Active Region (solar) -- A localized, transient volume of the solar atmosphere characterized by complex magnetic fields, often associated with sunspots, flares, coronal mass ejections, plages, faculae, and other solar phenomena.

Angstrom -- A unit of length = 1.0E-08cm. 

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

Aurora -- A colorful glow in the sky, often observed in a doughnut-shaped region around the magnetic poles ("auroral zone") and occasionally further equatorward.   The aurora is generally caused by fast electrons from space guided earthward by magnetic field lines. Its light comes from collisions between such electrons and the atoms of the upper atmosphere, typically 100 km (60 miles) above ground. 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, Aurora Australis (Southern Lights), and polar aurora.  

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

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

AWESOME Monitor -- Atmospheric Weather Electromagnetic System for Observation Modeling and Education instruments which monitor changes in the Earth’s ionosphere by tracking VLF signals as they bounce through the Earth’s waveguide. Developed by the Space, Telecommunications, and Radioscience (STAR) Laboratory, a research group within the Department of Electrical Engineering of Stanford University in association with the Stanford Solar Center and NSF’s Center for Integrated Space Weather Modeling (CISM). 

Bartels Rotation Number -- The serial number assigned to 27-day recurrence periods of solar and geophysical parameters. The equatorial rotation rate of the Sun is very close to 27 days. Rotation 1 Day 1 in this sequence was assigned arbitrarily by J. Bartels to February 8, 1832. 

Burst (radio) -- A transient enhancement of the solar radio emission, usually associated with an active region or flare.  

Chromosphere -- The layer of the solar atmosphere above the photosphere and beneath the transition region and the corona. It is seen during eclipses as a bright red ring around the Sun, with the term burning prairie used to describe it. 

CME – See Coronal Mass Ejection 

Coordinated Universal Time – see Universal Time

Corona (solar) -- 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

Coronal Hole -- an extended region of the corona, exceptionally low in density (essentially a large open gap), and associated with photospheric regions. Coronal holes are closely associated with those regions on the Sun that have an "open" magnetic geometry, that is, the magnetic field lines associated with them extend far outward into interplanetary space, rather than looping back to the photosphere. Ionized material can flow easily along such a path, and this in turn aids the mechanism that causes high speed solar wind streams to develop. 

Coronal mass ejection (CME)--a huge cloud of hot plasma occasionally expelled from the Sun. It may accelerate ions and electrons and travel through interplanetary space as far as the Earth’s orbit and beyond. The leading edges of fast-moving CMEs drive giant shock waves before them through the solar wind at speeds up to 1200 km per second. When the shock reaches Earth, a magnetic storm may result.  CMEs occurs on a time scale between a few minutes and several hours, and involve the appearance of a new discrete, bright, white light feature in a coronagraph field of view that displays a predominantly outward motion. The solar corona material is massive in size (they can occupy up to a quarter of the solar limb), and frequently accompanied by the remnants of an eruptive prominence, and less often by a strong solar flare. CMEs are the crucial link between a solar disturbance, its propagation through the heliosphere, and the effects on the Earth. 

Cosmic rays/radiation -- 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 of starlight. The origin of their huge energies is thought to come from expanding shock fronts created by huge cosmic explosions such as supernovae. 

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. 

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. 

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. 

Eclipse (solar) -- occurs when the Moon passes between Earth and the Sun, thereby totally or partially obscuring Earth's view of the Sun. This configuration can only happen during a new moon, when the Sun and Moon are exactly lined up as seen from the Earth.  Total solar eclipses are very rare events for any given place because totality is only seen where the Moon's shadow touches the Earth's surface, and only last for a few minutes. 

Ecliptic -- the plane in which the Earth orbits the Sun 

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.

Electric current--a continuous flow of electrons and/or ions through a material which conducts electricity. A current 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. 

