Space Weather Glossary +

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A fine, solid or liquid particle suspended in air, such as a dust mite

Aurorae that appear in the Earth’s southern hemisphere

Measures the amplitude of global geomagnetic activity during 3-hour intervals normalized to a geomagnetic latitude ±50°. The index was introduced to monitor geomagnetic activity over the longest possible time period. Current observatories are at Hartland, England, and Canberra, Australia. The aa values are in units of 1 nT. The index is available back to 1868

Active Region

Solar

A localized, transient volume of the solar atmosphere with features including plages, sun spots, faculae, flares, etc.,. Active regions are the result of enhanced magnetic fields; they may be complex if the region contains two or more bipolar groups

Observed conditions meeting or exceeding storm thresholds

a unit of measure often used in expressing distances in the heliosphere. The average Sun-Earth distance is 1 AU = 149,600,000 km = 92,957,000 miles

Friction produced when a spacecraft in low Earth orbit collides with atmospheric atoms and molecules. The result is a loss of mechanical energy and altitude

A sunspot group that has distinct positive and negative poles (hence, bipolar) with a simple division between the polarities. Often one polarity will dominate. The volume connecting the poles is called a magnetic flux tube.

Aurorae that appear in the Earth’s northern hemisphere

Particle-induced disturbances in the ionosphere-magnetosphere which are linked to the brightest and most dynamic aurorae

Diffused, glowing light emitted from atoms and molecules in the Earth’s upper atmosphere when incoming high energy particles from the Sun or the Earth’s magnetosphere collide with them

An oval-shaped band around each geomagnetic pole, ranging from 67°~75°magnetic latitude, in which most aurorae occur. The oval expands to higher and lower latitudes during the expansion phase of a auroral substorm.

A shock wave formed in a plasma stream when it comes upon a barrier such as the Earth’s magnetosphere. The bow shock deflects, slows and heats the incoming plasma.

Solar Wind

Magnetosphere

Plasma

A transient enhancement of the solar radio emission, usually associated with an active region or flare

The region of the Sun’s atmosphere above the photosphere and below the transition region, which exhibits temperatures between 4,000 K to 8,000 K. The chromosphere has visible (red) emission from excited hydrogen at ~656.3 nm (Balmer-alpha transition). Solar filaments, plage and spicules are evident in the chromosphere

A magnetic line of force that connects regions of opposite magnetic polarity, as for example, in magnetically-active areas on the Sun, or between the two magnetic poles of the Earth

A dark region in the inner corona where open magnetic field lines allow coronal particles to escape the Sun as fast solar wind, resulting in a region of partially-depleted density and hence reduced brightness

Segments of the corona expelled from the Sun into interplanetary space in the form of expanding clouds of solar plasma. The transient ejecta also carry matter flux into the heliosphere and are the primary sources of strong geomagnetic storms.

A coronal structure consisting of a magnetically-formed arch that narrows outward as a tapered shape or stalk. Streamers are a known source of coronal mass ejections. The bulb-arch structures are seen in eclipse photographs and in space-based white-light images

Highly energetic atomic nuclei stripped of most or all of their electrons, that continuously pass from all directions through intergalactic, interstellar and interplanetary space. The most energetic of these (called galactic cosmic rays) may originate in supernova explosions

The expanding sequence of secondary cosmic rays initiated by the collision of an incoming cosmic ray with an atom or molecule of air. This initial impact produces daughter particles that then collide, farther down in the atmosphere, to create a continuing series (or cascade) of similar events

Displays of the aurora borealis or australis which occur in the atmosphere above the daylit hemisphere of the Earth, and because of this go largely unseen

The apparent circular shape of the Sun when seen in the sky, resulting from a two-dimensional projection of a spherical object

The solar dynamo  harnesses the mechanical energy of differential rotation to twist polar field magnetic lines within the Sun into toroidal fields (perpendicular to the Sun’s axis of rotation.) These in turn give rise to sunspots when they are carried upward by convection to the solar surface

A region of space in which a detectable electric intensity is present at every point

The region of relatively constant high temperature in the Earth’s atmosphere above an altitude of about 600 miles. Here the few neutral atoms and molecules that remain are on their way out of the atmosphere, due to their high thermal velocities and the very low density of the region

The phase when going from solar maximum to solar minimum

A mass of gas suspended over the chromosphere by magnetic fields and seen as dark ribbons threaded over the solar disk. A filament on the limb of the Sun seen in emission against the dark sky is called a prominence.

