THEMIS E/PO : Glossary of Terms

A | B | C | D | E | F | G | H | I | J | K | L | M
N | O | P | Q | R | S | T | U | V | W | X | Y | Z

A

active region: An area of the solar atmosphere where the Sun's magnetic field is concentrated. The concentration and bipolar nature of the magnetic field results in the formation of dark areas such as sunspots and bright areas known as facula. These regions also produce flares and plages.

All-Sky imager: An instrument built to take images of the sky from horizon to horizon in order to observe the full sky from a point on Earth.

altitude: The vertical height above Earth's surface.

angstrom: A unit of length equal to one hundred-millionth of a centimeter.

astronomy: The science that studies the natural world beyond the Solar System.

astrophysics: The study of the history, structure and dynamics of objects in the universe.

Athabasca, AB, Canada: A town in the Alberta province in the country of Canada.

atom: A very small object with a diameter of approximately 0.1 nanometers that is the building block for everything in the Universe.

aurora: Light radiated by ions and atoms in the Earth's upper atmosphere, mostly in polar regions, the result of bombardment by energetic electrically charged particles from the ionosphere and magnetosphere.

Aurora Borealis: Aurora that occurs in the northern hemisphere (see Northern Lights).

auroral arc: Auroral light in the form of a regular bow, which often extends east-west.

auroral arc filaments: Very narrow auroral arcs on the order of 100 meters in width.

Auroral Australis: Aurora that occurs in the southern hemisphere (see Southern Lights).

auroral band: Auroral structure, either as "homogeneous bands" or as "rayed bands", resulting when an auroral arc loses its regular shape.

auroral eruption: A sudden change in the appearance of the aurora from a motionless single auroral arc to many moving auroral forms. This is also called aurora substorm onset.

auroral kilometric radiation: Intense radio waves whose wavelength is of the order of a kilometer, emitted from regions above the ionosphere where the aurora is (apparently) accelerated.

auroral oval: The instantaneous configuration of the global aurora.

auroral ray: Lines of auroral light, usually sharply defined, extending along Earth's
magnetic field lines.

B

bar magnet: A magnet in the shape of a bar with two poles (north and south).

bow shock: The shock wave that flanks the magnetosphere on the day side. It causes the solar wind flow to slow down and flow around the magnetosphere.

breakup phase: A phase in the development of aurora known as an auroral substorm in which many forms of aurora move rapidly and the auroral oval expands both towards Earth's magnetic poles and equators.

C

Cal Day: A day in April when departments at the University of California at Berkeley
showcase their instructional products and give information about their programs.

charged particles: Particles with positive or negative charge, for example electrons, protons, or ions. Particles with like charges repel and opposite
charges attract.

chromosphere: The part of the Sun's (or another star's) atmosphere between the photosphere and the corona.

compass: A tiny magnet that aligns itself with Earth's magnetic field and points north.

convection: Circulation of a fluid or gas.

corona: The Sun's outer atmosphere, with a temperature of greater than a million degrees, that gives rise to the solar wind.

coronal mass ejection: A vast magnetic blob of plasma that erupts from the Sun's corona and travels through space at high speed. Coronal mass ejections may cause intense geomagnetic storms when they strike the Earth and accelerate vast quantities of energetic particles in both interplanetary space and the magnetosphere.

cosmic ray: An extremely energetic (relativistic) charged particle.

cosmology: The study of the history, structure and dynamics of the Universe.

Cro-magnon: An early form of modern human who lived Europe in the late Paleolithic Period.

cow magnet: A strong little bar magnet with smooth edges used to treat a cow or bull who has eaten scraps of metal. All the metal (nails, bits of barbed wire) in the leathery first stomach clumps together around the cow magnet so it does not harm the insides of the cow.

current: Moving charged particles such that opposite charges are moving relative to one another, i.e. not at the same speed or in the same direction.

current disruption: A process that occurs in Earth's magnetosphere whereby current on the equatorial plane is disrupted and forced to move along Earth's
magnetic field into the upper atmosphere into the nighttime auroral oval.

D

data: The outcome of a set of measurements from which inferences may be drawn, theories constructed, and so forth.

dawn: The time of day immediately preceding sunrise.

dipole: A compact source of magnetic force, with two magnetic poles.

disk: The visible surface of the Sun projected against the sky.

dome: A vaulted roof having a circular, polygonal, or elliptical base and a generally hemispherical or semispherical shape.

dusk: The time of day immediately following sunset.

E

electric field: A map of the direction and magnitude of electric forces around charged particle.

excited state: A state in which an atom or molecule has absorbed a discrete amount of energy from a particle (including light particles).

extreme ultraviolet: Electromagnetic radiation, invisible to the naked eye, with wavelengths shorter than ultraviolet radiation and longer than x-rays. These wavelengths are mainly responsible for the existence of the ionosphere.

