Go to our Level 1 Glossary

Return often to out new terms and figures.

Go Back to Education and Outreach Page   

ACE      

The amount of ionization of the ions of the solar wind (i.e., the number of electrons stripped from their original atoms) depends on the temperature of the region on the Sun where the ions originated. Because of the high temperatures on the Sun, these ions usually have many more electrons stripped away than the atoms of the same elements in the ionosphere of Earth, say, where the temperatures are much lower. Considering oxygen, for example, we may observe O+7 or +8 (meaning 7 or 8 electrons stripped away) in the solar wind, but only O+1 or +2 (1 or 2 electrons stripped away) for oxygen in the ionosphere. For each positive ion in the solar wind, including the protons, which are the dominant constituents, there is an electron. So the solar wind is electrically neutral on average, just as is the Sun itself.

Alfvén Wave      

See the figure. An Alfvén wave in its simplest form is a sine-wave shaped magnetic field variation which acts like a plucked string confined to a plane and carrying attached fluid with it (plasma in this case). In this "plane polarized" case the wave is in the x-z plane and travels along the x-axis. The magnetic field variation (given by b, with amplitude b_o, in the figure) is denoted by the solid blue curve, and the plasma variation (given by v) is denoted by the dashed red curve. We see that they are proportional to each other, i.e., b proportional to v. Notice that the background magnetic field, B_o, is considered here to be constant in time and uniform in space throughout the area (actually the volume) of the figure. The total magnetic field (B) is the vector sum of the background field (B_o) and the varying field (b), so that (where B_o, and b are perpendicular to each other): B = B_o + b, (1)
and where b itself is b = b_o sin(2pt/T), (2)
and therefore, B = B_o + b_o sin(2pt/T), (3)
where we see that the total field, B, is a function of time B(t), even though B_o is not.
Now we consider the value of b (from equation (2) for various t's shown in the top of the figure, where T is the period, and T/4 is a quarter period, etc.:
At t = 1 (meaning at t = 1 x T/4), sin(2pt/T) is sin(p/2) = sin(90ø) = + 1, so b = + b_o.
And at t = 2 (meaning at t = 2 x T/4), sin(2pt/T) is sin(p) =sin(180ø) = 0, so b = 0.
And at t = 3 (meaning at t = 3 x T/4), sin(2pt/T) is sin(3p/2) =sin(270ø) = - 1, so b = - b_o.
And at also for t = 4 (t = T, not shown in fig), sin(2pt/T) is sin(2p) =sin(360ø) = 0, so b = 0, again, as for t = 2. At t = 5 a full cycle has been completed, and again b= + b_o.
The time from t = 1 to t = 3 is a half period, T/2. Then equation (3) gives the following B(t)'s for t's of 1 through 5:
B (1) = B_o + b_o, B (2) = B_o + 0 = B_o, B (3) = B_o - b_o, B (4) =B_o , and B (5) = B_o + b_o.
The first three of these are shown at the bottom of the figure in vector form. If we start with only the uniform field B_o in a uniform plasma and "pluck" the field slightly in some way, like a string on a violin, it will go into such an Alfvén mode where the change in field is perpendicular to the vector B_o. This is called a transverse wave. The wave will move to the right or the left depending on the source of the energy causing the wave. All of these same remarks hold for the velocity, V, so that V = V_o + v_o sin(2pt/T), and similar vector diagrams can be drawn for the V's.

Atom      

See the figure for an example of the simplest atom, the hydrogen atom. The classical view of the hydrogen atom has a proton at the center and concentric shells, or orbits, in which a single electron can be located. These shells are designated by the orbit number n=1, 2, 3, 4,...in the left side of the figure, where the higher the number the greater the energy associated with the electron. If an electron jumps to a lower level, i.e., to an orbit closer to the proton, a photon is created which emanates from the atom. The photon's energy is equal to the loss of the electron's energy. The higher the photon's energy the higher is its frequency. The modern view (right side of the figure) has some similarities, but some big differences. The main difference is that the electron's position is never exactly known and it is not even exactly knowable. We believe instead that we can know only the probability of where the electron is for any energy level (n). In this case n is called the principal quantum number. The wave-like curves (right side) represent an amplitude related to this probability. There are many higher energy levels (higher n's) that are allowed than those shown in the figure - an infinite number.

