adiabatic invariant--An invariant of a motion is a quantity which does not change as time advances. For instance, the energy of a system is often an invariant (for a swinging pendulum, or a planet and the Sun), and knowing that it stays constant is a great help in calculating the motion.

Adiabatic invariants are quantities associated with approximately periodic motions which almost do not change, and are similarly useful. They are important in calculating the way ions and electrons move in a magnetic field.

alpha particle--A type of fast ion emitted by many heavy radioactive nuclei, such as uranium. Actually, the nucleus (atom stripped of all electrons) of the gas helium.

ampere--see electric current.

attitude (of a satellite)--The direction in which the satellite is oriented in space.

aurora (short for polar aurora)--A glow in the sky, seen often in a ring-shaped region around the magnetic poles ("auroral zone") and occasionally further equatorward. The name comes from an older one, "aurora borealis," Latin for "northern dawn," given because an aurora near the northern horizon (its usual location when seen in most of Europe) looks like the glow of the sky preceding sunrise. Also known as "northern lights," although it occurs both north and south of the equator.

The aurora is generally caused by fast electrons from space, guided earthward by magnetic field lines, and its light comes from collisions between such electrons and the atoms of the upper atmosphere, typically 100 km (60 miles) above ground.

auroral oval--the region in which aurora can be seen at any single time, as observed (for instance) by satellite cameras. 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 rarely seen.

astronomical unit (AU)--The mean Sun-Earth distance, a unit of distance widely used in expressing distances in the solar system. 1 AU = km = miles.

Birkeland currents--electric currents linking the Earth's ionosphere with more distant regions, flowing along magnetic field lines. Named for Kristian Birkeland, a pioneer of auroral research who first proposed such currents around 1900, these currents are often associated with the polar aurora.

Bow shock--a sharp front formed in the solar wind ahead of the magnetosphere, marked with a sudden slowing-down of the flow near Earth. It is quite similar to the shock forming ahead of the wing of a supersonic airplace. After passing near Earth, the slowed-down flow gains speed again, to the same value as the surrounding solar wind.

Chromosphere--a reddish layer in the Sun's atmosphere, the transition between the photosphere and the corona

Corona--the outermost layer of the Sun's atmosphere, visible to the eye during a total solar eclipse; it can also be observed through special filters and best of all, by X-ray cameras aboart satellites. The corona is very hot, up to 1-1.5 million degrees centigrade, and is the source of the solar wind.

Coronal mass ejection (CME)--a huge cloud of hot plasma, expelled sometimes from the Sun. It may accelerate ions and electrons, and may travel through interplanetary space as far as the Earth's orbit and beyond it, often preceded by a shock front. When the shock reaches Earth, a magnetic storm may result.

Cusps (of the magnetosphere)--two regions of weak magnetic field, on the sunward boundary of the magnetosphere, one on each side of the equator. They separate magnetic field lines closing on the front from those swept into theearth's magnetotail.

Diffuse aurora--a spread-out glow often covering much of the auroral oval. It is not seen by the eye but can be observed well by satellite cameras. See discrete aurora.

Dipole--a 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.

Discrete aurora (or "auroral arcs") are the typical ribbon-like structures of aurora observed from the ground. From space they may appear as brighter spots in the diffuse aurora.

Drift--A 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 magnetic 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.

Dynamo process--the generation of an electric currents by the flow of an electrically conducting fluid through a magnetic field. For instance, the magnetic field originating inside the Earth is believed to come from a dynamo process involving the flow of molten iron in the Earth's hot core. The energy required by the current is obtained from the motion of the flow.

Earth 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.

Edison effect--the flow of an electric current through a laboratory vacuum, between two metal wires, one of which is heated. The current flows only when the heated wire is more negative, because it is carried by free electrons released from the wire by its heat. The Edison effect was the principle behind "vacuum tubes" used in radio and television receivers and transmitters before the invention of the transistor.

Electric charge--that which causes electrons and ions to attract each other, and the repel particles of the same kind. The electric charge of electron is called "negative" (-) and that of ions "positive" (+). Materials such as glass, fur and cloth acquire and electric charge by rubbing them egains 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.

Electric current--a continuous flow of electrons and/or ions, through a material with conducts electricity. A currents 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. their drifts.

Electromagnetic field (EM field)--the regions of space near electric currents, magnets, broadcasting antennas etc., regions in which electric and magnetic forces may act. Generally the EM field is regarded as a modification of space itself, enabling it to store and transmit energy.

Electromagnetic wave--an electric field spreading in wavelike-fashion through space at a speed of about 300 000 km.sec, with its direction and intensity at any point in space oscillating rapidly back and forth. James Clerk's Maxwell's theory in 1864 suggested that light was such a wave, and today we know that such waves include all forms of light--also infra-red and ultra-violet, as well as radio waves, microwaves, x-rays and gamma rays.

Electron--a lightweight particle, carrying a negative electric charge and found in all atoms. Electrons can be energized or even torn off atoms by light and collisions, and are responsible for most electric phenomena and light emission in solid matter and in plasmas.

Electron 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 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 1,000,000,000 times more.

