends both on the strength of the solar wind and on the properties of the local interstellar medium.
Heliosphere -- The region in space that extends to the heliopause. The heliosphere is the cavity around the Sun in the local interstellar medium that is produced by the solar wind. The heliosphere contains most of the solar system, but not the most distant comets such as in the Oort cloud.
Frequency (HF) -- that portion of the radio
frequency spectrum between 3 and 30 MHz.
Interplanetary Magnetic Field
polarity -- the general direction of interplanetary magnetic
field lines in a certain location (e.g. near Earth), i.e. whether the
field lines head away from the Sun ("away polarity") or towards it ("towards
polarity"). The IMF polarity determines which of the polar caps of the
Earth is magnetically linked to the Sun and gets polar rain guided towards
(electrical) -- electrical noise induced upon the signal wires
that obscures the wanted information signal; inhibition or prevention
of clear reception of broadcast signals.
Heliophysical Year (IHY) 2007-9 -- an international science
and education based celebration of the 50th anniversary of
the International Geophysical Year 1957. Its goal us to advance our understanding
of the fundamental processes that govern the Sun, Earth, and heliosphere
and to demonstrate the beauty, relevance, and significance of space and
Earth science to the world. See http://ihy2007.org/.
magnetic field (IMF) -- The magnetic field carried with the
wind. The IMF is kept out of most of the Earth’s magnetosphere, but
the interaction of the two plays a major role in the flow of energy from
the solar wind to Earth’s environment.
shock -- the abrupt boundary formed at the front of a plasma
cloud (e.g. one from a coronal mass ejection) if it pushes its way through
interplanetary space much faster than the rest of the solar wind.
-- Usually, an atom from which one or more electrons has been torn
off, leaving a positively charged particle. "Negative ions" are atoms
which have acquired one or more extra electrons. Clusters of atoms can
also become ions.
-- The process by which a neutral atom, or a cluster of such atoms, becomes
an ion. This may occur, for instance, by absorption of light ("photoionization")
or by a collision with a fast particle ("impact ionization").
-- A region covering the highest layers in the Earth’s atmosphere containing
an appreciable population of ions and free electrons. The ions are created
by sunlight ranging from the ultra-violet to X-rays and from cosmic rays.
The ionosphere significantly influences radio wave propagation of frequencies
less than about 30 MHz.
LASCO – The Large Angle and Spectrometric Coronagraph Experiment aboard the Solar and Heliophysics Observatory (SOHO) spacecraft. See http://lasco-www.nrl.navy.mil/
– a super-magnetized neutron star.
field lines -- lines in space, used for visually representing
magnetic fields. At any point in space, the local field line points in
the direction of the magnetic force which an isolated magnetic pole at
that point would experience. In plasma, magnetic field lines guide the
motion of ions and electrons and direct the flow of some electric currents.
poles -- A term with two meanings: 1) the points
on the surface of the Earth towards which the compass needle points. (Several
slightly different definitions exist, because the field is not exactly
that of a dipole.) 2) A concentrated source of magnetic force,
e.g. a bar magnet has two magnetic poles near its ends.
storm -- A large-scale disturbance of the magnetosphere, often
initiated by the arrival of an interplanetary shock originating at the
Sun. A magnetic storm is marked by the injection of an appreciable number
of ions from the magnetotail into the ring current, a process accompanied
by increased auroral displays. The strengthened ring current causes a
world-wide drop in the equatorial magnetic field, taking perhaps 12 hours
to reach its greatest intensity, followed by a more gradual recovery.
Magnetogram -- a graphic representation of solar magnetic field strengths and polarity.
– an instrument for measuring magnetic fields.
--The boundary of the magnetosphere, separating plasma attached to Earth
from the one flowing with the solar wind.
-- The region between the magnetopause and the bow shock, containing
solar wind which has been slowed down by passage through the bow shock.
As the magnetosheath plasma streams away from the bow shock, it gradually
regains its former velocity.
region around Earth, bounded by the magnetopause, whose processes are
dominated by the Earth’s magnetic field.