Electrical interference – see interference 

Electromagnetic field/wave -- a combination of oscillating magnetic and electric fields, spreading in wavelike fashion through space at a speed of about 300 000 km/sec. Such waves include all forms of light--infra-red and ultra-violet, visible light, 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.  

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. 

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).  

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. 

Extremely Low Frequency (ELF) -- that portion of the radio frequency spectrum from 30 to 3000 hertz. 

Extreme ultraviolet light/energy/radiation (EUV) -- a portion of the electromagnetic spectrum from approximately 100 to 1000 angstroms.

  Facula, pl. faculae -- A bright cloud-like feature located a few hundred km above the solar photosphere near sunspot groups, seen in white light. Faculae are clouds of emission that occur where a strong magnetic field creates extra heat (about 300 degrees K above surrounding areas). 

Farside (solar) – The half of the Sun not seen from Earth at any given time is colloquially referred to as the “farside”.  It is somewhat akin to the back side of the Moon, although the Sun does not keep one side locked towards the Earth as the Moon does.

  Filament (solar) -- a mass of gas suspended over the photosphere by magnetic fields and seen as dark lines threaded over the solar disk. A filament on the limb of the Sun seen in emission against the dark sky is called a prominence.

 Flare (solar) -- a sudden eruption of energy on the solar disk, lasting minutes to hours, from which radiation and particles are emitted. Flares usually occur in the vicinity of active regions or sunspots. Their sudden brightening 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.  

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.

Gamma ray bursts (GRB) -- short-lived sudden explosions of gamma-ray photons, the most energetic form of light. Lasting anywhere from a few milliseconds to several minutes, gamma-ray bursts shine hundreds of times brighter than a typical supernova and about a million trillion times as bright as the Sun, making them briefly the brightest source of cosmic gamma-ray photons in the observable universe. They can be associated with supernovae, pulsars, convergence of neutron stars or Black Holes, and other energetic cosmic cataclysms.

Gamma rays -- electromagnetic waves of the smallest wavelengths and highest frequencies known; the most energetic of waves in the electromagnetic spectrum. Gamma rays are generated by transitions within atomic nuclei such as those in radioactive atoms and nuclear explosions. This high energy radiation (energies in excess of 100 keV) is observed during large, extremely energetic solar flares. 

Gauss -- The unit of magnetic induction in the cgs (centimeter-gram-second) system 

Geomagnetic Field -- The magnetic field observed in and around the Earth. The intensity of the magnetic field at the Earth's surface is approximately 0.32 gauss at the equator and 0.62 gauss at the North Pole.  

Geomagnetic Storm -- A worldwide disturbance of the Earth's magnetic field, distinct from regular diurnal variations. 

Geosynchronous – The term applied to any equatorial satellite with an orbital velocity equal to the rotational velocity of the Earth. The net effect is that the satellite is virtually motionless with respect to an observer on the ground. 

GMT -- Greenwich Mean Time. See Universal Time 

GOES satellites – A collection of Geostationary Operational Environmental Satellites circling the Earth in a geosynchronous orbit, allowing them to hover continuously over one position on the surface. The geosynchronous plane is about 35,800 km (22,300 miles) above the Earth, high enough to allow the satellites a full-disc view. They provide a constant vigil for the atmospheric triggers for severe weather conditions such as tornadoes, flash floods, hail storms, and hurricanes.  They also monitor solar flares and activity on the Sun.  

Heliopause -- The region in space where the Sun's atmosphere merges with interstellar space. The position of the heliopause depends both on the strength of the solar wind and on the properties of the local interstellar medium.  

Heliosphere -- The region in space that extends to the heliopause. The heliosphere is the cavity around the Sun in the local interstellar medium that is produced by the solar wind. The heliosphere contains most of the solar system, but not the most distant comets such as in the Oort cloud. 

High Frequency (HF) -- that portion of the radio frequency spectrum between 3 and 30 MHz. 

IMF—See Interplanetary Magnetic Field 

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

Interference (electrical) -- electrical noise induced upon the signal wires that obscures the wanted information signal; inhibition or prevention of clear reception of broadcast signals. 