F

facula: Brighter-than-average regions on the Sun's surface that typically appear near a group of sunspots just before the sunspots themselves appear.

flare: A sudden outburst of energy from the Sun that occurs near concentrated magnetic fields (known as active regions) on the Sun's visible surface. Flares emit high-energy charged particles (ions and electrons) and all forms of electromagnetic radiation into space.

G

gamma rays: High-energy electromagnetic radiation, invisible to the naked eye, with wavelengths shorter than x-rays. Gamma rays are emitted from the Sun during the most energetic flares on the solar surface.

gauss: A unit of magnetic field strength.

GEMS: An acronym that stands for Great Explorations in Math and Science. This is a
program at the Lawrence Hall of Science at the University of California at Berkeley for science educators. It includes professional development for educators around the United States of America as well as teacher guides in a variety of science topics.

GEONS: An acronym that stands for Geomagnetic Event Observation Network by Students. This is a network of schools, teachers, and students participating in the magnetometer science of the THEMIS NASA mission.

geomagnetic field: The Earth's magnetic field.

geomagnetic storm: A worldwide large disturbance in the Earth's magnetic field, accompanied by intense auroras in the northern and southern polar regions and intensifications of the particle radiation trapped in Earth's magnetosphere (the Van Allen belts).

geophysics: The study of the nature of processes occurring on Earth.

growth phase: The first phase in a substorm when the magnetotail becomes thinner and the auroral oval expands southward.

granulation: The cellular structure of the photosphere. "Granules" are formed by convection, each one is quite large, about 700 to 1000 km (400 - 600 miles) in diameter.

H

hard drive: A device found in a computer used to store information even when the
computer is turned off.

heliopause: The outer edge of the heliosphere, where the solar wind runs into the interstellar medium. At the heliopause, the pressure of the solar wind balances that of the interstellar medium.

homogeneous arc: An auroral arc that is uniform in structure throughout.

Hydrogen: The smallest and most common atom in the Universe.

I

infrared radiation: Electromagnetic radiation, invisible to the naked eye, with wavelengths longer than visible light and shorter than microwaves.

interplanetary magnetic field: The magnetic field among and between planets, brought out from the Sun by the solar wind.

interstellar medium: Electrified gas and dust between the stars.

ionosphere: The region of the Earth's upper atmosphere containing a small percentage of free electrons and ions produced by photoionization of the constituents of the atmosphere by solar ultraviolet radiation at very short wavelengths (<100 nm).

K

Kelvin (K): A unit of temperature with a magnitude equal to that of the degree Celsius, and based on a scale in which 0 K is absolute zero (-273.15 °C). A temperature expressed in Kelvins is equal to the Celsius temperature minus 273.15.

L

limb: The edge of the Sun or planet visible to an observer or instrument.

M

magnet: The magnetic field among and between planets, brought out from the Sun by the solar wind.

magnetic field: A map of the direction and strength of magnetic forces around any object (such as the Sun or planet) that is magnetic. Magnetic fields are caused by electric currents in the Sun.

magnetic field line: Lines drawn to represent the magnetic field around an object.

magnetic pole: A magnetic pole of a magnet is point near the end of the magnet, at which the magnetic force seems concentrated.

magnetic reconnection: A process that takes place in plasmas with very few collisions such that the charged particles are attached to magnetic fields until the fields reconnect and the particles can move from one magnetic field line to another.

magneto-hydrodynamic: A mathematical theory to describe charged particles and magnetic fields when there are very few collisions.

magnetometer: An instrument to measure magnetic fields.

magnetopause: The boundary of the magnetosphere, lying inside the bow shock, usually about 10 Earth radii toward the Sun. The solar wind is deflected inside the bow shock to flow around the magnetopause in the magnetosheath.

magnetosheath: The region between the bow shock and the magnetopause, characterized by very turbulent plasma. For Earth, along the Sun-Earth axis, the magnetosheath is about two Earth radii thick.

magnetosphere: The region surrounding a planet within which the planetary magnetic field is the dominant force on electrically charged particles that can be trapped within the magnetosphere, making radiation belts like Earth's Van Allen belts.

magnetotail: A comet-like extension of a planet's magnetosphere formed on the planet's dark night side by the interaction of the solar wind and the magnetosphere. It can extend hundreds of planetary radii away from the Sun.

mass: Stuff in the Universe that has weight in a gravitational field.

model: A description of a system or phenomenon that accounts for its known or inferred properties and may be used for further study of its characteristics.

molecule: Multiple atoms that are connected to one another.

N

noctilucent clouds: Clouds formed at extremely high-altitude that shine at night. A bit of a mystery, scientists aren't sure why or how they are formed.

Northern Lights: The common name for Aurora Borealis, aurora seen in the Northern Hemisphere.

O

onset: The time in a substorm between the growth phase and breakup phase when the aurora changes from a stationary arc or auroral oval to an active aurora that expands both toward the poles and the equator.

orbit: The path of an object in space as it revolves around another object.

