A Glossary of Space Weather Terms

  • AE index: A geomagnetic index describing the auroral electrojet.
  • ap index: A mean, 3-hourly "equivalent amplitude" of magnetic activity based on K index data from a planetary of 11 Northern and 2 Southern Hemisphere magnetic observatories between the geomagnetic latitudes of 46 and 63 degrees. ap values are given in units of 2 nT.
  • Ap index: A daily index determined from eight ap index values.
  • Aurora: A sporadic, faint visual phenomena associated with geomagnetic activity that occurs mainly in the high-latitude night sky. Auroras occur within a band of latitudes known as the auroral oval, the location of which is dependent on geomagnetic activity. Auroras are a result of collisions between atmospheric gases and precipitating charged particles (mostly electrons) guided by the geomagnetic field from the magnetotail. Each gas (oxygen and nitrogen molecules and atoms) gives out its own particular color when bombarded, and atmospheric composition varies with altitude. The auroral altitude range is 80 to 1000 km, but typical auroras are 100 to 250 km above the ground; the color of the typical aurora is yellow-green, from a specific transitions of atomic oxygen. Auroral light from lower levels in the atmosphere is dominated by blue and red bands from spectral line of atomic oxygen. The patterns and forms of the aurora include quiescent arcs, rapidly moving rays and curtains, patches, and veils.
  • Auroral electrojet: A current that flows in the ionosphere in the auroral zone.
  • Bartel' rotation number: The serial number assigned to 27-day rotation periods of solar and geophysical parameters. Rotation 1 in this sequence was assigned arbitrarily by Bartels to begin in January 1833, and the count has continued by 27-day intervals to present. (For example, rotation 2195 began on April 17, 1994). The 27-day period was selected empirically from the observed recurrence of geomagnetic activity attributed to corotating features on the sun.
  • Bow shock: A collisional shock wave in front of the magnetosphere arising from the interaction of the supersonic solar wind with earth's magnetic field.
  • Coronal hole: An extended region in the corona, exceptionally low in density and associated with unipolar magnetic photospheric regions having "open" magnetic field topology. Coronal holes are largest and most stable at or near the solar poles, and a source of high-speed solar wind. Coronal holes are visible in several wavelengths but most notably in solar x-rays.
  • Coronal mass ejection: An observable change in coronal structure that (1) occurs on a time scale between a few minutes and several hours and (2) involves the appearance of a new, discrete, bright white-light feature in the coronograph field of view. They are associated with the large-scale, closed magnetic structures in the corona. At times of coronal mass ejections large quantities of material (10^15 - 10^16 g) are sporadically ejected from the Sun into the interplanetary space. The speed of the leading edge of the coronal mass ejection may vary from 50 km/s to 1200 km/s. Average speed is about 400 km/s. The average heliocentric width is about 45 degrees. The largest geomagnetic storms are caused by coronal mass ejections.
  • Coronal neutral line: The line in the corona that separates solar magnetic fields of opposite polarity. It is calculated from solar observations of the photospheric magnetic field. Extension of the neutral line radially outward by the solar wind defines the current sheet in the heliosphere.
  • Coronal streamer: A feature of the white light corona that looks like a ray extending away from the sun out to about one solar radius, having an arch-like base.
  • 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 nT or deeper indicates a storm-level disturbance, and an index of -200 nT or deeper is associated with middle-latitude auroras.
  • Geomagnetic activity: Natural variations in the geomagnetic field classified into quiet, unsettled, active, and geomagnetic storm levels.
  • Geomagnetic field: The magnetic field in and around earth. The intensity of the magnetic field at the earth's surface is approximately 31 000 nT (0.31 gauss) at the equator and 62 000 nT (0.62 gauss) at the north pole. The geomagnetic field is dynamic and undergoes continual slow secular changes as well as short-term disturbances (geomagnetic activity). The magnetic field can be approximated by a centered dipole field, with the axis of the dipole inclined to the earth's rotational axis by about 11.5 degrees.
  • Geomagnetically induced currents: According to Faraday's law of induction, a temporal change of a magnetic field is always accompanied by an electric field. Hence an electric field is associated with geomagnetic activity. The geomagnetic variation and the geoelectric field observed at the earth's surface depend primarily on ionospheric-magnetospheric currents, and secondarily on currents and charges induced in earth. A part of the earth currents can flow into man-made conductors, like power transmission systems, pipelines, telecommunication cables, and railroads. Such currents are called geomagnetically induced currents (GICs).
