Glossary

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. They almost do not change, and thus also help in calculating the motion, to a very good degree of accuracy. They are often important in calculating the way ions and electrons move in a magnetic field.

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

Ampere--see electric current.

Argus (project)--A 1958 experiment by the US military, to create artificial radiation belts by exploding small nuclear bombs above the atmosphere.

Astronomical unit (AU)--The mean Sun-Earth distance, a unit of distance widely used in expressing distances in the solar system. 1 AU = 149,600,000 km = 92,957,000 miles.

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

AU -- acronym for Astronomical Unit, mean distance to the Sun.

Aurora (short for polar aurora)--A glow in the sky, often observed 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.

Aurora, diffuse --see diffuse aurora

Aurora, discrete--see discrete aurora

Auroral acceleration--The process by which auroral electrons acquire their energies, typically 1-10 keV. May be associated with parallel voltage drops or with an interaction between particles and plasma waves, and may be related to magnetic reconnection in the plasma sheet.

Auroral electrojet--see electrojet, auroral

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. Since the waves are even longer than those of the AM radio band, they are stopped by the ionosphere and do not reach the ground, but they are readily observed from spacecraft.

Auroral oval--the region in which aurora appears at the same time, corresponding to the "ring of fire" around the magnetic pole, often observed 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 a rare occurence.

Auroral zone--the region on Earth where auroras are common--a smeared-out average (over time and distance from the magnetic pole) of the auroral oval. Typical magnetic latitude is 63-65 degrees.

Barium release --the firing from a rocket or spacecraft above the atmosphere of a charge of barium, evaporated by a thermite process. Usually produced shortly after sunset, when the sky is already dark but sunlight still reaches the high altitude where the release occurs. The barium atoms are released as a vapor, they spread rapidly and are readily ionized by sunlight. The ion cloud then moves with the local plasma and is therefore a useful tracer of plasma flows.

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 and with substorms.

Boundary layer --a transition layer between two neighboring regions in the magnetosphere. The plasma sheet boundary layer (PSBL) is the transition from the plasma sheet and the tail lobes. The low latitude boundary layer (LLBL), just inside the magnetopause, is the transition between the equatorial magnetosphere and the solar wind (more accurately, the magnetosheath, solar wind slowed down by passage through the bow shock).

Bow shock --a sharp front formed in the solar wind ahead of the magnetosphere, marked by a sudden slowing-down of the flow near Earth. It is quite similar to the shock forming ahead of the wing of a supersonic airplane. 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

Convection (magnetospheric)--large-scale plasma flow, circulating in the magnetosphere and driven by the solar wind. Plasma physics requires such circulation to be associated with an electric field. Assuming that the electric field propagates along magnetic field lines (as it would along good conductors of electricity) and reaches the polar ionosphere, corresponding electric fields should be observed above the polar caps, and such fields exist.
    In the view proposed in 1961 by Axford and Hines, plasma near the flanks is dragged tailwards by the adjoining solar wind flow, through the action of "viscous-like forces"; in the view suggested that same year by Dungey, plasma travels tailward on "open" field lines following reconnection. Evidence suggests both processes contribute. In both models the plasma returns earthward in the plasma sheet near midnight, a process which could be not continuous but intermittent, associated with substorms.

Corona--see solar corona.

Coronal mass ejection (CME)--a huge cloud of hot plasma, occasionally expelled 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.

Cosmic rays --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 starlight. The origin of their huge energies is uncertain, but may come from expanding shock fronts created by supernova explosions.

Crab nebula --a cloud-like nebula observed in the Crab constellation, the remnant of a supernova explosion observed in China in 1054. It contains a very rapidly rotating (and hence, young) pulsar, which is probably the remnant of the supernova. The emissions of radio waves and light from this nebula suggest the presence of high energy particles.

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 the Earth´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 quite 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, located near the Earth´s center.

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 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.

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 to create 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 due to free electrons released from the wire by heat. The Edison effect made possible "vacuum tubes" used in radio and television equipment before the invention of the transistor.

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. About Ben Franklin's role in studying and naming electrical charges, click here.

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.

Electric 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.

Electrojets, auroral--two intense electric currents, flowing around the auroral oval from the day side towards the night side and meeting somewhat west of midnight. Associated with Birkeland currents and caused by the unusual electric conductivity properties of ionospheric plasma, the electrojets are responsible for practically all of the magnetic disturbance observed on the ground due to substorms. Their magnitude (derived by analyzing such disturbances) often serves as a convenient gauge of the intensity of substorm activity.

Electromagnet--a magnet powered by an electric current. Usually the current flows in a coil, which may or may not contain a core of iron or of some other magnetic material.

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--a combination of oscillating magnetic and electric field, spreading in wavelike fashion through space at a speed of about 300 000 km.sec. James Clerk 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 from atoms by light and by collisions, and they are responsible for many electric phenomena in solid matter and in plasmas. (About the discovery of the electron in 1897, click here.

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´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.