P

parallax: The difference in apparent direction of an object as seen from two different locations.

patchy aurora: Aurora that appears as large patches in the sky that turn on and off every 3 seconds.

pedagogical: Of, relating to, or characteristic of the art or profession of teaching.

penumbra: A dark region that surrounds an even darker central area of a sunspot.

photosphere: The visible portion of the Sun.

plage: Bright regions of gases with concentrated magnetic fields in the solar chromosphere. They appear near groups of sunspots and in the regions surrounding sunspots that are about to emerge.

plasma: A low-density gas in which the individual particles are charged and which contains an equal number of positively charged ions and negatively charged electrons.

plasmasphere: A region of the inner magnetosphere that contains relatively cool (low energy) and dense plasma. This area can be considered an outer extension of the ionosphere.

preliminary design review: The first NASA review during the second phase of a mission in which the designs of the mission, from instruments to the satellite bus (body), are evaluated by an independent review team.

prominence: Complex structures of relatively cool, dense solar material that extend into the outer chromosphere and inner corona. They are generally static and believed to be supported by magnetic forces. They can appear as loops on the edge of the solar disk or limb. Their shape is probably controlled by the Sun's magnetic field. Sometimes they erupt, often in association with coronal mass ejections.

psychology: The study of mental processes and behavior.

radiation: Radiation has two different meanings. One is the stream of particles emitted by decaying nuclei such as uranium. This energy often takes the form of alpha or beta particles, or neutrons. A second use is as part of the term “electromagnetic radiation," which refers to energy traveling in the form of electromagnetic waves or photons. For example, yellow light is a form of electromagnetic radiation, as are radio waves and x-rays.

radiation belts: Regions of of high-energy particles traped by the magnetic influence of the Earth. These belts are sometimes called "Van Allen" belts because of their discovery in 1958 by Professor J.A. Van Allen. Radiation belts are composed of electrons, protons, and smaller numbers of other ions.

radio-tomographic imaging: A technique for the imaging of the spatial distribution of ionospheric plasma density.

rayed arc: An auroral arc that looks like a curtain with distinct vertical lines along the arc.

real-time image: An image that can be studied or observed as they are being generated.

recovery phase: The final phase in a substorm.

relaxed state: The state of an atom or molecule when it has released all of its discrete energy initially absorbed by a particle that collided with the atom or molecule.

S

satellite: An object that was launched into orbit around Earth or around another celestial body.

simulation: A mathematical description of a system or phenomenon that is studied using a computer.

solar activity: Activity of sunspots, flares and CME's which follow the solar cycle.

solar cycle: A predictable 11-year cycle when defined by solar activity, including the number of sunspots, flares, and CMEs, which follow this cycle. When defined by the solar magnetic field directions, the cycle is 22 years long.

solar flare: An explosive release of energy of the Sun.

solar maximum: A period of increased solar activity when the number of sunspots reaches a maximum in the 11-year solar cycle.

solar minimum: A period of decreased solar activity when the number of sunspots reaches a minimum in the 11-year solar cycle.

solar wind: The charged particles (plasma), primarily protons and electrons, that are continuously emitted from the Sun and stream outward throughout the solar system at speeds of hundreds of kilometers per second.

Southern Lights: The aurora seen in the southern hemisphere.

space science: The study of the universe from Earth's upper atmosphere to the far reaches of the Solar System.

space weather: The state of space in the Solar System (the solar wind, Earth's
magnetosphere…etc) at a given time and place, with respect to variables such as plasma density and temperature, and magnetic field strength and direction.

spatial resolution: The smallest discrete step in space that an instrument can measure.

star: A hot ball of gas that shines from its own energy.

substorm: A global development of the aurora and Earth's magnetosphere that has three main phases: growth, break-up, and recovery.

sunspot: A region of the solar surface that is dark and relatively cool; it has an extremely high magnetic field.

T

temporal resolution: The smallest discrete step in time that an instrument can measure.

terrella experiment: An experiment created by K. Birkland to prove that the aurora was due to charged particles following magnetic field lines to the upper atmosphere.

triangulation: Measurement of the distance of a planet or nearby star by sighting its apparent position against background stars from two or more separate locations.

Tribal College: A college located on Native American Tribal lands.

U

ultraviolet light: See ultraviolet radiation.

ultraviolet radiation: Electromagnetic radiation, invisible to the naked eye, with wavelengths shorter than violet light and longer than x-rays.

umbra: The dark central area of a sunspot.

V

visible light: Light with wavelengths that can be observed by a human eye.

W

white light: Electromagnetic radiation composed of all wavelengths of light that is visible to the naked eye (red through violet).

X

x-rays: High-energy electromagnetic radiation, invisible to the naked eye, with wavelengths shorter than ultraviolet radiation and longer than gamma rays.