Attitude Stabilized     

Attitude stabilized spacecraft are useful when imaging instruments are carried onboard to keep their "look directions" as steady as possible as they take their pictures. For example, the Voyager spacecraft were attitude stabilized and they carried a television camera an ultraviolet sensor and an infrared detector, all concerned with taking images of the planets as the spacecraft fly by the planets. You can imagine what a challenge it would be to takes images of a planet using instruments on a spinning spacecraft.

Aurora   

Sometimes called the Northern Lights, which is colorful light coming from high up in the atmosphere in the Northern (and Southern) regions near Earth's poles. These occur during times of active space weather. Usually the altitudes of these auroras are from 100 to 250 km (or about 60 to 160 miles) above the Earth's surface. During periods of very strong space activity these lights can be seen far away from the polar regions. This happens during so called great (strong) magnetic storms. A recent great magnetic storm occurred on February 7, 1986. See the figures.

Comet   

As the comet gets close to the Sun its core heats up, and some of the dust and gas escapes forming two types of tails, an ion tail and a dust tail. Sometimes the Sun's pull damages the core and parts of it break up.

Coronal Mass Ejections   

The magnetic field lines carried by CME's are unusual in structure, often appearing like stretched­out bed springs, called magnetic clouds. Magnetic clouds and effects of CME's are sometimes seen by spacecraft far from the Sun.

Coulomb's Law   

A mathematical law that allows a determination of the force between two point charges at rest. It states that the magnitude of the force (Fcoul) is proportional to the product of the charges (q1 and q2) and inversely proportional to the square of the distance (r) between them: Fcoul µ q1q2                         (1)                             r2                         The constant of proportionality is frequently denoted k and in mks units k » 9 X 109 N-m2/coul2           (2) The force, positive if repulsive and negative if attractive, operates along the vector (r) connecting the two point charges, so that the more general form of (1) is a vector relation: Fcoul = kq1q2 r                       (3)                         r3             (Notice that because r/r is a unit vector, equation 3 still represents an inverse squared dependence on r.) Note the similarity between Coulomb's Law and Newton's inverse square law for gravitation.

Earth   

Earth is the only planet whose name did not derive from Greek or Roman mythology, it comes from Old English and Germanic descent. For centuries, Earth was thought to be the center of the universe, but that was disproved by Copernicus in the sixteenth century. Earth is 71% covered by liquid or solid water. That leaves only 29% of our planet to be land! Planet Earth is surrounded by satellites, only one of which is natural.

Galaxy      

Compared to the diameter of a typical galaxy, galaxies are rather close to each other in space, especially when contrasted to the relatively large distance between stars, when compared to the diameter of a typical star. Our galaxy, like most, is very large. It takes light about one hundred thousand years to travel from one edge of our galaxy to the opposite one, that is, 100,000 years.

GEOTAIL    

This spacecraft is named after the Earth's magnetotail where it spent most of its time early in its mission. It was built by the Japanese Space Agency, called ISAS, but has many scientists and engineers from around the world involved. It was launched by NASA from Cape Canaveral for Japan on July 24, 1992. It is part of a big worldwide program of scientific space studies called the International Solar Terrestrial Physics Program, and was the first spacecraft launched in this program. In its early orbit for a few years it moved around the Earth and then far down the magnetotail of the Earth after being slung toward the tail by the Moon as the spacecraft went close by the Moon. This orbit is very complicated because of this and is sometimes called a figure 8 orbit, because part of it looks a little like the number 8. It is similar to the Wind orbit. Now Geotail has an orbit a little closer to that of the IMP 8 spacecraft.

International Solar Terrestrial Physics (ISTP) Program    

In the ISTP program, there also are teams of scientists who create models and pictures of what is happening in space based on what is observed and on basic scientific knowledge already known. In some cases the individual participating spacecraft are shared by several countries. The program utilizes a modern fast data distribution system, which helps scientists all over the world.

Jupiter   

Jupiter is more than twice as massive as all the other planets combined. See table of planets. Jupiter, like the other Giant Planets, has rings, but they are so dim that they can only be seen with infrared telescopes. The big red spot on Jupiter is actually a massive storm raging in the atmosphere and is so large that it can be seen in small, backyard telescopes. Jupiter's name is derived from the Greek Zeus - King of the Gods.