Electroscope--a simple instrument, indicating the presence of electric charge by the spreading-apart of two leaves of metal foil, hanging next to each other inside a glass jar. The rate at which an electroscope in dry air loses its charge is a rough indicator of the prevailing level of ionizing radiation.

Energetic 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. of x-rays). See also radiation belt, cosmic rays, solar energetic particles.

Energy--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 matter.

Explorer 1--first satellite launched by the US, around midnight of January 31, 1958. Carrying Geiger counters, Expolorer 1 and the similar Explorer 3 (launched two months later) discovered the existence of a belt of magnetically trapped energetic particles around Earth.

Field line preservation--a property of fluids which are perfect conductors of electricity (including "ideal plasmas"), by which two particles which initially share the same field line, continue to do so into the future. The opposite also holds for such fluids: two particles which start out on different field lines will always be on different field lines.

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 scientist who first produced and observed radio waves in the lab. 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--see solar flare

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

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

GCM--guiding center motion

Geiger counter--a simple electronic detector of energetic particles. It consists of a thin straight wire at a high positive voltage (usually close to 1000 volts) relative to a cylindrical electrode surrounding it. Geiger counters can detect ("count") high-energy particles, but they cannot identify their type or distinguish their energy.

Geocorona--the outermost layer of the Earth´s neutral atmosphere, a huge cloud of hydrogen surrounding our planet. Its density diminishes with distance and it has been observed up to distances of 5-6 Earth radii.

GSM coordinates--geocentric solar magnetospheric coordinates, the system in which locations in the large-scale magnetosphere are usually given. The main axis ("noon-midnight") points at the Sun, and the plane of symmetry ("noon-mignight plane") contains the Earth´s magnetic axis, which however needs not be exactly perpendicular to the sunward direction.

Guiding center--An ion and electron in a magnetic field, of suitably low energy, is constrained to circle ("gyrate") around a local magnetic field line, while the center of its circular motion slides up or down along the line and also slowly shifts from one guiding field line to its neighbor, following certain rules. The center of that circle is known as the particle´s guiding center and the entire mode of mottion is called guiding center motion. See gyration, magnetic mirroring, drift.

Gyration--a term used in plasma studies for the circular motion of an ion or electron around its guiding center.

IMF--interplanetary magnetic field (see below).

IMF polarity--the general direction of interplanetary magnetic field lines in a certain location (e.g. near Earth), i.e. whether they 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 it. See interplanetary sector.

Inner magnetosphere--the region of the magnetosphere in which ions and electrons are relatively stably trapped. Approximately the region threaded by field lines which cross the equator within synchronous orbit, i.e. within 6.6 Earth radii.

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 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 the Earth´s environment.

Interplanetary 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. See bow shock. For an article about the impact of a large shock on the magnetosphere, click here.

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"). The Earth´s orbit typically contains 4 sectors, but 2 or 6 are not unusual; they are caused by waviness of the current sheet separating magnetic field lines from opposite polar regions of the Sun.

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.

Ionic Theory--in chemistry, the theory (by Svante Arrhenius, 1884) which first explained the behavior of acids, alkalis (bases) and salts when dissolved in water. By the ionic theory, each molecule of such materials consists of molecular or atomic groupings charged with positive or negative electricity ("ions"), held together by their mutual electrical attraction. In water these electrical forces are greatly weakened, the groupings often get separated, and if an electric current flows, positive and negative ones migrate with it in opposite directions.

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.

LLBL--low latitude boundary layer. See boundary layers.

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 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 a plasma, magnetic field lines also guide the motion of ions and electrons, and direct the flow of some electric currents.

Magnetic latitude--geographic latitude of a location, in a system of latitudes and longitudes whose axis is not the rotation axis of the Earth but the magnetic axis, i.e. the axis of the dipole at the Earth´s center which best fits the internal magnetic field. The auroral zone, for instance, is near magnetic latitude 65 degrees. See magnetic local time.

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

Magnetic local time (MLT)--in the a system of latitude and longitude whose axis is the dipole axis, magnetic local time is the longitude, measured not in degrees but in hours (1 hour = 15 degrees).
   The zero of this longitude is not fixed relative to Earth (the way the Greenwhich meridian is for geographic longitude), but rather relative to the Sun: the line of magnetic longitude facing the Sun always has MLT = 12 hours ("magnetic noon"), and the opposite one has MLT = 0 or 24 hours ("magnetic midnight"). See magnetic latitude

Magnetic mirroring--the process by which an ion or electron, constrained by its guiding center motion to follow a magnetic fields line, slows its advance down that line as it enters a region of stronger magnetic field, and is ultimately turned back ("mirrors") at a certain "mirror point."
    Mirroring is what makes possible long-term trapping of ions and electrons in the Earth´s radiation belts. In the inner magnetosphere, ions and electrons are confined between two mirror points, one north of the equator and one south of it. These turn them back before their motion along the guiding field line reaches the atmosphere, where they might otherwise have been lost by colliding with molecules of air.

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.

Magnetic 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.