Forecasts are the predictions of future events, based on analysis and modeling of the past and present conditions of the environment. Space weather forecasts are typically for conditions 1-3 days in advance

Radiation consisting of oscillating electric and magnetic fields, including gamma rays, visible light, ultraviolet and infrared radiation, radio waves and microwaves

NOAA Space Weather Scale ranging from 1 to 5 for geomagnetic storm disturbances (G). It is based on the Kp index and is associated with disturbances in the geomagnetic field, aurora and sometimes power grid upsets.

High-energy radiation (energies in excess of 100 keV) observed during large, extremely energetic solar flares. Gamma ray flashes are also associated with some lightning discharges at Earth

Natural variations in the geomagnetic field classified qualitatively into quiet, unsettled, and active/storm. Geomagnetic activity may be described quantitatively by one of several geomagnetic indices including the Kp, auroral electrojet (AE) and/or Disturbance Storm Time (Dst) indices, or by the location of visible aurora

The magnetic field in and around the Earth. The intensity of the magnetic field at the Earth’s surface is approximately 32,000 nT at the equator and 62,000 nT at the north pole (the place where a compass needle points vertically downward). The geomagnetic field is dynamic and undergoes continual slow secular changes as well as short-term disturbances .

A severe but transitory fluctuation in the Earth’s magnetic field, evident initially as a sharp decrease in the strength of the horizontal component of the Earth’s magnetic field, felt around the world and lasting a few hours, followed by a recovery phase lasting a day to several days. Geomagnetic storms are most often initiated when regions of enhanced solar wind flow compress the steady-state form of the magnetosphere on its Sun-facing side

A circular, equatorial Earth orbit, whose orbital period is equal to the Earth’s sidereal rotation period. A satellite in geostationary orbit appears stationary to an observer on Earth. A geostationary orbit is at type of geosynchronous orbit

Earth Orbit with an orbital period equal to the Earth’s sidereal rotation period. The orbit is an inclined ellipse and the orbital height varies. A satellite in geosynchronous orbit is above the same spots of the Earth's surface once per sidereal day

The outer boundary of the heliosphere, at which place the solar wind becomes indistinguishable from the local interstellar medium

The region of subsonic flow that stands between the termination shock in the extended solar wind and the outer boundary of the heliosphere

The vast region surrounding the Sun dominated by atomic particles and magnetic fields that are carried away from the Sun by the solar wind

A feature of the solar wind having velocities exceeding approximately 600 km/s (about double average solar wind values). High-speed streams that originate in coronal holes are less dense than those originating in the average solar wind.

The innermost of the two concentric Van Allen belts of trapped atomic particles that surround the Earth in the equatorial region. The inner belt contains more energetic electrons, protons and heavier ions and extends upward from the top of the atmosphere to a height of about 12,000 miles

The extension of the magnetic field of the Sun throughout the heliosphere

The electrically-conducting region in the upper atmosphere made up of three horizontal layers extending from about 35 to more than 1000 miles above the surface, which are produced by the ionization of neutral atoms of air by shortwave solar radiation.

A disturbance in the F region of the ionosphere, which occurs in connection with geomagnetic activity. In general, there are two phases of an ionospheric storm, an initial increase in electron density (the positive phase) lasting a few hours, followed by a decrease lasting a few days. At low latitudes only the positive phase is usually seen. Individual storms can vary, and their behavior depends on geomagnetic latitude, season, and local time.

