Explanatory list of terms and acronyms frequenly used in Space Weather. At the end of some explanations is provided a link to a more extensive description of the term.


  • ACE - Advanced Composition Explorer
  • Active region - Part of the solar atmosphere where can be observed phenomena associated with the Sun's magnetic activity: sunspots, prominences, filaments, plages, facular regions, flares, coronal mass ejections, etc.
  • Active prominence - Prominence that moves and changes its appearance over a time scale of minutes.
  • ap index - Formally determined from the eight daily ap indices. However, for daily operational uses, NOAA/SWPC estimates the value of the Ap index by measuring the geomagnetic field in near-real time at several magnetometer stations around the world. The real-time station indices are used to estimate the official Kp and Ap based on historical data. The value of this estimated Ap index is reported in NOAA daily and weekly summaries of geophysical activity.
  • Ap index - Formally determined from the eight daily ap indices.
  • Astromical ephemeris - Almanac providing the positions of astronomical objects at given times. (more)
  • Aurora - Light seen in the nocturnal skies generally at high latitudes. This result of collisions between atmospheric gases and charged particles from the solar wind as they fall guided a planet's magnetic field (phenomena not exclusive to Earth). Coronal mass ejections can increase the intensity of the solar wind, enhancing this effect, while the resulting magnetic storms can decrease the latitude at which auroras are seen. (more)


  • Bartels rotation number - Number of 27-day intervals (close to the Sun's rotation period) since 1 of January of 1833.
  • Bright point - A short-lived brightening of flare or near flare intensity, less than ten millionths of the solar hemisphere in area.
  • Burst - Transient increase of solar radio emission, generally associated with active regions or solar flares.
  • Butterfly diagram - Plot of the latitude of solar active regions against time. In this diagram can be seen that, on a solar cyle, the formation of active regions drifts from higher to lower latitudes, hence its butterfly shape.


  • Chromosphere - The layer of the solar atmosphere above the photosphere and beneath the transition region and the corona. The chromosphere is the source of the strongest lines in the solar spectrum, including the Balmer alpha line of hydrogen and the H and K lines of calcium, and is the source of the red color often seen around the rim of the moon at total solar eclipses. (more)
  • Convection - Heat transport process by vertical the movement of fluids. This is the same process involved when water is boiled in a pot. (more)
  • Corona - Outermost and largest layer of the Sun (extending for millions of kilometers) characterised by low densities ( < 109 cm-3) and high temperatures (> 106 K). Its outermost layers are constanly being lost, originating the solar wind. In this region we find phenomena such as: coronal holes. coronal loops, solar flares, and coronal mass ejections. The corona is better seen during total eclipses but can also be observed using coronographsor from space in the x-rays. (more)
  • Coronal hole - Extended region of the solar corona. with particularly low density ( 100 times less than the average), associated to magnetic fields with an "open" topology thus being a source of faster solar winds. (more)
  • Coronal loop - Typical structure of the transition region and lower corona, than can be seen in the Extreme ultraviolet lines and soft x-rays. Coronal loops represent the "closed" magnetic topology. (more)
  • Coronagraph - Device attached to telescopes that blocks the direct light from the solar disk making possible the observation of the solar corona. (more)
  • Coronal mass ejection (CME) - Large release of magnetized plasma from the solar corona. These events are associated with magnetic reconnection phenomena. CMEs vary widely in structure, density, and velocity. Large and fast CMEs can approach densities of 1016 g and velocities of 2000 km/s. The interaction between material released by CMEs and Earth's magnetic field can originate strong geomagnetic storms (more)
  • Cosmic rays - High energy particles that generally originate from outside the solar system (albeit some can be produced in more energetic solar proton events). (more)


  • Disk - The visible surface of the Sun projected against the sky.
  • Disturbance storm time (DST) index - A measure of variation in the geomagnetic field due to the equatorial ring current. It is computed from the H-components at approximately four near-equatorial stations at hourly intervals. At a given time, the Dst index is the average of variation over all longitudes. The reference level is set so that Dst is statistically zero on internationally designated quiet days. An index of -50 or deeper indicates a storm-level disturbance, and an index of -200 or deeper is associated with middle latitude aurorae. Dst is determined by the World Data Center C2 for Geomagnetism, Kyoto University, Kyoto, Japan. (more)
  • Diurnal variation of the geomagnetic field (Sq) - Daily variation of the geomagnetic field (derived from measurements of its declination and horizontal and vertical components) due to the ionospheric currents induced by solar radiation.