Magnetotail -- The long stretched-out night side of the magnetosphere, the region in which substorms begin. It starts about 8 Earth radii (RE) nightward of the Earth and has been observed to distances of at least 220 RE.
Doppler Imager (MDI) – an instrument on the SOHO spacecraft,
designed to study the interior structure and dynamics of the Sun. MDI
is a project of the Stanford-Lockheed Institute for Space Research and
is a joint effort of the Solar Oscillations Investigation (SOI)
in the W.W. Hansen Experimental Physics Laboratory of Stanford University
and the Solar and Astrophysics Laboratory of the Lockheed-Martin Advanced
Technology Center. See http://soi.stanford.edu/
Lights--an older name for the polar aurora.
-- the line followed by a spacecraft or a celestial body in motion around
-- in general, a charged component of an atom; that is, an ion or
--colloquially, a "packet of light." Although light spreads as an electromagnetic
wave, it can be created or absorbed only in discrete amounts of energy,
known as photons. The energy of a photon is greater the shorter the wavelength--smallest
for radio waves, larger for visible light, largest for X-rays and gamma
layer of the Sun from which all visible light reaches us. The Sun is too
hot to have a solid surface and the photosphere consists of a plasma at
about 6000 degrees centigrade. Sunspots
are observed in the photosphere.
– a bright feature found in the vicinity of most active sunspot
groups; occurs on a larger scale and is brighter than a facula.
Plage is French for "beach," because each plage looks like light-colored
sand against the darker structures around them.
-- any gas containing free ions and electrons, and therefore capable of
conducting electric currents. A partially ionized plasma such as the Earth’s
ionosphere is one that also contains neutral atoms.
Polar orbit --a satellite orbit passing over both poles of the Earth. During a 12-hour day, a satellite in such an orbit can observe all points on Earth.
-- an ion of hydrogen and one of the fundamental building blocks from
which atomic nuclei are made.
--a term with two broad meanings: 1) In the narrow sense,
some type of electromagnetic wave: radio, microwave, light (infra-red,
visible or ultra-violet), X-rays or gamma rays are all types of radiation.
2) Colloquially, the full term is "ionizing radiation" and means any spreading
emission which can penetrate matter and ionize its atoms. That includes
X-rays and gamma rays, but also high-energy ions and electrons emitted
by radioactive substances, accelerated by laboratory devices or encountered
in space (e.g. the radiation belt and cosmic rays, also known as cosmic
belt --The region of high-energy particles trapped in the Earth’s
waves -- Radio waves have the longest wavelengths in the electromagnetic
spectrum, ranging from about the size of a football to many miles in length.
--Instability of some atomic nuclei, causing them to change spontaneously
to a lower energy level or to modify the number of protons and neutrons
they contain. The 3 "classical" types of radioactive emissions are (1)
alpha particles, nuclei of helium (2) beta-rays, fast electrons and (3)
gamma-rays, high-energy photons.
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 trapped charged particles. The ring current is greatly
augmented during magnetic storms because of the hot plasma injected from
-- To throw or bend back (light, for example) from a surface.
– To bend or change direction, as in the light refracted into a rainbow.
-- a jargon term for radio signals induced by lightning.
-- A sudden transition at the front of a fast flow of plasma or gas when
that flow moves too fast for the undisturbed gas ahead of it to get out
of its way. Also occurs when a steady fast flow hits an obstacle.
Monitor – see Sudden Ionospheric Disturbance Monitor
Cycle -- The approximately 11-year quasi-periodic
variation in frequency or number of solar active events. See
also “Sunspot cycle.”
energetic particles -- high energy particles occasionally emitted
from active areas on the Sun, usually associated with solar flares and
coronal mass ejections. The Earth’s magnetic field keeps them out of regions
close to Earth (except for the polar caps) but they can pose a hazard
to space travelers far from Earth.
flare – see Flare
wind -- The outward flux of solar particles and magnetic fields
from the Sun. The solar wind is produced primarily in the cooler regions
of the corona,
known as coronal
holes, and flows along the open magnetic field lines. Typically,
solar wind velocities are 300-500 km per second.
Weather--the popular name for energy-releasing phenomena from
the Sun and its effects on planetary systems. Space weather
conditions can influence the performance and reliability of space-borne
and ground-based technological systems and endanger human life or health.
