A region of low intensity at a particular wavelength in a spectrum, formed when a cool, tenuous gas
between a hot radiating source and the observer absorbs electromagnetic
radiation of that wavelength. The opposite effect is
seen as an emission line.
Abbreviated Å. A unit of length equal to 10-8
cm (a one-hundred millionth of a centimetre). An
Angstrom is about the size of an atom, and 10 Angstroms
are equal to one nanometre.
Abbreviated arcmin. A unit of angular measure
in which there are 60 arc minutes in 1 arc
degree The single apostrophe is usually used as the symbol
for an arcminute. For example 3' means 3 arcminutes.
Abbreviated arcsec. A unit of angular measure in
which there are 60 arc seconds in 1 arc minute and
therefore 3600 arc seconds in 1 arc
degree.
The double quotation mark is usually used as the symbol
for an arcsecond. For example 3" means 3 arcseconds.
When looking at the Sun from Earth, one arc second is equal
to about 725 km on the Sun's surface.
Often defined as the average distance between the Earth and Sun, about 150 million
kilometres. But if you really want to impress (or bore) people,
insist on the correct definition which is that the Astronomical
Unit is the radius of a circular orbit in which an object of
negligible mass, and free of other disturbances, would revolve
around the Sun in 2pi/k days, where k is the Gaussian gravitational
constant! Now you can see why people cheat and use the first
"definition"!
The basic building block of all matter. All atoms consist
of a nucleus containing protons and neutrons surrounded by
different numbers of electrons. The different number of protons
in the nucleus is what make the atoms appear as
different elements
(for example, Helium, Oxygen, Carbon).
A display of coloured light given off by collisions between charged
particles trapped in a planet's
magnetic fields and atoms of atmospheric gases
near the planet's magnetic poles. Aurora are visible on Earth as
the aurora borealis or northern lights and the
aurora australis or southern lights.
A region of space that has so much mass concentrated in
it, and therefore such a strong gravitational pull, that there
is no way for a nearby object, or even light, to escape the
gravitational pull.
Coronal Diagnostic Spectrometer,
or in other words it is a
Spectrometer designed to Diagnose the solar Corona - so maybe
it should have been called the SDC!
An ultraviolet spectrometer aboard
SOHO, designed to study the solar spectrum
from 18 to 80 nanometres.
Many of the spectra and images
illustrated on these web pages were obtained with the CDS.
Abbreviated C. A unit of temperature. Zero degrees
Celsius is equal to 273 Kelvin.
Also known as centigrade. Water freezes at 0° C
and boils at 100° C.
Abbreviated C. A unit of temperature. Zero degrees
Centigrade is equal to 273 Kelvin.
Also known as celsius. Water freezes at 0° C
and boils at 100° C. In the SunBlock pages the distinction
between Kelvin and Centigrade is usually ignored since temperatures
cannot be measured accurately enough to make the difference
important.
The layer of the solar
atmosphere that is located above the photosphere and beneath the
transition region and the corona.
The chromosphere is hotter than the photosphere but not
as hot as the corona.
The physical up-welling of hot matter, thus transporting
energy from a lower, hotter region to a higher, cooler
region. A bubble of gas that is hotter than its
surroundings expands and rises. When it has cooled by
passing on its extra heat to its surroundings, the bubble
sinks again. Convection can occur when there is a
substantial decrease in temperature with height, such as
in the Sun's convection zone.
A layer in a star in which convection
is the main mechanism by which energy is
transported outward. In the Sun a convection zone
extends from just below the photosphere
to about seventy percent of the solar radius.
The outermost layer of the solar atmosphere. The corona
consists of a highly rarefied gas with a
temperature greater than one million degrees Kelvin. It is visible to the naked eye
during a solar eclipse.
Telescope for observing the corona by
producing an artificial eclipse.
It contains an occulting disk which covers the
disk of the Sun
so that the faint corona may be more easily observed. Before
coronagraphs were built astronomers had to wait for a natural solar
eclipse to blot out the Sun's disk in order to let them study
the corona.
An area of the corona which appears dark in X-rays and ultraviolet light.
They are usually located at the poles of the Sun, but can occur other
places as
well. The magnetic field lines
in a coronal hole extend out into the solar wind rather than
coming back down to the Sun's surface as they do in other parts
of the Sun.
The amount of mass or number of particles per unit
volume. In cgs units mass density has
units of gm cm-3. Number
density has units cm-3
(particles per cubic centimetre).
The change in rotation rate with latitude. Solid bodies like the
Earth do not show this effect but the giant planets (eg Jupiter and
Saturn) and the Sun do because they are composed of gas. Low
latitudes rotate at a faster angular rate. For the Sun the
difference is such that low latitudes near the equator rotate at
14 degrees per
day but the high latitudes at only 12 degrees per day.