A temperature directly related to the average speed of atoms or molecules in a substance such as air

The Planetary K-index quantifies the disturbance in the horizontal component of Earth's main magnetic field. A network of geomagnetic observatories record the maximum fluctuations of horizontal components observed by magnetometers during three-hour intervals. The Kp from 0 (quiet) to 9 (extremely disturbed) in 28 steps

A gradual reduction in brightness of the Sun’s disk (or star) from center to limb. At disk-center, an observer sees into the Sun’s warmer layers that emit the most light. At the limb, the emissions are from the upper, cooler layers that produce less light. Limb darkening allows solar observers to investigate the solar atmospheric temperature structure. Limb darkening also occurs in some radio wavelengths

Positions in space where objects sent there tend to ‘settle’. At Lagrange points, the gravitational pull of two large masses precisely equals the centripetal force required for a small object to move with them. These locations can be used by spacecraft to reduce fuel consumption needed to remain in position.Solar wind monitors are often placed at Lagrange point 1 (L1). The L1 point of the Earth-Sun system affords an uninterrupted view of the Sun

The apparent, circular edge of the Sun as seen in the sky. In visible light the edge is very sharp. In other wavelengths of solar light, it is much thicker.

Earth Orbit with an apogee altitude that does not exceed 2,000 km

The portion of space near a magnetic body (such as the Sun) or a current carrying body (such as an electric power line) in which there is a detectable magnetic force at every point in the region

Imaginary lines (like the arrows used to show wind flow direction on a meteorological chart) that indicate the direction of the magnetic force at any point in a magnetic field.

Either of two non-fixed points on the Earth, close to but not coincident with the north and south rotational poles, where the Earth’s magnetic field is most intense and where magnetic field lines are most nearly perpendicular to the Earth’s surface

The outer boundary of the Earth’s magnetosphere, where the strength of the solar wind magnetic field surpasses that of the Earth. Though highly variable, it is typically 40,000 to 60,000 miles away from the Earth on the Sun-facing side, and much farther away on the down-wind side

The region between the bow shock and the magnetopause, characterized by very turbulent plasma. For the Earth, along the Sun-Earth axis, the magnetosheath is about 2 Earth radii thick.

The region around the Earth occupied by its magnetic field

The extension of the magnetosphere in the antisunward direction as a result of interaction with the solar wind. In the inner magnetotail, the field lines maintain a roughly dipolar configuration. At greater distances, the field lines are stretched into northern and southern lobes, separated by a plasmasheet. There is observational evidence for traces of the Earth’s magnetotail as far as 1000 Earth radii downstream.

Earth orbit with apogee altitude that is greater than 2,000 km but does not exceed 36,000 km

The upper boundary of the mesosphere and lower boundary of the thermosphere, which lies just above it

The upper part of the middle atmosphere of the Earth, extending from about 30 to 53 miles above the surface, in which air temperature falls monotonically from about plus 200 to minus 135° F

Generically, any radio frequency of 500 MHz or more

One of two regions in the magnetotail where stretched-out open magnetic field lines of opposite polarity, attached at the Earth’s north and south magnetic poles, are brought in contact, allowing a return path for captive particles in the magnetotail to be channeled back toward the planet

A magnetic line of force in the magnetic field of either the Sun or the Earth, one end of which is rooted in the photosphere or at the surface of the Earth, and the other drawn away and detached by dynamic forces

The outer of the two concentric Van Allen belts of trapped atomic particles that surround the Earth in the equatorial region. The outer belt is separated from the inner belt by a 4000 mile gap, and extends above the surface of the planet from about 16,000 to 24,000 (and at times as far as 36,000) miles. Within it are the lighter and less energetic trapped particles: primarily weaker electrons with energies in the range of 10,000 to about one million electron volts

The region in the Earth’s middle atmosphere, between altitudes of about 25 and 65 miles above the surface, where almost all atmospheric ozone is found. The remainder is created in the form of air pollution at ground level in the photosphere

A gas that is ionized sufficiently to be a good electrical conductor and can be affected by magnetic fields.