(auroral) -- a process by which plasma in the Earth’s magnetotail
becomes energized at a fast rate, flowing earthward and producing bright
auroras for typical durations of half an hour.
Sudden Ionospheric Disturbance (SID) – an abnormally high plasma density in the Earth’s ionosphere caused by an occasional sudden solar flare; SIDs often interrupt or interfere with telecommunications systems.
Ionospheric Disturbance (SID) Monitor - an instrument
which tracks sudden changes to the Earth’s ionosphere caused by solar
-- the star at the center of our solar system. The Sun keeps
Earth warm and sustains life on it, and it also emits the solar wind and
occasional bursts of solar energetic particles.
-- concentrations of magnetic flux, typically occurring in bipolar (i.e.
two-part with positive and negative poles, like a magnet) clusters or
groups. They appear dark because they are cooler than the surrounding
photosphere. They are cooler than the surrounding photosphere because
the magnetic field interferes with the outflow of solar heat in that region.)
Sunspots tend to be associated with violent solar outbursts of various
cycle (or solar cycle)--an irregular cycle, averaging about
11 years in length, during which the number of sunspots (and of their
associated outbursts) rises and then drops again. Like sunspots, the cycle
is magnetic in nature, and the polar magnetic field of the Sun also reverses
each solar cycle, making the true cycle about 22 years long.
orbit --a near-Earth orbit resembling that of a polar satellite,
but inclined to it by a small angle. With an appropriate value for the
inclination angle, the equatorial bulge causes the orbit to rotate during
the year once around the polar axis. Such a satellite then maintains a
fixed position relative to the Sun and can, for instance, avoid entering
the Earth’s shadow.
a large explosion at the end of the evolutionary process of a very massive
star.2) an explosion of material from a white dwarf star after it has
been accumulating mass from a binary companion. An enormous amount of
energy is released in these explosions.
orbit -- a circular orbit around the Earth?s equator, at a
distance of 6.6 Earth radii. At this distance the orbital period is 24
hours, keeping the satellite "anchored" above the same spot on Earth.
This feature makes the synchronous orbit useful for communication satellites
and satellites transmitting TV programs.
gamma-ray flashes (TGFs) -- bursts of gamma
rays in the Earth's atmosphere, probably caused by electric
fields produced above thunderstorms
. TGFs have been recorded to last 0.2 to 3.5 milliseconds,
and have energies
of up to 20 MeV.
-- the dividing line between the bright and shaded regions (usually shaded
from the light of the Sun) of the disk of a moon or planet.
High Frequency (UHF) -- Those radio frequencies
exceeding 300 MHz.
Ultraviolet (UV) -- electromagnetic radiation lying in the ultraviolet range , i.e. wavelengths shorter than visible light but longer than X-rays. UV cannot be seen by the eye.
Low Frequency (VLF) -- That portion of the
radio frequency spectrum from 3 to 30 kHz.
light – see White Light
(Earth’s ionosphere) – a structure that guides electromagnetic
waves along its length. The space between the Earth’s surface and the
ionosphere makes an excellent waveguide for VLF radio transmissions.
Light, visible light -- Sunlight integrated over the visible
portion of the spectrum (4000 - 7000 angstroms) so that all colors are
blended to appear white to the eye.
Burst – In solar-terrestrial terms, a temporary sudden enhancement
of the X-ray emission of the Sun. These bursts can also be caused
by thermonuclear explosions on the surface of a neutron star accreting
material from a binary companion.
X-rays-- a type of high-energy electromagnetic radiation with wavelengths of about 10-10 meters. X-ray photons are generated by energetic electron processes. The hot outer atmospheres, or coronas, of normal stars such as our Sun produce X-rays, as do the cataclysmic explosions of supernovae, accreting or merging neutron stars, and Black Holes.
Whistlers -- A type of VLF electromagnetic signal generated by some lightning discharges. Whistlers propagate along geomagnetic field lines and can travel back and forth several times between the Northern and Southern Hemispheres. So named from the sound they produce in radio receivers.
< Back •