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Absorption Line
- 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. A spectral line is seen in emission when the radiating source is seen directly.
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Active Region
- An area of the Sun with a particularly large cluster of strong magnetic field. At ultraviolet and
X-ray wavelengths they appear bright. In visible light they often exhibit sunspots.
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Angstrom
- Abbreviated Å. A unit of length equal to 10^-8 cm (a one-hundred millionth of a
centimeter). An Angstrom is about the size of an atom, and 10 Angstroms are equal
to one nanometer.
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Arc Degree
- A unit of angular measure in which there are 360 arc degrees in a full circle.
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Arc Minute
- 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 arc minute.
For example 3' means 3 arc minutes.
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Arc Second
- 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. At the Earth-Sun distance, one arc second is equal to about
725 km on the Sun's surface.
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Astronomical Unit
- Often defined as the average distance between the Earth and Sun, about 150 million
kilometers. 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/G days, where G is the Gaussian gravitational constant! Now
you can see why people cheat and use the first "definition"!
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Atom
- The basic building block of all matter. An atom 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). If the atom is charge-neutral, it has the same number of electrons and protons.
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Aurora
- A display of colored 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.
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Black Hole
- A region of space that has so much mass concentrated in it, and therefore such a
strong gravitational pull, that a nearby object, or even light, cannot
escape the gravitational pull.
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Blueshift
- See Doppler Shift.
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Bow Shock
- A collisionless shock wave in front of the magnetosphere arising from the interaction of the supersonic solar wind with the planet's magnetic field. (Suess, p.154)
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CDS
- 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 nanometers. Many of the spectra and images illustrated on these web
pages were obtained with the CDS.
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Celsius
- 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 at sea level.
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CELIAS
- Charge, Element, and Isotope Analysis System.
Instrument aboard SOHO which analyzes the constituents of the solar wind.
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Centigrade
- 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.
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CGS
- Centimeter-Gram-Second. The system of measurement that uses these units for
distance, mass, and time.
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Charge-Coupled Device (CCD)
- An electronic device used for data acquisition, composed of many tiny pixels, each of which records a buildup of charge to measure the amount of light striking it. (See schematic picture.)
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Chromosphere
- The layer of the solar atmosphere that is located above the photosphere and beneath
the transition region and the corona. The chromosphere is about 10,000K, which is hotter than the
photosphere but not as hot as the corona.
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Conduction
- The transfer of energy via collisions of randomly moving atoms and electrons.
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Convection
- The physical overturning of a fluid or gas driven by temperature gradient, thus transporting energy from a lower,
hotter region to a higher, cooler region. Convection can occur when there is a
substantial decrease in temperature with height, such as in the Sun's convection
zone. This is where gas becomes opaque so that energy needs to be transported by other means than radiation.
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Convection Zone
- An interior layer of the Sun, which can extend deep into the Sun, where energy is transported outward by convective motions.
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Core
- In solar astronomy, the innermost part of the Sun, where energy is generated by
nuclear reactions.
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Corona
- 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.
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Coronagraph
- 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 (when the moon blots out the Sun's disk) to study the
corona.
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Coronal Hole
- 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.
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Coronal Mass Ejection
- A huge bubble of plasma that erupts from the Sun's corona and travels through
space at high speed.
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Coronal Streamer
- Large scale magnetic structures observed in the Sun's corona.
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Cosmic Ray
- High energy charged particles traveling through interstellar space at nearly the
velocity of light.
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COSTEP
- Comprehensive Suprathermal and Energetic Particle Analyzer.
Instrument aboard SOHO which analyzes high energy particles in the solar wind.
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Cusps
- The cusps mark the division between geomagnetic field lines on the sunward side and the field lines in the polar cap that are swept back into the magnetotail by the solar wind. (Suess, p.156)
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Deep Space Network
- A NASA network of communication dishes used to communicate with spacecraft
beyond Earth's orbit.
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Density
- 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
centimeter).
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Differential Rotation
- The change in rotation rate with latitude. Solid bodies like the Earth do not show
this effect but the giant planets (e.g. 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. Thus, the gas at the equator takes 25.7 days, while the gas at the poles takes 30 days to make a single rotation.
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Disk
- The visible surface of the Sun (or any heavenly body) projected against the sky.
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Doppler Shift
- 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). An analogous effect in sound waves is what causes a higher pitch from a siren as an emergency vehicle moves toward you and a lower pitch as it moves away.
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Eclipse Season
- This is when TRACE goes through a day/night cycle every orbit rather than seeing the Sun 100% of the time.
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Ecliptic
- The plane of Earth's orbit around the Sun.
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EIT
- 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.
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Electromagnetic Radiation
- Visible 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.
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Electromagnetic Spectrum
- The entire range of all the various kinds or wavelengths of electromagnetic
radiation, including (from short to long wavelengths) gamma rays, X-rays,
ultraviolet, optical (visible), infrared, and radio waves.
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Electron
- A negatively charged elementary particle that normally orbits the nucleus of an
atom.
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Emission Line
- A bright section of a spectrum at a particular wavelength. Usually emitted by a hot
gas.