The outer boundary of the Earth’s plasmasphere and inner boundary of the magnetosphere

The central and densest part of the Earth’s magnetotail, consisting of a compressed sheet that extends downwind of the Sun for at least 950,000 miles from the Earth, separating the northern and southern lobes of the tail, which have opposite magnetic polarity. The plasmasheet is a major storage region for ionized particles in the geomagnetic tail

The upward extension of the Earth’s ionosphere into the exosphere, within and co-existing with the magnetosphere, which reaches on its Sun-facing side from about 1000 miles to as much as a million miles above the surface. It consists of a relatively low-energy plasma and takes its form as charged particles from the ionosphere flow upward to fill the relative vacuum of space surrounding the Earth

The region of the solar atmosphere from which all visible light and heat are radiated into space. The intangible surface we see when we look at the Sun in visible light

In auroral nomenclature, the area around either the north or south magnetic pole of the Earth bounded by the inner boundary of the auroral oval. Here, poleward of the auroral oval, auroras are more frequent but weaker, more diffuse and less variable than in the auroral oval itself

The singular regions over the Earth’s magnetic poles where magnetic field lines are nearly perpendicular to the Earth’s surface, creating an “opening” in the magnetosphere that allows charged particles paths of easier entry into the upper atmosphere

A spacecraft orbit that passes over the polar (as opposed to restricted to middle and equatorial) latitudes of the Earth. For spaceborne instruments that observe the Sun, a polar orbit offers the opportunity for continuous 24-hour monitoring.

A high-energy atomic particle that arrives at the Earth from beyond the planet, as opposed to the secondary cosmic rays that are formed as a result of a collision of a primary with an atom or molecule of air in the Earth’s atmosphere

A measure of the severity of solar x-ray bursts that cause radio blackouts at Earth

Two concentric areas of trapped electrons, protons and ions held within the closed part of the Earth’s magnetic field, from about 600 miles to 25,000 miles above the surface
Same as Van Allen Belts

A process through which oppositely-directed closed magnetic field lines come into contact, sever, and join to form new magnetic field structures. In the process part of the magnetic energy contained in the magnetic field is converted into thermal or kinetic energy

A tendency of some solar and geophysical parameters to repeat a trend and sometimes the actual value of the parameter itself every 27 days (the approximate rotation period of the Sun).

An electrical current produced in the equatorial plane within the closed part of the Earth’s magnetic field where properties of the magnetic field cause ions and electrons to drift in opposite directions

The phase between a solar minimum and a solar maximum

A measure of the severity of solar proton events as depicted in the NOAA Space Weather Scales.

The dispersal of a beam of light into a spread of directions as a result of physical interactions: in the daytime sky, the redirection of incoming sunlight across the dome of the sky by its interaction with molecules of air; in the white-light corona, the redirection of photospheric radiation by free electrons.

A flickering of electromagnetic radiation caused by its passage through turbulent media

A secondary or “daughter” particle produced by collisions between primary cosmic rays from space and the atomic nuclei of atoms and molecules in the Earth’s atmosphere

Discrete, wedge-shaped segments, centered on the Sun, in the expanding solar wind in which the magnetic polarity of the source region on the Sun is carried outward in the plasma and preserved. They are sensed at the Earth (as they sweep by with solar rotation) as distinct changes in the prevailing polarity of the solar wind

An abrupt change in temperature, speed, density and pressure in a moving plasma, produced by the movement of an object traveling through the medium at a speed greater than the local speed of sound, that can accelerate energetic particles and trigger geomagnetic phenomena.

Phenomena on the Sun such as sunspots, plages, flares, and CMEs whose frequency of occurrence is related to the 11-year sunspot cycle.

The photosphere, chromosphere, and corona of the Sun

The total amount of radiant energy received from the Sun per unit time per unit area at the top of the Earth’s atmosphere, at mean Sun-Earth distance. Once thought, erroneously, to be constant, the term has now been supplanted by the more precise term, total solar irradiance

A proton ejected from the Sun with an energy in the 1 to 500 meV range, which is potentially damaging due to its heavy mass and high speed

A sudden and highly-localized increase in the brightness and the energy released from a restricted area on the solar surface which is most often located within a complex solar active region; thought to be provoked by instabilities in magnetic structures that cause opposite field lines to reconnect in a very small volume of material.

The month(s) during a sunspot cycle when the smoothed sunspot number reaches a maximum.

The month(s) during a sunspot cycle when the smoothed sunspot number reaches a minimum.