A change in the wavelength of
radiation received
from a source
because of its motion along the line of sight. A Doppler shift in the
spectrum of an astronomical object is commonly known as a
redshift when the shift is towards longer wavelengths
(the object is moving away) and as a blueshift when the
shift is towards shorter wavelengths (the object is approaching).
Extreme ultraviolet Imaging Telescope.
Telescope aboard SOHO which obtains
images of the
Sun at ultraviolet wavelengths.
Many of the pictures featured on these web pages were taken with the
EIT.
Light we see is one kind of electromagnetic radiation - it has
a special range of wavelengths that our eyes can respond to.
In general, electromagnetic radiation is radiation that travels
through vacuous space at the speed
of light and propagates by the interplay of oscillating
electric and magnetic fields.
This radiation has a wavelength
and a frequency and transports energy.
The great circle on the surface of a body formed by
the intersection of the surface of the body and the plane
passing through the centre of the body at right angles to the
axis of rotation.
A cgs unit of energy equal to work
done by a force of 1 dyne acting over a distance of 1 cm.
107 (ten million) erg s-1 (ergs per
second) = 1 watt. Also, 1 Calorie = 4.2 × 1010
(42 billion) ergs.
A structure in the corona consisting
of cool plasma supported by
magnetic fields. Filaments are
dark structures when seen against the bright solar
disk,
but appear bright when seen over the solar limb,
Filaments seen over the limb are also known as
prominences.
The number of repetitions per unit time of the
oscillations of an
electromagnetic wave (or other wave). The higher the
frequency, the greater the energy of the radiation and the
smaller the wavelength.
Frequency is measured in Hertz.
The orbit of a satellite that travels
above the Earth's equator from west to east so that it
has a speed matching that of the Earth's rotation and
remains stationary in relation to the Earth (also called
geostationary). Such an orbit has an altitude of about
35,900 km (22,300 miles).
A roughly circular region on the Sun whose bright centre indicates
hot gases rising to the surface, and whose dark edges indicate
cooled gases that are descending towards the
interior. Individual granules appear and disappear on time
scales of about 5 minutes and are typically about 1000 km.
Light emitted at a wavelength of 656.3
nanometres
from an atomic transition in hydrogen. This wavelength is in
the red portion of the visible spectrum
and is emitted in the solar
chromosphere.
The part of the electromagnetic
spectrum whose radiation has
longer wavelengths than
optical radiation, but shorter wavelengths than
radio waves . Because much infrared light
is absorbed by the Earth's atmosphere, infrared astronomy is
often performed in space.
The process by which ions are
produced, typically occurring by collisions with atoms or
electrons ("collisional
ionization"), or by interaction with electromagnetic
radiation ("photoionization").
The region of the Earth's upper atmosphere containing a
small percentage of free
electrons and ions produced by photoionization of the
constituents of the atmosphere by solar ultraviolet
radiation. The ionosphere significantly influences radiowave
propagation of frequencies less than about 30 MHz.
International Solar-Terrestrial Physics Science Initiative.
Collaborative effort by US, European, and Japanese space agencies
to obtain coordinated, simultaneous investigations of
the Sun-Earth space environment over an extended period of time.
SOHO is a part of this program.
Abbreviated K. A unit of absolute temperature. Zero degrees
Celsius is equal to 273 Kelvin. Zero Kelvin is
"absolute zero" - the coldest temperature possible.
The five "gravitational balance" points between two orbiting masses.
The first Lagrange Point (L1) is in between the
two bodies. The SOHO spacecraft orbits the Sun at a place
near the (L1) point. It is often wrongly stated that
the (L1) point is where the gravitational pull of the
Sun and the Earth balance. From your knowledge of the mass of the
Sun and Earth and how the strength of gravity changes with distance
can you show why that statement is wrong?
A north-south coordinate measured on the surface of a sphere.
It is the angular distance from the equator in the direction
of one of the rotational poles.
A field of force that is generated by electric currents.
The Sun's average large-scale magnetic field, like that
of the Earth, exhibits a north and a south pole linked by
lines of magnetic force.
Imaginary lines that indicate the strength and direction
of a magnetic field. The
orientation of the line and an arrow show the direction
of the field. The lines are drawn closer together where
the field is stronger. Charged particles move freely
along magnetic field lines, but are inhibited by the
magnetic force from moving across field lines.
Michelson Doppler Imager/Solar Oscillations Investigation. Helioseismology
instrument aboard SOHO which analyzes
the vibrational modes of the Sun. Also measures the Sun's
magnetic field in the
photosphere.
A unit of distance equal to 3.16 light years.
The name is derived from PARallax and SECond since
one parsec is the distance at which the Earth's orbit
subtends an angle of one arcsecond.