Energy--loosely, anything that can cause a machine to move. For example, energy is contained by 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 matter.

Frequency--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 discoverer of radio waves. Alternating current in homes in the US goes through 60 cycles each second, hence its frequency is 60 Hz; in Europe it is 50 cycles and 50 Hz.

Flare (or "solar flare")--an rapid outburst on the Sun, usually in the vicinity of active sunspots. A sudden brightening (usually seen only through special filters) may be followed by the signatures of particle acceleration to high energies--x-rays, radio noise and often, a bit later, the arrival of high-energy ions from the Sun.

Gamma rays--electromagnetic waves of the highest frequencies known, originally discovered as an emission of radioactive substances.

Gamma ray bursts--brief bursts of gamma rays from the distant universe, observed by satellites.

Guiding center--the center point of the gyration of ion and electrons in a magnetic field. The guiding center of radiation belt particles bounces back and forth between mirror points and also undergoes a slow drift from one field line to the next.

Gyration--a term used in plasma research for the circular motion of ions and electrons around magnetic field lines.

Interplanetary magnetic field (IMF)--the weak magnetic field filling interplanetary space, with field lines usually connected to the Sun. The IMF is kept out 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 the Earth's environment.

Interplanetary shock--the abrupt boundary formed at the front of a plasma cloud (e.g. from a coronal mass ejection) moving much faster than the rest of the solar wind, as it pushes its way through interplanetary space. See bow shock.

Interplanetary sector--a region of interplanetary space in which all magnetic field lines point either away from the Sun ("away sector") or towards the Sun ("towards sector"). Typically the interplanetary magnetic field consists of 4 sectors, but 2 or 6 are not unusual.

Ion--usually, an atom from which one or more electrons have been torn off, leaving a positively charged particle. "Negative ions" are atoms which have acquired one or more extra electrons, and clusters of atoms can also become ions.

Ionization--the process by which a neutral atom, or a cluster of such atoms, becomes an ion. This may occur, for instance, by absorbtion of light ("photoionization") or by a collision with a fast particle ("impact ionization"). Also, certain molecules (such as table salt or sodium chloride, NaCl) are formed by natural ions (like Na+ and Cl-) held together by their electric attraction, and they may fall apart when dissolved in water (which weakens the attraction), enabling the solution to conduct electricity.

Ionosphere--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. In the lowest and least rarefied layer of the ionosphere, the D-layer (around 70 km or 45 miles), as soon as the Sun sets the ions and electrons recombine, but in the higher layers, collisions are so few that its ion layers last throughout the night

Lagrangian point--in a system dominated by two attracting bodies (such as Sun and Earth), a point at which a third, much smaller body (such as a satellite) keeps the same position relative to the other two. Theoretically, the Sun-Earth system has 5 Lagrangian points, but only two are important: L1 (L-one), on the sunward side of Earth, about 4 times the distance of the Moon, and L2 at approximately the same distance on the midnight side.

Magnetic field--a region in which magnetic forces can be observed. See "electromagnetic field," a more general field also including electric forces.

Magnetic field lines--lines everywhere pointing in the direction of the magnetic force, used as a device to help visualize magnetic fields. In a plasma, magnetic field lines also guide the motion of ions and electrons, and direct the flow of some electric currents.

Magnetic lines of force--Michael Faraday's original term for what is now widely called magnetic field lines.

Magnetometer--intrument for measuring magnetic fields. Spacecraft often carry fluxgate magnetometers, which measure components of the magnetic field (3 of them are combined to give its strength and direction) but need to be calibrated. Rubidium-vapor and similar instruments measure only the strength, but their reading is absolute, related to atomic constants.

Magnetopause--The boundary of the magnetosphere, separating plasma attached to Earth from the one flowing with the solar wind.

Magnetic poles--Two meanings: (1) the points on 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 end.

Magnetic reconnection--In a plasma, the process by which plasma particles riding along two different field lines find themselves sharing the same field line. For instance, solar wind particles on an interplanetary field line, and magnetospheric ones on a field line attached to Earth, finding themselves united on an "open" field line, which has one end anchored on Earth and the other in distant space.

Magnetic reconnection can occur when plasma flows through a neutral point or a neutral line where the intensity of the magnetic field is zero and its direction is not defined. It is an important concept in the theories of energy transfer from the solar wind to the magnetosphere, and of energy release in substorms.

Magnetic storm--A large-scale disturbance of the magnetosphere, usually initiated by the arrival of an interplanetary shock, originating on 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 an auroral disturbance. The strengthened ring current causes a world-wide drop in the equatorial magnetic field, over perhaps 12 hours, followed by a more gradual recovery.

Magnetosphere--The region around Earth whose processes are dominated by the Earth's magnetic field, bounded by the magnetopause.

Magnetotail--The region of the magnetosphere containing field lines stretched away from the Sun. It starts about 8 Earth radii (RE) nightward of the Earth and has been observed to distances of at least 22 RE.

Mirror point--For a particle spiraling around a field line of the magnetosphere, the point where the stronger field (of the Earth, usually) causes the spiral to flatten and then unwind again in the opposite directions. In the inner magnetosphere, ions and electrons are trapped between two mirror points, one north of the equator and one south of it; because of the "mirroring", ions and electrons are turned back before they reach the atmosphere, where they might have got lost.