  • Eclipse - Obscuring of one astronomical object by another. When the Moon's shadow touches our planet's surface we have in those regions a solar eclipse. If the Moon covers completely the solar disk it is called a total eclipse, otherwise it is called a partial eclipse. A lunar eclipse occours when Earth's shadow covers the Moon. (more)
  • Eruptive prominence - Prominence associated with a coronal mass ejections.


  • F10.7 index - Daily index that consists of the solar 2800 MHz (10.7 cm) radio flux, which originates in the chomosphere and lower corona. For this reason a good indicator of the degree of solar activity, being well correlated with the sunspot number and visible and untraviolet solar irradiation levels. Reference values of the 10.7 cm radio flux are obtained at noon at the Dominion Radio Astrophysical Observatory (Canada).
  • Facular regions (or faculae) - Bright active regions on the solar photosphere. Faculae have good correspondence with concentrated magnetic fields that presage the formation of sunspots and solar plages located on the solar chromosphere above. Facular regions contribute even more than sunspots towards the Sun's total irradiation. (more)
  • Filaments - Long loop like structures on the solar surface, supported by strong magnetic fields, that are seen against the e solar disk (thus seen as dark filaments). The disrupture of some of filaments give rise to coronal mass ejections. When these structures are seen on the edge of the solar disk they are known as prominences. (more)
  • Flare - Sudden release of electromagnetic radiation near the solar surface, lasting minutes to hours, assocated with magnetic reconnection phenomena. The x-ray and extreme ultraviolet radiation generated by strong flares can ionise the upper layers of atmosphere, disrupting high frequency communications and global positioning singnals. (more)


  • Gamma radiation - High-energy radiation (more than 100 keV) than can be observed during large and extremely energetic solar flares. (more)
  • Geomagnetically induced current (GIC) - Quasi-direct current induced on electrical conductors by strong variations of the geomagnetic field that occur during geomagnetic storms. (more).
  • Geomagnetic field - Earth's magnetic field. (more)
  • Geomagnetic storm - Temporary disturbance of the geomagnetic field on global level, which is different from its regular daily variations. These events result from interactions between strong solar winds and Earth's magnetic field. By definition a geomagnetic storm occours when the planetary Kp index is equal or greather than 5. (more)
  • GOES - Geostationary Operational Environmental Satellites.
  • GNSS - Global Navigation Satellite System.


  • H-alpha (or Ha) - 656.3 nm wavelength red light associated with the first atomic transition in the hydrogen Balmer series (between the third and second atomic levels). This radiation is frequently used in the monitorization solar activity and in the study of several astronomical objects. (more)
  • Halo coronal mass ejection - Bright ring seen around all (or most) of the area covered by a coronagraph indicate a particularly large CME. Full halo CMEs on the front side of the Sun almost always result in geomagnetic storms, especially if accompanied by solar proton events.
  • Heliosphere - Region around the Sun, extending well beyond Pluto's orbit, that is dominated by the Sun and its solar wind. (more)


  • INTERMAGNET - International Real-time Magnetic Observatory Network.
  • Ionogram - Graphic showing the height at which the ionosphere reflects radio waves of a given frequency, providing information on the ammount of electrons and ions in the ionosphere. (more)
  • Ionosphere - Region of Earth's upper atmosphere (between 60 km and 1,000 km) constituted by free electrons and ions (resulting from the atmospheric ionization by solar ultraviolet radiation) and infalling energetic particles. The ionosphere plays a key role in the propagation of some radio waves (with frequencies smaller than 300 MHz) (more)
  • Ionospheric storm - Disturbance of the ionosphere that occours in connection with geomagnetic activity and higher fluxes of solar energetic electrons. (more)
  • ISWI - International Space Weather Initiative.