A discrete quantity of electromagnetic energy. Short wavelength (high frequency)
photons carry more energy than long wavelength (low frequency) photons. See Electromagnetic
Radiation.
The visible surface of the Sun. It consists of a zone in
which the gaseous layers change from being completely
opaque to radiation to being transparent. It is the layer
from which the light we actually see (with the human eye)
is emitted.
Plasma consists of a gas heated to sufficiently high
temperatures that the atoms ionize.
The properties of the gas are controlled by
electromagnetic forces among constituent ions
and electrons, which results in a
different type of behaviour. Plasma is often considered
the fourth state of matter (besides solid, liquid, and
gas). Most of the matter in the Universe is in the plasma
state.
Bright structure of out-flowing gas which occur along
magnetic field lines in
coronal holes. These field
lines extend into the solar system. Although plumes usually
occur at the poles, they can appear anywhere there is a coronal
hole.
A structure in the corona consisting
of cool plasma supported by
magnetic fields. Prominences are
bright structures when seen over the solar limb,
but appear dark when seen against the bright solar
disk. Prominences seen on the disk are also known as
filaments.
A ring-shaped region around a planet in which
electrically charged particles (usually electrons and protons)
are trapped. The particles follow spiral trajectories
around the direction of the magnetic
field of the planet. The radiation belts surrounding
Earth are known as the Van
Allen belts.
An atmosphere is generally the
outermost gaseous layers of a planet, natural satellite,
or star. Only bodies with a strong gravitational pull can
retain an atmosphere. Atmosphere is used to describe the
outer layer of the Sun because it is relatively
transparent at visible wavelengths.
Parts of the solar atmosphere include the photosphere, chromosphere, and the corona.
A satellite dedicated to observing the Sun, especially solar
flares, which occur more frequently during solar maximum.
It was in orbit throughout the 1980s.
A stream of particles, primarily electrons
and protons, flowing outward
from the Sun at speeds as high as 900 km/s. The solar wind is
essentially the hot solar corona expanding
into interplanetary and interstellar space.
The region over the South Atlantic Ocean where the lower Van Allen belt of energetic,
electrically charged particles is particularly close to
the Earth's surface. The excess energy in the particles
presents a problem for satellites in orbit around the
Earth.
A line in a spectrum due to the
emission or absorption of electromagnetic
radiation at a discrete wavelength.
Spectral lines result from discrete changes in the energy
of an atom or molecule. Different atoms or molecules can
be identified by the unique sequence of spectral lines
associated with them.
An instrument that spreads light or other electromagnetic
radiation into its component wavelengths
(a spectrum), recording the results
photographically or electronically. From that it is possible to
measure the intensity of radiation as
a function of wavelength.
Electromagnetic
radiation arranged in order of
wavelength.
A rainbow is a natural spectrum of visible light from the
Sun. Spectra are often punctuated with emission or
absorption lines, which can be examined to reveal the
composition and motion of the radiating source.
Solar Ultraviolet Measurements of Emitted Radiation.
An ultraviolet spectrometer aboard
SOHO. A companion instrument to
CDS, but
which studied the solar spectrum at longer wavelengths (up to
160 nanometres).
A temporary disturbed area in the solar photosphere that
appears dark because it is cooler than the surrounding
areas. Sunspots consist of concentrations of strong magnetic flux. They usually
occur in pairs or groups of opposite polarity that move
in unison across the face of the Sun as it rotates.
Solar Wind Anisotropies.
Instrument aboard SOHO which analyzes
large scale variations in the solar wind
by observing
radiation given off by neutral hydrogen.
The combination of atomic nuclei
at high temperatures to form more massive nuclei with the
simultaneous release of energy.Thermonuclear
fusion is the power source at the core of the Sun.
Controlled thermonuclear fusion reactors, when
successfully implemented, could become an attractive
source of power on the Earth.
The Transition Region
and Coronal Explorer.
Launched in April
1998, this satellite is providing some of the best images of the
Sun's outer atmosphere ever seen.
The layer in the Solar Atmosphere located
above the chromosphere and below the
corona. In this region the
temperature appears to rise dramatically from ten thousand Kelvin to
over one million kelvin as you enter the lower corona. Recent
observations from SOHO
have found this region to be very dynamic in nature.
The part of the electromagnetic
spectrum whose radiation has
shorter wavelengths than
optical radiation, but longer wavelengths than
X-rays . Because ultraviolet light is absorbed
by the Earth's atmosphere, ultraviolet astronomy is performed in
space.
Abbreviated UT. A time system used internationally
for recording the timing of events, particularly in astronomy.
The same as Greenwich Mean Time (GMT).
The part of the electromagnetic
spectrum whose radiation has somewhat greater frequencies and smaller wavelengths than those of
ultraviolet radiation. Because x-rays are absorbed by the
Earth's atmosphere, x-ray astronomy is performed in
space.