Lines of Magnetic Field   

A magnetic force's strength depends on the speed of the charge, the amount of charge, and the strength of the field lines. The strength of field lines is usually represented by how close the lines are show to each other: if they are drawn far apart the field will be weak (and the resulting force weak) and if they are close the field is strong and the force strong. See the figure. There are also lines of gravitational force and lines of electrical force. In these cases force and field are along the same line and in that sense are simpler to understand than magnetic field.

Magnetic Cloud   

Magnetic clouds often have internal magnetic field structures resembling stretched out bed springs which slowly expand as they leave the Sun. They are believed to be the result of a rapid expulsion of matter into the solar wind from prominences on the Sun.

Mars   

Mars is the seventh largest planet in the solar system. At one time, Mars was believed to be criss-crossed with deep canals of water. Mars also has the honor of hosting the largest mountain in the solar system, Olympus Mons, which is 24 km (78,000 ft) high. The name, Mars, is the from the Greek Ares, god of War.

Mercury   

Mercury is the eighth largest planet in the solar system. The Hubble Space Telescope cannot be turned on Mercury because its orbit causes Mercury to always have too much of the Sun in the frame. That kind of exposure to the Sun would destroy the Hubble. Mercury has had many names throughout history. Apollo or Hermes was the name depending on whether the planet was seen in the morning or evening sky. In Roman mythology, Mercury was the god of commerce, travel, and thievery.

Neptune   

Neptune is most often the outermost planet, but because Pluto was discovered last and its orbit takes it out beyond Neptune's, Pluto is given the routine designation of outermost. The surface winds of Neptune are the fastest in the solar system, often reaching 2,000 km per hour. Each of the rings of Neptune have been named and have such distinctive arcs within them that the arcs have been named as well. The rings are Adams (with arcs named Liberty, Equality, and Fraternity), Galatea, Leverrier (with arcs Lassell and Arago) and Galle. The name Neptune is Poseidon in Greek, and means god of the Sea in Roman mythology.

Pluto   

Pluto was discovered completely by accident. The mathematics predicting the presence of another planet out beyond Neptune was faulty. However, Clyde Tombaugh (the discoverer of Pluto) did not know this and used the calculations to find the planet anyway. It was just a fluke that Pluto was in the right place at the right time for Tombaugh to sight it. Pluto is the only planet in our solar system not to be visited by any man-made spacecraft. Because of the erratic nature of Pluto's orbit, some scientists insist that it is an asteroid. Also, Pluto rotates in the opposite direction of most of the other planets. However, Pluto has its own moon, Charon. Pluto's name in Roman mythology was the name of the god of the Underworld.

Quantum Number    

In quantum mechanics three integer quantum numbers are required to describe the electron probability cloud. The principal quantum number, denoted n, is related to the total energy of the atom, higher energies having larger n's. The orbital quantum number, denoted   l, describes the electron's angular momentum and can have any value from zero through n-1 with larger values of  l corresponding to progressively less spherical probability clouds with larger angular momentum. Finally, the magnetic quantum number, denoted m, takes on values between -l and   l and describes the orientation of the electron's angular momentum. In addition, an electron has an intrinsic spin, leading to a fourth, non-integer, spin magnetic quantum number.

Saturn   

Saturn is the second largest planet in the solar system but has the most moons. Saturn has 18 known moons, Jupiter only 16 (Uranus has 17). Together these three planets and Neptune are known as the Giant Planets. Saturn is best known to the lay person for its beautiful ring structure, made up of rocks and dust orbiting the planet. These rings can be seen from your backyard with even a small telescope. Saturn was known in Roman mythology as the god of Agriculture.

Spin Stabilized    

Spinning spacecraft are useful for making measurements of charged gases, waves, and electric and magnetic fields in space, because of the need to "look" in many directions, which occurs naturally as the spacecraft spins at least in the plane perpendicular to the spin axis. This means of looking in different directions, during the period of spin, is acceptable provided that the measurements do not have to be done at the same time. When simultaneity of measurements is necessary more complicated instruments are needed.