Interference (waves) -- The variation of wave amplitude that occurs when waves of the same or different frequency come together. 

International Heliophysical Year (IHY) 2007-9 -- an international science and education based celebration of the 50^th anniversary of the International Geophysical Year 1957. Its goal us to advance our understanding of the fundamental processes that govern the Sun, Earth, and heliosphere and to demonstrate the beauty, relevance, and significance of space and Earth science to the world.  See http://ihy2007.org/. 

Interplanetary magnetic field (IMF) -- The magnetic field carried with the solar wind. 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 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.  

Ion -- Usually, an atom from which one or more electrons has been torn off, leaving a positively charged particle. "Negative ions" are atoms which have acquired one or more extra electrons. 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 absorption of light ("photoionization") or by a collision with a fast particle ("impact ionization").  

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 and from cosmic rays. The ionosphere significantly influences radio wave propagation of frequencies less than about 30 MHz. 

LASCO – The Large Angle and Spectrometric Coronagraph Experiment aboard the Solar and Heliophysics Observatory (SOHO) spacecraft. See http://lasco-www.nrl.navy.mil/

 Low Frequency (LF) -- That portion of the radio frequency spectrum from 30 to 300 kHz. 

Magnetar – a super-magnetized neutron star. 

Magnetic field --a region in which magnetic forces can be observed. See also "electromagnetic field."  

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

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 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.  

Magnetogram -- a graphic representation of solar magnetic field strengths and polarity.

Magnetometer – an instrument for measuring magnetic fields.  

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 night side 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.

 MDI – see Michelson Doppler Imager 

Medium Frequency (MF) -- That portion of the radio frequency spectrum from 0.3 to 3 MHz. 

Mercator projection (maps) -- The Mercator projection is a cylindrical map projection onto a flat surface; devised by the Flemish geographer and cartographer Gerardus Mercator in 1569. 

Michelson Doppler Imager (MDI) – an instrument on the SOHO spacecraft, designed to study the interior structure and dynamics of the Sun. MDI is a project of the Stanford-Lockheed Institute for Space Research and is a joint effort of the Solar Oscillations Investigation (SOI) in the W.W. Hansen Experimental Physics Laboratory of Stanford University and the Solar and Astrophysics Laboratory of the Lockheed-Martin Advanced Technology Center. See http://soi.stanford.edu/ 

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

Orbit -- the line followed by a spacecraft or a celestial body in motion around an object.  

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

Photon --colloquially, a "packet 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. Sunspots and faculae are observed in the photosphere. 

Plage – a bright feature found in the vicinity of most active sunspot groups; occurs on a larger scale and is brighter than a facula. Plage is French for "beach," because each plage looks like light-colored sand against the darker structures around them. 

Plasma -- any 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. 

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.

 Prominence -- A term identifying magnetic field induced cloud-like features in the solar atmosphere. The features appear as bright structures in the corona above the solar limb and as dark filaments when seen projected against the solar disk.  

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:  1) 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. 2) 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 cosmic radiation). 

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

Radio propagation -- a term used to explain how radio waves behave when they are transmitted from one point on the Earth to another. 

Radio waves -- Radio waves have the longest wavelengths in the electromagnetic spectrum, ranging from about the size of a football to many miles in length. 

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. 

Ring current -- In the magnetosphere, a region of current that flows in a disk-shaped region near the geomagnetic equator in the outer of the Van Allen radiation belts. The current is produced by the gradient and curvature drift of trapped charged particles. The ring current is greatly augmented during magnetic storms because of the hot plasma injected from the magnetotail. 

Reflect -- To throw or bend back (light, for example) from a surface. 

Refract – To bend or change direction, as in the light refracted into a rainbow. 

Sferics -- a jargon term for radio signals induced by lightning. 