The turning of the Sun( in about 27 days) about an axis that passes through it

The Sun together with the planets and all other objects that revolve about it

The continual release of atomic particles and embedded magnetic fields (plasma) from the Sun resulting from the thermal expansion of the corona

The variable state of the magnetosphere, ionosphere and near-Earth space as perturbed by solar activity and the solar wind: the counterpart of meteorological weather

Current conditions

The upper limit of the Earth’s stratosphere and lower limit of the mesosphere

The atmosphere extending above the troposphere to an altitude of about 30 miles that exhibits warming with height, the result of the absorption of solar radiant energy by stratospheric ozone

Flight at an altitude and velocity that would result in a trajectory incapable of circling the Earth at least once.

A geomagnetic perturbation lasting 1 to 2 hours, which tends to occur during local post-midnight nighttime. The magnitude of the substorm is largest in the auroral zone, potentially reaching several thousand nanotesla. A substorm corresponds to an injection of charged particles from the magnetotail into the auroral oval.

A distinctive, activity-related region in the photosphere, the embodiment of a very strong magnetic field that is cooler and hence darker than the surrounding photosphere

The roughly 11-year cyclic variation in the state of activity on the Sun, most apparent in annual averages of the number of sunspots seen on its white-light surface

An historical index of solar activity defined in the 1860s as the number of spots that are visible on the Sun at any time plus ten times the number of groups of sunspots, multiplied by a factor intended to correct for differences in telescopes, observing sites, and observers

The region of the Sun’s atmosphere above the transition region with a temperature between 500,000 K and 2M K. Magnetic arches and coronal holes are often imaged in the corona. Disturbances such as solar flares and coronal mass ejections are also evident in the corona.

A non-self-sustaining discharge (sometimes visible) due to ionization of the gas surrounding a conductor around which exists a voltage gradient exceeding a certain critical value for a gaseous medium. Often used in describing spacecraft charging effects

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A shock wave that forms at the place in the outer heliosphere where the solar wind first begins to feel the competing force of stellar winds. In passing through it, the solar wind slows from supersonic (about a million miles per hour) to subsonic speeds

The uppermost layer of the Earth’s atmosphere, extending from an altitude of about 50 to more than 1000 miles, where absorption of short-wave solar radiation heats the gas to very high temperatures

Electromagnetic energy in all wavelengths received from the Sun at the top of the Earth’s atmosphere

The region of the Sun’s atmosphere above the chromosphere and below the corona, which exhibits temperatures between 8,000 K to 500,000 K

The upper limit of the troposphere and lower limit of the stratosphere, at an altitude of about seven miles above sea-level, though somewhat higher in the tropics

The lowest region of the Earth’s atmosphere, extending from the surface to the tropopause (about 7 miles high) and characterized by decreasing temperature with increasing altitude; the locus of all weather and climate

That part of the electromagnetic spectrum between 5 - 400nm.

A large-scale photospheric region where the magnetic elements are predominantly of one polarity (for example, the solar polar regions)

Two concentric areas of trapped electrons, protons and ions held within the closed part of the Earth’s magnetic field, from about 600 miles to 25,000 miles above the surface
same as radiation belts

Storm is imminent with high probability

Conditions are favorable for storm

The combination of light of all colors in the visible spectrum of electromagnetic radiation. The disk of the Sun, which appears white to us, is an example, as is the white color of clouds or the solar corona, both of which represent scattered light from the white photosphere

A major flare in which small parts become visible in white light. This rare continuum emission is caused by energetic particle beams bombarding the lower solar atmosphere. Such flares are usually strong x-ray, radio, and particle emitters.

Relatively cool, quiescent plasma anchored in the photosphere but extending into the solar corona. Prominences mark regions in the upper solar atmosphere where magnetic fields exert forces on partially ionized plasma to counteract gravity and suspend the plasma above the solar surface.Erupting prominences comprise most of the mass in Coronal Mass Ejections. When seen on the solar disk, prominences are called “filaments” in reference to their long, thread-like structure

Radiation of extremely short wavelength (generally less than 1 nm).

A temporary enhancement of the x-ray emission of the Sun. The time-intensity profile of soft x-ray bursts is similar to that of the H-alpha profile of an associated flare.