Stratosphere   

The cold region of a planetary atmosphere above the convecting regions (the troposphere), usually without vertical motions but sometimes exhibiting strong horizontal jet streams.

Torque    

See the figure. Just as the two activated jets of gas, by acting in OPPOSITE directions, can exert a torque about a spacecraft's axis(left side of the figure), when operating in the SAME direction (right side), the jets will change the velocity of the spacecraft and contribute to its "linear motion". In the latter case there is no net torque imposed on the spacecraft by these jets. Notice that we can send commands to make the spacecraft speed up, slow down, or change course, by this process. We are assuming here that the two jets exert exactly the same force. If one is stronger than the other upon activation, then we have the interesting situation of a new linear motion arising, AND some net torque, even if small. Usually spacecraft jets are not supposed to do that. When operating correctly, in the same direction, the jets can put the spacecraft on a different trajectory. This was important for the WIND spacecraft as it approached the Moon, because the operators (on the ground) had to carefully adjust the spacecraft motion so it would pass the Moon at just the right place - especially so it WOULD NOT ACCIDENTALLY HIT THE MOON!

Troposphere   

The lower regions of a planetary atmosphere where convection keeps the gas mixed and maintains a steady increase of temperature with depth. Most clouds are in the troposphere.

Uranus   

Uranus is tipped over so far that it actually rotates on its side. It is unique in this. The color of blue that is seen in the telescope is caused by the methane in the upper atmosphere which absorbs the red light. Only the blue escapes to reach our telescopes. Uranus, like the other Giant Planets, has rings, but they are very faint and cannot usually be seen from Earth unless with a large telescope. The name comes form the ancient Greek deity of the Heavens, the earliest supreme god.

Van de Graaff Generator     

It is a well known property of a good conductor that when charge is added to it, the charge must rapidly move to its outer surface, in order to maintain a zero electrical field within the conductor. If this were not the case, any unbalanced charge in the conductor would set up an electric field that would act on the charges to move them to the outer surface. This principle is essential in the performance of the Van de Graaff electrostatic field generator, because the belt which adds electrons to the hollow conducting sphere adds them to the INSIDE surface of the sphere. Immediately they leave that surface and go to the outside surface allowing more electrons to be added to the inside. If this were not the case, i.e., say that instead we tried to add the electrons directly to the outside surface, those already on the sphere would start to reject the new electrons on the belt through the natural tendency of electrostatic repulsion. So the Van de Graaff generator is capable of being charged (i.e., "pumped") to very high voltages in this way. Can we go on forever and charge the sphere to anything that we want? No. Eventually the voltage between the sphere and the ground will get so strong that mini-lightning will occur (as we see in the figure). This is nature's means of discharging the sphere. At what voltage this occurs will depend on the conditions of the air, especially the level of its moisture. For this reason, it helps to have the generator in a vacuum. Since charged particles have forces on them in an electric field, and such an electric field is created between the charged sphere of the generator and the ground, a charged particle in this field will be accelerated. Hence, a Van de Graaff generator can be set up to be a charged particle accelerator. This was important for studies in nuclear physics.

Venus   

Next to the Sun and the Moon, Venus is the brightest object in Earth's sky. The interior is very similar to Earth's iron core of about 3,000 km in radius. Venus is the sixth largest planet in the solar system. Its name in Greek mythology was Aphrodite.

Voyager Spacecraft      

The highly successful Voyager 2 mission is sometimes referred to as the Grand Tour of the Outer Planets. The Voyager 1 and 2 planetary encounters were carefully targeted so that these spacecraft could get the necessary energy boost at the previous planet through a sling­shot effect to make it to the next.

WIND      

WIND orbits around the Earth and sometimes moves far in front of it, out toward the Lagrangian Point after being slung outward by the Moon as the spacecraft goes close by the Moon. This orbit is very complicated because of this and is sometimes called a figure 8 orbit, because part of it looks a little like the number eight.

Go Back to Education and Outreach Page     

Site created: April 25, 1997 Last Modified: December 18, 2003
Curator: Albert E. Davison, International Technology and Management, Inc. E-mail: Albert.E.Davison@gsfc.nasa.gov
Responsible NASA Representative: Dr. Michael Collier E-mail: Michael.R.Collier@nasa.gov