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

SID Monitor – see Sudden Ionospheric Disturbance Monitor 

SOHO – see Solar and Heliospheric Observatory Solar and Heliospheric Observatory (SOHO) – A joint ESA/NASA spacecraft launched in 1995 and containing a battery of instruments to study the structure and dynamics of the Sun.  See http://sohowww.nascom.nasa.gov/ 

Solar corona— see Corona 

Solar Cycle -- The approximately 11-year quasi-periodic variation in frequency or number of solar active events.  See also “Sunspot cycle.”  

Solar energetic particles -- high energy particles occasionally emitted from active areas on the Sun, usually 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 – see Flare 

Solar wind -- The outward flux of solar particles and magnetic fields from the Sun. The solar wind is produced primarily in the cooler regions of the corona, known as coronal holes, and flows along the open magnetic field lines. Typically, solar wind velocities are 300-500 km per second.  

Space Weather--the popular name for energy-releasing phenomena from the Sun and its effects on planetary systems.  Space weather conditions can influence the performance and reliability of space-borne and ground-based technological systems and endanger human life or health. 

Substorm (auroral) -- a process by which plasma in the Earth’s magnetotail becomes energized at a fast rate, flowing earthward and producing bright auroras for typical durations of half an hour. 

Sudden Ionospheric Disturbance (SID) – an abnormally high plasma density in the Earth’s ionosphere caused by an occasional sudden solar flare; SIDs often interrupt or interfere with telecommunications systems. 

Sudden Ionospheric Disturbance (SID) Monitor -  an instrument which tracks sudden changes to the Earth’s ionosphere caused by solar flares. 

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 -- concentrations of magnetic flux, typically occurring in bipolar (i.e. two-part with positive and negative poles, like a magnet) clusters or groups. They appear dark because they are cooler than the surrounding photosphere. They are cooler than the surrounding photosphere because the magnetic field interferes with the outflow of solar heat in that region.) 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 sunspots, the cycle is magnetic in nature, and the polar magnetic field of the Sun also reverses each solar cycle, making the true cycle about 22 years long. 

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—1) a large explosion at the end of the evolutionary process of a very massive star.2) an explosion of material from a white dwarf star after it has been accumulating mass from a binary companion. An enormous amount of energy is released in these explosions.  

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 and satellites transmitting TV programs. 

Terrestrial gamma-ray flashes (TGFs) -- bursts of gamma rays in the Earth's atmosphere, probably caused by electric fields produced above thunderstorms . TGFs have been recorded to last 0.2 to 3.5 milliseconds, and have energies of up to 20 MeV. 

Terminator -- the dividing line between the bright and shaded regions (usually shaded from the light of the Sun) of the disk of a moon or planet. 

Ultra High Frequency (UHF) -- Those radio frequencies exceeding 300 MHz.  

Ultraviolet (UV) -- electromagnetic radiation lying in the ultraviolet range , i.e. wavelengths shorter than visible light but longer than X-rays. UV cannot be seen by the eye.

Universal Time (UT) -- By international agreement, the local time at the prime meridian, which passes through Greenwich, England; also known as Greenwich Mean Time or Coordinated Universal Time.

 Very High Frequency (VHF) -- That portion of the radio frequency spectrum from 30 to 300 MHz. 

Very Low Frequency (VLF) -- That portion of the radio frequency spectrum from 3 to 30 kHz.  

Visible light – see White Light 

Waveguide (Earth’s ionosphere) – a structure that guides electromagnetic waves along its length. The space between the Earth’s surface and the ionosphere makes an excellent waveguide for VLF radio transmissions.  

White Light, visible light -- Sunlight integrated over the visible portion of the spectrum (4000 - 7000 angstroms) so that all colors are blended to appear white to the eye. 

X-ray Burst – In solar-terrestrial terms, a temporary sudden enhancement of the X-ray emission of the Sun. These bursts can also be caused by thermonuclear explosions on the surface of a neutron star accreting material from a binary companion.