Neutral point or neutral line--A point or line along which the magnetic intensity is zero. Plays an important role in magnetic reconnection.

Northern lights--an older name for the polar aurora.

Orbit--see Sun synchronous orbit, Synchronous orbit

Particle--in general, a charged component of an atom, that is, an ion or electron.

Photon--colloquially, a "particle 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 photons increases, the shorter the wavelength--smallest for radio waves, larger for visible light, largest for x-rays and gamma rays.

Photosphere--The 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.

Planetary magnetospheres--the magnetospheres of planets, especially of Jupiter, Saturn, Uranus and Neptune, all of which have dipole-like magnetic fields stronger than the Earths. Mercury has a weak magnetic field, Mars and the Moon are magnetized in patches (probably on their surfaces) and Venus, although non-magntic, has its own interaction with the solar wind, through its thick ionosphere.

Plasma--a 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.

Plasmasphere--A region of relatively dense but cool plasma, surrounding Earth, extending to distances of about 5 Earth radii (RE). The plasmasphere is the upward extension of the Earth's ionosphere, getting less and less dense with increasing distance, and it shares the Earth's rotation.

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.

Proton--an ion of hydrogen and one of the fundamental building blocks from which atomic nuclei are made.

Radiation--a term with two broad meanings. 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. Colloquially (the full term is "ionizing radiation") it means any spreading emission which can penetrate matter. 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 radiation").

Radiation belt--The region of high-energy particles trapped in the Earth's magnetic field.

Radioactivity--Instability of the nuclei of atoms, 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.

Radio waves--Electromagnetic waves of relatively low frequency.

Reconnection--see "Magnetic reconnection"

Ring current--A very spread-out electric current circling around the Earth, carried by trapped ions and electrons.

Shock--A sudden transition at the front of fast flow of plasma or gas, when the flow moves too fast for the undisturbed gas to flow out of the way. Also occurs when a steady fast flow hits an obstacle.

Solar corona--see "corona"

Solar flare--see "flare"

Solar wind--hot solar plasma spreading from the Sun's corona in all directions, at a typical speed of 300-700 km/sec. It is caused by the great heat of the corona.

Space tether--an experiment in which a satellite was released from the space shuttle at the end of a long insulated cable. Theplan was for the dynamo processdue to the motion of the tether through the Earth's magnetic field to generate a large current in it.

Space Weather--the popular name for energy-releasing phenomena in the magnetosphere, associated with magnetic storms, substorms and shocks.

Substorm--a process by which plasma in the magnetotail becomes energized at a fast rate, flowing Earthward and producing bright auroras and large Birkeland currents, for typical durations of half an hour.

Sunspot--An intensely magnetic area on the Sun's visible face. For unclear reasons, it is slightly cooler than the surrounding photosphere (perhaps because the magnetic field somehow interferes with the outflow of solar heat in that region) and therefore appears a bit darker. Sunspots tend to be associated with violent solar outbursts of all kinds.

Sunspot 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 the sunspots, the cycle is probably magnetic in nature, and the polar magnetic field of the Sun also reverses each solar cycle.

Sun-synchronous orbit--a near-Earth orbit resembling that of a polar satellite, but inclined to it by a small angle. With the proper 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.

Synchronous orbit--a circular orbit around the 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 makes the synchronous orbit useful for communication satellites: a satellite transmitting TV programs to the US, for instance, will always be in touch with the US if "anchored" above it, and receiving antennas will only need to point to a fixed spot in the sky.

Tail lobes--the two bundles of nearly-parallel magnetic field lines which stretch into the magnetotail, on opposite sides of the plasma sheet. The northern lobe contains field lines entering the north polar region of Earth, while the southern lobe contains lines emerging from the southern polar region.

Terrella--a small magnetized sphere, used as laboratory model of the Earth.

Ultraviolet (UV)--electromagnetic radiation resembling visible light, but of shorter wavelength. The eye cannot see UV, and much of it is absorbed by ozone, a variant of oxygen, at altitudes of 30-40 km; satellite telescope, however, can and do view stars and the Sun in UV, and even in the extreme UV (EUV), the range between UV and X-rays.

Vector--a physical quantity having both magnitude (= strength, intensity) and direction. The magnetic field is a vector; other examples are velocity, acceleration, force and the electric field, which maps the electric force on ions and electrons. Equations involving vectors tend to be more complicated, as they have to take account of their three-dimensional nature.

Voltage--a sort of "electric pressure", gauging the electric force acting on ions or electrons (or more accurately, the amount of energy they might obtain from that force). In electric devices such as are used in the home, increasing the voltage increases the electric current--just as increasing the pressure driving water through a pipe increases its flow rate. (The scientific term is "potential" or "potential difference".)

X-rays--electromagnetic waves of short wavelength, capable of penetrating some thickness of matter. Medical x-rays are produced by letting a stream of fast electrons come to a sudden stop at a metal plate; it is believed that X-rays emitted by the Sun or stars also come from fast electrons.