| Alpha particle |
A highly ionizing form of particle radiation which have low penetration. They consist of two protons and two neutrons bound together into a particle identical to a helium nucleus; hence, it can be written as He2+. |
| Apoapsis |
The point in an orbit when two objects are furthest apart. The opposite of periapsis. |
| Apogee |
The point in its orbit where an Earth satellite is farthest from the Earth. Opposite of perigee |
| Astronomical unit (AU) |
149,597,870 km; the average distance from the Earth to the Sun |
| Atmosphere |
The gas that surrounds a planet or star. The Earth's atmosphere is made up of mostly nitrogen, while the Sun's atmosphere consists of mostly hydrogen. |
| Corona |
The uppermost level of a star's atmosphere. In the sun, the corona is characterized by low densities and high temperatures (> 1,000,000 degrees K). |
| Coronal Holes |
Areas where the Sun's corona is darker. These were found when X-ray telescopes were flown above the earth's atmosphere to reveal the structure of the corona. Coronal holes are linked to open magnetic field lines and are mostly found at the Sun's axis. Fast-moving solar wind is known to come from coronal holes. |
| Coronal Mass Ejections (CME) |
An ejection of material from the solar corona, is plasma consisting primarily of electrons and protons (in addition to small quantities of heavier elements such as helium, oxygen, and iron), and the coronal magnetic field. |
| Cosmic rays |
Atomic nuclei (mostly protons) and electrons that are observed to strike the Earth's atmosphere with exceedingly high energies. |
| Ecliptic |
The plane of Earth's orbit about the Sun. |
| Electron |
A negatively charged particle commonly found in the outer layers of atoms. The electron has only 0.0005 the mass of the proton. |
| eV (Electron volt) |
The change of potential energy experienced by an electron moving from a place where the potential has a value of V to a place where it has a value of (V+1 volt). This is a convenient energy unit when dealing with the motions of electrons and ions in electric fields; the unit is also the one used to describe the energy of radiation belt particles, X-rays, and gamma rays. A keV (or kiloelectron volt) is equal to 1000 electron volts. An MeV is equal to one million electron volts. |
| Flux |
A measure of the amount of energy given off by an astronomical object over a fixed amount of time and area. Because the energy is measured per time and area, flux measurements make it easy for astronomers to compare the relative energy output of objects with very different sizes or ages. |
| Geomagnetic storm |
A temporary disturbance of the Earth's magnetosphere, usually associated with solar coronal mass ejections (CME), coronal holes, or solar flares. A geomagnetic storm is initiated by a solar wind shock wave (like a sonic boom) which typically strikes the Earth's magnetic field 24 to 36 hours after the event leaves the Sun (assuming that the shock wave travels in a direction toward Earth). When the solar cloud reaches the Earth, it may disrupt the Earth's magnetosphere, compressing it on the dayside and extending the nightside tail. Geomagnetic storm events, along with solar flares, can disrupt radio transmissions, cause power outages (blackouts), and cause damage to satellites and electrical transmission lines. Magnetic storms usually last 24 to 48 hours, but some may last for many days. |
| Inclination |
The inclination of the orbit of a satellite around the Earth is the angle between the plane of its orbit and the equator. |
| keV (Kilo electron volt) |
is equal to 1000 electron volts. |
| Magnetic field |
A description of the strength of the magnetic force exerted by an object. Bar magnets have "di-polar" fields, as the force is exerted from the two ends of the bar. In simple terms, the earth, the sun, stars, pulsars all have dipolar magnetic fields. |
| Magnetosphere |
The region of space in which the magnetic field of an object (e.g., a star or planet) dominates the radiation pressure of the stellar wind to which it is exposed. |
| Magnetotail |
The portion of a planetary magnetosphere which is pushed in the direction of the solar wind. |
| MeV (Mega electron volt) |
is equal to one million electron volts. |
| Periapsis |
The point in an orbit when two objects are closest together. The opposite of apoapsis. |
| Perigee |
The point in its orbit where an Earth satellite is closest to the Earth. Opposite of apogee. |
| Plasma |
A low-density charged gas of electrons and ions (individual atoms that are ionized and therefore charged), even though the total number of positive and negative charges is equal, maintaining an overall electrical neutrality. |
| Proton |
A particle with a positive charge commonly found in the nucleus of atoms. |
| Radiation |
Energy emitted in the form of waves (photons) or particles (electrons, protons, etc.). |
| Radiation belts |
Regions of high-energy charged particles in a magnetosphere |
| Solar flares |
Violent eruptions of gas on the Sun's surface. |
| Solar wind |
The ejection of gas off the surface of the Sun. Many different types of stars, including our Sun, have stellar winds; however, a star's wind is strongest near the end of its life when it has consumed most of its fuel. |
| South Atlantic Anomaly (or SAA) |
The region where Earth's inner van Allen radiation belt makes its closest approach to the planet's surface. The result is that, for a given altitude, the radiation intensity is higher over this region than elsewhere. The SAA is produced by a "dip" in the Earth's magnetic field at that location, caused by the fact that the center of Earth's magnetic field is offset from its geographic center by 450 kilometers (280 miles). The South Atlantic Anomaly is of great significance to satellites and other spacecraft that orbit at several hundred kilometers altitude and at orbits tilted (inclined) between 35° and 60° against Earth’s equator and having altitudes of a few hundred miles; these orbits take satellites through the Anomaly periodically, exposing them to several minutes of strong radiation each time. The International Space Station, orbiting with an inclination of 51.6°, required extra shielding to deal with this problem. The Hubble Space Telescope does not take observations while passing through the SAA.
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