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Equator
- The great circle on the surface of an approximately spherical body formed by the intersection of the surface
of the body and the plane passing through the center of the body at right angles to
the axis of rotation.
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ESA
- The European Space Agency.
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Erg
- A cgs unit of energy equal to work done by a force of 1 dyne acting over a distance
of 1 cm.
10^7 (ten million) erg s^-1 (ergs per second) = 1 watt. Also, 1 Calorie = 4.2 × 1010
(42 billion) ergs.
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ERNE
- Energetic and Relativistic Nuclei and Electron experiment.
Instrument aboard SOHO which analyzes high energy nuclei and electrons in the
solar wind.
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Filament
- 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. Filaments were first observed in the non-coronal layers of the Sun.
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Flare (Solar)
- Rapid release of energy from a localized region on the Sun in the form of
electromagnetic radiation, energetic particles, and mass motions.
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Free Electron
- An electron that has broken free of its atomic bond and is therefore not bound to an
atom.
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Frequency
- 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.
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Gamma Ray
- The highest energy (shortest wavelength) photons in the electromagnetic spectrum.
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Gauss
- A unit of magnetic field strength. The Earth's magnetic field has a strength of
about 0.5 Gauss at the surface.
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Geomagnetic Storm
- A worldwide disturbance of the Earth's magnetic field, associated with solar
activity.
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Geosynchronous Orbit
- 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).
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GOES
- A flare classification scheme that uses a number following a letter. The number acts as a multiplier while the letter signifies the importance. A B flare has a peak flux of 10^-6 watts per meter^2, a C flare has a peak flux of 10^-5 watts per meter^2, a M flare has a peak flux of 10^-4 watts per meter^2, and a X flare has a peak flux of 10^-3 watts per meter^2. An example of how this works is a C2 flare stands for a flare with a peak flux of 3 * 10^-5 watts per meter^2.
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GOLF
- Global Oscillations at Low Frequencies.
Helioseismology instrument aboard SOHO which analyzes the vibrational modes
of the Sun.
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Granule
- A roughly circular region on the Sun whose bright center 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.
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Gyroscope
- A gyroscope, or gyro, is any instrument which uses a rapidly spinning mass to sense and respond to changes in the inertial orientation of its
spin axis. TRACE uses rate sensing gyros which measure the spacecraft rotation in each of three axes. The measured values are sent to the
ACS computer which accumulates these measurements over time to determine how far the spacecraft has moved.
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Helioseismology
- The study of the interior of the Sun by the analysis of its natural modes of
oscillation.
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Heliosphere
- The region around the Sun where the solar wind dominates over the interstellar
medium.
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Hertz
- Abbreviated Hz. A unit of frequency equal to one cycle per second. One kHz = 1000
Hz. One MHz = 10^6 (one million) Hz. One GHz = 10^9 Hz.
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Hydrogen Alpha
- Light emitted at a wavelength of 656.3 nanometers from an atomic transition in
hydrogen. This wavelength is in the red portion of the visible spectrum and is
emitted in the solar chromosphere.
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Image Stabilization
- Image stabilization is the process of removing unwanted image motion. TRACE uses the guide telescope to examine how far the edge of the
Sun moves and then uses this measurement to move the secondary mirror a corresponding amount, thus stabilizing the image on the CCD.
The instrument processor samples data from the guide telescope 300 times a second stabilizing the images from up to 20 arcseconds down to
0.1 arcsecond!
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Infrared
- 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.
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Ion
- An atom that has lost or gained one or more electrons and has become electrically
charged as a result. 99% of the atoms in the universe are ions.
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Ionization
- The process by which ions are produced, typically occurring by collisions with
atoms or electrons ("collisional ionization"), or by interaction with electromagnetic
radiation ("photoionization").
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Ionosphere
- The region of the Earth's upper atmosphere containing a small percentage of free
electrons and ions produced by photoionization of the constituents of the
atmosphere by solar ultraviolet radiation. The ionosphere significantly influences
radiowave propagation of frequencies less than about 30 MHz.
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Isotope
- One of two or more atoms having the same number of protons in its nucleus, but a
different number of neutrons and, therefore, a different mass.
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ISTP
- 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.
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Kelvin
- Abbreviated K. A unit of absolute temperature. Zero degrees Celsius is equal to 273
Kelvin. Zero Kelvin is "absolute zero" - the coldest temperature possible.
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Kilometer
- Abbreviated km. 1 km = 1000 meters = 0.62 mile.
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Lagrange Points
- The five "gravitational balance" point for 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?
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LASCO
- Large Angle and Spectrometric Coronagraph. Set of three Coronagraphs aboard SOHO, that image the solar corona from 1.1 to 32 solar radii.
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Latitude
- 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.
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Light Year
- The distance light travels in 1 year (approximately 9.5 million million kilometers).
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Limb
- See Solar Limb.
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Longitude
- An east-west coordinate measured on the surface of a sphere.
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Magnetic Field
- 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.
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Magnetic Field Lines
- 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.
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Magnetogram
- A map showing the strength of the magnetic field in different locations. (See accompanying example, taken for the solar disk by MDI)
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Magnetometer
- A magnetometer senses the Earth's magnetic field in three dimensions. The Earth's magnetic field is fairly constant and can be mapped.