  • Kp index - Index used to indicate the severity of the global magnetic disturbances in near-Earth space. Kp is an index based on the average of weighted K indices at 13 ground magnetic field observatories. It is based on the range of the magnetic field variation within 3 hour intervals that is caused by phenomena other than the diurnal variation and the long-term components of the storm time variations. The values of the Kp range from 0 (very quiet) to 9 (very disturbed) in 28 discrete steps, resulting in values of 0, 0+, 1-, 1, 1+,...9.


  • LASCO - Large Angle Spectrometric COronagraph detector on board od the SOHO satelite capable of imaging CMEs.
  • Limb - Edge of the solar disk.
  • Limb darkening - Optical effect observed at certain wavelengths in which, as we move from the center to the edge of the solar disk, this appears darker. This effeito happens because of projection efects which, near the center of the solar disk, the optical depth allows us to see deeper (and warmer) layers of the Sun. (more)


  • Magnetic reconnection - Process in which the magnetic field lines with different directions form new connections among themselves, re-arranjing the magnetic field and converting part of the magnetic field's energy into kinetic and thermal energy. This phenomena is associated with the occurrence of solar flares and coronal mass ejections. (more)
  • Magnetogram - In heliophysics a magnetogram is a visual representation of the spatial distribution of the solar magnetic field strength, while in geophysics a magnetogram is a graphic representation of the temporal variations of the intensity of the geomagnetic field (or of one of its components).(more)
  • Magnetosphere - Region of space around an astronomical object in which charged particles are controlled by that object's magnetic field. (more)
  • Microwave - Electromagnetic waves with a frequency between 300 MHz and 300 GHz. (more)


  • Neutron monitor - Intrument used to measure the amount of neutrons generated on the atmosphere by cosmic rays of solar or extrasolar origin. (more)
  • NOAA - National Oceanic and Atmospheric Administration. Reference institution for the monitorization and study of Space Weather.



  • Penumbra - Lighter area that surrounds (or almost surrounds) the umbra of larger sunspots
  • Photosphere - Lowest visible layer of the Sun's atmosphere, corresponding to the solar surface observable in white light. Sunspots and facular regions are observed in this region. (more)
  • Plages - Bright active regions on the solar chromosphere that a distribution of the facular regions on the photosphere. Magnetic fields and temperatures are higher in plages than in the surrounding quiet regions. (more)
  • Plasma - Gas partially or totally ionized to a point that it is good electrical conductor and can be affected by magnetic fields. (more)
  • Prominence - Long loop like structure on the solar surface that is supported by strong magnetic fields. The disrupture of some of prominences give rise to coronal mass ejections. When these structures are seen against the solar disk they are known as filaments. (more)
  • Proton flare - flare that produces protons with energies above 10 MeV (see also proton storms).


  • Quiet day - Day without any storm.
  • Quiet day curve - Graphic of the daily variation of the components of the geomagnetic field in absence of geomagnetic activity
  • Quiet Sun - Sun when dispalying minumum degree of activity.


  • Radio waves - Electromagnetic waves with wavelengths between 1 cm and 10 km. The solar radio emission is well correlated with the degree of solar activity. (more)

S )