Magnetic reconnection--In a plasma, the process by which plasma particles riding along two different field lines can be made to share the same field line (see field line preservation). For instance, following reconnection, solar wind particles on an interplanetary field line, and magnetospheric ones on a field line attached to Earth, may find themselves sharing the same "open" field line, which has one end anchored on Earth and the other extending to distant space.
   Magnetic reconnection can occur when plasma flows through a neutral point or a neutral line at which 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, 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.

Magnetometer--an instrument 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 they need to be calibrated.
    Rubidium-vapor and similar instruments measure only the field 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.

Magnetosheath--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.

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

Magnetotail--The long stretched-out nightside 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. See plasma sheet, tail lobes,

NENL--near-earth neutral line (see below).

Neutral point --A point at which the magnetic intensity is zero. Plays an important role in magnetic reconnection.

Neutral line (NL)--A line along which the magnetic intensity is zero. Like a neutral point, a NL can play an central role in magnetic reconnection, but because of physical reasons, it may be a more likely setting for the actual reconnection process.

Neutral line, near-earth (NENL)--a neutral line which many believe forms in the plasma sheet during magnetic substorms.

Neutral line, distant --A neutral line in the distant magnetotail where (by Dungey´s theory) interplanetary field lines which were split apart by magnetic reconnection when they first encountered the magnetosphere are once more re-united.
   There is little doubt such re-uniting takes place: what is unclear is the exact manner in which it happens.

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

Orbit--the line followed by a spacecraft or a celestial body. See Sun synchronous orbit, synchronous orbit.

Parallel voltage drops ("parallel electric fields")--voltage drops along magnetic field lines.

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 a photon is greater 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 Earth´s. Mercury has a weak magnetic field, Mars and the Moon are magnetized in patches (probably on their surfaces) and Venus, although non-magnetic, has its own interaction with the solar wind, by means of 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.

Plasma physics --the study of plasma phenomena--in the laboratory, where it may one day help extract energy from hydrogen fusion, in the Sun and the distant universe, in the Earth´s ionosphere and in the magnetospheres of Earth and other planets.

Plasma sheet --a near-equatorial layer of denser plasma in the tail of the Earth´s magnetosphere. It separates the two tail lobes, the two bundles of magnetic field lines connected to the regions around the Earth´s magnetic poles.

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 caps --in magnetospheric usage, the regions around the Earth´s magnetic poles, inside the auroral oval. The field lines in these regions extend into the tail lobes of the Earth; they reach great distances and do not close in the magnetosphere.

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.

Polar rain --a drizzle of electrons observed inside the polar caps, apparently from the high end of the energy distribution of solar wind electrons. Its origin in the solar corona is revealed by the fact that in general only one polar cap receives it at any time--the one which (depending on IMF polarity) is linked to the Sun.

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" 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 the "cosmic radiation").

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

Radioactivity --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.

Radio Astronomy--the observation of radio waves from the Sun, planets and the distant universe. In many cases these are signature of energetic particles.

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 that flow moves too fast for the undisturbed gas to move out of its way. Also occurs when a steady fast flow hits an obstacle.

Solar 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 aboard satellites. The corona is very hot, up to 1-1.5 million degrees centigrade, and is the source of the solar wind

Solar energetic particles--high energy particles occasionally emitted from active areas on the Sun, 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.

Solar flare--a rapid outburst on the Sun, usually in the vicinity of active sunspots. A sudden brightening (only rarely seen without 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.

Solar wind--hot solar plasma spreading from the solar 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--see tether, space

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

Sun--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.

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 various 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 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.

Supernova--a large explosion at the end of the evolutionary process of many stars. (Strictly speaking, all that follows applies to a "type II" supernova.)
   A star such as the Sun is kept "puffed up" to its apparent size by the heat which nuclear reactions create in its core. Once its nuclear fuel is used up, the pull of gravity overcomes all other forces and makes the star contract to a very small size. The star´s atoms or even its nuclei are then crushed, and the process may turn it into a pulsar or black hole.
   An enormous amount of energy is released in this last collapse, blowing off the star´s outer layers as a rapidly expanding cloud of gas. It is widely believed that powerful shock fronts form ahead of this cloud´s advance, and through them some ions get accelerated to the very high energies of cosmic rays.

Synchronous 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: 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 on the ground only need to point to one 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. About Birkeland's terrella experiments (1896), here

Tether, space--an experiment in which a satellite was released from the space shuttle at the end of a long insulated cable. The plan was for the dynamo process due to the motion of the tether through the Earth´s magnetic field to generate a large current in the cable.

Ultraviolet (UV)--electromagnetic radiation resembling visible light, but of shorter wavelength. UV cannot be seen by the eye, and much of it is absorbed by ozone, a variant of oxygen, at altitudes of 30-40 km. Satellite telescopes, 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 or intensity) and direction. The magnetic field observed at any point in space is a vector; other examples are velocity, acceleration, force and the electric field, which maps the electric force acting on ions and electrons. Equations involving vectors tend to be more complicated, as they have to describe three-dimensional structure.

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 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.