Similar to the way a compass senses the north pole, a magnetometer combined with a good map of the Earth's field strength stored in the
flight computer can provide a coarse attitude knowledge.
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Magnetosheath
- The region between a planet's bow shock and the magnetopause, characterized by turbulent plasma. The plasma has been heated and slowed as it passed through the bow shock. (Suess, p.163)
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Magnetosphere
- Area around a planet in which the planet's magnetic field is stronger than the
magnetic field carried by the solar wind.
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Magnetotail
- The extension of the magnetosphere in the antisunward direction as a result of interaction with the solar wind. (Suess, p.163)
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MDI/SOI
- 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.
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Moss
- A thin layer with a temperature between
0.5 and 2 million Kelvin, located only a few thousand kilometers above
the solar surface, underneath much hotter material overhead. The emission
pattern of the moss shows the interface between the hot corona and the much cooler chromosphere. This interface changes continually as the
magnetic fields and the atmospheric heating shift around. The resemblance
to moss on any single image gave it its name.
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Naked eye
- Using only the human eye for observations. That is with no artificial help, e.g.
binoculars or a telescope.
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NASA
- The National Aeronautics and Space Administration
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Neutrino
- An elementary particle with no charge and no mass or almost no mass, which interacts very
weakly with other matter.
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Neutron
- An electrically neutral elementary particle. A neutron is 1839 times heavier than an
electron.
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Nucleus
- The positively charged core of an atom, consisting of protons and neutrons (except
for hydrogen), around which electrons orbit.
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Optical Radiation
- Electromagnetic radiation (light) that is visible to the human eye.
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Orbiting Solar Observatories
- A series of eight Sun observing satellites launched from 1962 to 1975.
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Orbit
- The path of an object revolving around another object or point.
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Orbital Period
- The amount of time it takes a spacecraft or other object to travel once around its
orbit.
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Parsec
- 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.
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Penumbrae
- The outer region of a sunspot, surrounding the umbra, which is not as dark, and not as cool as the central region.
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Photodiodes
- Photodiodes are semiconducting devices that convert light into electrical signals. All photodiodes work on the same principle which is based
on photoconductivity. Photoconductivity is the increase of current in a semiconductor due to the absorption of photons. A photoconductor
converts information and energy from an optical form to an electronic form. The electric signals generated by the guide telescope
photodiodes correspond to attitude errors which the ACS nulls out.
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Photon
- A discrete quantity of electromagnetic energy. Short wavelength (high frequency)
photons carry more energy than long wavelength (low frequency) photons. See
Electromagnetic Radiation.
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Photosphere
- 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.
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Planetary Nebula
- The ejected envelope of a red giant star, spread over a volume roughly the size of our solar system
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Plasma
- 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 behavior than that of neutral (unionized) matter.
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.
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Polar Plume
- Bright structure of out-flowing gas which occurs 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.
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Pole
- The intersections of the surface of a body and its axis of rotation.
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Prominence
- 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.
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Proton
- A positively charged elementary particle. A proton is 1836 times heavier than an
electron.
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Quiet Sun
- Any area of the Sun that is covered with relatively little
magnetic field. It is characterized by a pepper-and-salt mixture of
north and south polarities embedded in the network formed by the
large convective cells of the supergranulation. The corona over
the quiet Sun is relatively dim and cooler than in active regions.
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Radiation
- See Electromagnetic Radiation
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Radiation Belt
- 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.
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Radiative Zone
- An interior layer of the Sun that extends into the core and tends to be below the convection zone. In this layer energy travels outward by radiation.
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Radio waves
- The lowest energy (longest wavelength) photons in the electromagnetic spectrum.
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Red Giant
- A giant star whose surface temperature is relatively low, so that it glows with a red color.
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Redshift
- See Doppler Shift.
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Resistance
- Opposition to the flow of electric current.
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Resistor
- A device that is designed to have a specific resistance.
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Skylab
- A space station which orbited Earth in the 1970s. On Skylab, astronauts performed many important
observations of the Sun.
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SOHO
- The SOlar and Heliospheric Observatory, a joint ESA/NASA mission. The largest and most comprehensive
solar observatory ever put into space. It was launched in December 1995.
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Solar Atmosphere
- 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.
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Solar Cycle
- The approximately 11-year variation in frequency or number of sunspots, coronal
mass ejections, solar flares, and other solar activity.
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Solar Disk
- The glowing surface of the Sun that we see with our eyes or through a heavily filtered telescope.
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Solar Limb
- The apparent edge of the Sun as it is seen in the sky.
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Solar Maximum
- The month(s) during the solar cycle when the number of sunspots reaches a
maximum. The next solar maximum will occur in about the year 2000.
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Solar Maximum Mission
- A satellite dedicated to observing the Sun, especially solar flares, which occur more
frequently during solar maximum. It was in orbit throughout the 1980s.
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Solar Minimum
- The month(s) during the solar cycle when the number sunspots is lowest. The most
recent minimum occurred in 1996.
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Solar Wind
- A stream of particles, primarily