  • Scintillation - Degradation of the propagation of radio waves due to abrupt variations of electron density along their path, causing rapid variation of radio signal's amplitude and/or phase. (more)
  • Single event upsets (SEU) - Change of digital information on space borne micro-electronic devices caused by single energetic particle.
  • SOHO - Solar and Heliospheric Observatory.
  • Solar cycle or solar magnetic activity cycle - Corresponds to the nearly periodic 11-year variation of the solar magnetic activity, starting with an increase of the level activity (and that of phenomena associated with it), followed by a slower decrease. In each cycle the polarity pattern of the solar magnetic field reverses. (more)
  • Solar energetic particles (SEP) - High energy (keV to GeV) protons, electrons and ions that come from the Sun. (more)
  • Solar flux unit (sfu) - Unit of energy flux used in solar observations.
    1 SFU = 10-19 erg s-1 cm-2 Hz-1
  • Solar proton events (or proton storms) - Emission of high energy particles (mostly protons) that can penetrate Earth's magnetic field (and cause ionisation in the ionosphere like electrons do during auroras), that may occour during more energetic solar flares). These events can pose a threat to spacecrafts and astronauts. (more)
  • Solar wind - Outward flow of charged particles from the solar corona, constituted mainly by electrons, protons and alpha particles with thermal energies between 1.5 and 10 keV. Typical solar wind velocity is around 375 km/s (but can be up to 700 km/s) and proton and electron densities are near 5 particles/cm3. Intense solar winds can affect Earth's megnetic field. (more)
  • Space weather - Field of Space Sciences which deals with the variable conditions in the Sun, the interplanetary medium and our planet's vicinity, which have an impact on human safety and technological systems. (more)
  • Sudden ionospheric disturbance (SID) - Radio propagation anomalies due to changes in the ionosphere created by solar or geophysical events.
  • Sunspots - Small areas of the solar surface that, in contrast with the rest of the surface, appear as dark spots. These result from small concentrations of the Sun's magnetic field that, as they erupt on the solar surface (or are very close to it), inhibit the energy transport by convection, resulting in regions slightly colder (2700-4200 ℃) than their surroundings (5500 ℃). Larger sunspots can be larger than Earth, being divided into a darker central region (the umbra), surrounded by a lighter penumbra. Smaller spots only have an umbra. Sunspots typically occur in groups or in pairs associated with magnetic fields of opposite intensity. Their lifespan ranges from days to a few months. Since this phenomenon is due to the solar magnetic field activity, the number of visible sunspots is a good indicator of the solar magnetic activity. Most solar flares and coronal mass ejections originate in magnetically active regions associated with sunspot groups. (more)
  • Sunspot number (SSN) - Daily index of sunspot activity, which indicates the ammount of sunspots visible. This is a good indicator of the degree of solar activity. Currently the reference sunspot number is computed in Brussels


  • Total Electron Content (TEC) - Total number of electrons integrated between two points. The ionospheric TEC has impact on satellite communication and navigations systems, being strongly affected by solar and geomagnetic activity. The TEC maps presented here correspond to the vertical TEC (or vTEC), i.e. TEC measurement in the vertical direction (more).


  • Ultraviolet (UV) - Electromagnetic radiation with wavelength between 10 nm and 400 nm (shorter and more energetic than visible light). Ultraviolet radiation with wavelengths comprised between 10 nm and 100 nm are called extreme ultraviolet (EUV)radiation. (more)
  • Umbra - Dark central area of larger sunspots, or sunspot without penumbra.
  • Universal Time (U.T.) - Time standard corresponding to a continuation of the Greenwich Mean Time, i.e. the mean solar time at the Greenwich Meridian. It coincides with the Portuguese mainland standard (i.e. winter) time. (more)



  • White light - Combination of all visible radiation (wavelengths between 400 nm and 700 nm) of the electromagnetic spectrum. (more)


  • X-ray - Electromagnetic radiation with wavelength between 0.01 nm and 10 nm (more energetic than ultraviolet radiation). X-rays are used to classify the intensity of solar flares. (more)
  • X-ray flare class - Classification of the intensity of solar flares according to their x-ray emission in the 0.1 to 0.8 nm band: B class (under 10-6 W/m2), C class (10-6 W/m2 to 10-5 W/m2), M class (10-5 W/m2 to 10-4 W/m2) and X class (greater or equal to 10-4 W/m2).