  • Geomagnetic storm: A worldwide disturbance of the earth's magnetic field, distinct from regular diurnal variations.
      An operational definition:
      A storm occurs when the Ap > 29, a minor storm when 29 < Ap < 50, a major storm when 50 <= Ap < 100 and a severe storm when Ap >= 100.
      A physical definition:
      An interval of time when a sufficiently intense and long-lasting interplanetary convection electric field leads, through a substantial energization in the magnetosphere-ionosphere system, to an intensified ring current sufficiently strong to exceed some key threshold of quantifying storm time Dst index.

  • Heliospheric current sheet: A current layer that separates adjacent interplanetary magnetic field regions with opposite magnetic polarity.
  • K index: A 3-hourly quasi-logarithmic local index of geomagnetic activity relative to an assumed quiet-day curve for the recording site, range is from 0 to 9. The K index measures the deviation of the most disturbed horizontal component.
  • Kp index: A 3-hourly planetary index of geomagnetic activity calculated by the Institut fur Geophysik der Gottingen Universitat, Germany, from the K indices observed at 13 stations primarily in the Northern Hemisphere.
  • L1: Lagrangian point designated L1, the point about one one-hundredth of the way from the Earth to the Sun, where the gravitational pull of the Earth and Sun balance in such a way as to give an orbit of exactly one Earth year. A spacecraft at the Lagrangian point will orbit the Sun with the Earth without revolving about the Earth, and therefore never experiences night. The Lagrangian point is located at a distance of about 1.5 x 109 m towards the Sun. The ACE and SOHO spacecraft are both near this point.
  • Magnetopause: The boundary surface between the solar wind and the magnetosphere, where the pressure of the magnetic field of the object effectively equals the dynamic pressure of the solar wind.
  • Magnetosheath: The region between the bow shock and the magnetopause, characterized by very turbulent plasma.
  • Magnetosphere: The magnetic cavity surrounding a magnetized body, carved out of the passing solar wind by virtue of the magnetic field, which prevents, or at least impedes, the direct entry of the solar wind plasma into the cavity.
  • Magnetotail: The extension of the magnetosphere in the antisunward direction as a result of the interaction with solar wind. In the inner magnetotail, the field lines maintain a roughly dipolar configuration. But at greater distances in the antisunward direction, the field lines are stretched into northern and southern lobes, separated by a plasmasheet. There is observational evidence for traces of earth's magnetotail as far as 1000 earth radii downstream.
  • Plasma: A plasma is a quasineutral gas of charged and neutral particles which exhibit collective behavior. A plasma must satisfy three conditions: (1) the Debye length must be much less than a characteristic distance of the system, (2) the number of particles in a Debye sphere must be much larger than one and (3) the plasma frequency times the mean time between collisions with neutral atoms must be larger than one. Plasmas behave sometimes like fluids and sometimes like a collection of individual particles.
  • Radiation belts: Regions of the magnetosphere roughly 1.2 to 6 earth radii above the equator in which charged particles are stably trapped by closed geomagnetic field lines. There are two belts. The inner belt is part of the plasmasphere and corotates with earth. The outer belt extends on to the magnetopause on the sunward side (10 earth radii under normal quiet conditions) and to about 6 earth radii on the nightside. The radiation belts are often called the "Van Allen radiation belt" because the were discovered in 1968 by J.A. Van Allen.
  • Reconnection: A plasma process by which differently directed field lines link up, allowing topological changes of the magnetic field to occur, determining patterns of plasma flow, and resulting in conversion of magnetic energy to kinetic and thermal energy of plasma. Reconnection is invoked to explain the energization and acceleration of the plasmas that are observed in coronal mass ejections, magnetic substorms, and elsewhere in the solar system.
  • 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.
  • Solar wind: The outward flow of solar particles and magnetic fields from sun. Typically at 1 AU, solar wind velocities are near 450 km/s and proton and electron densities are near 5 cm^-3. The total intensity of the interplanetary magnetic field is nominally 5 nT. The fast solar wind originates from coronal holes and the slow wind is assumed to originate from regions near the coronal neutral line.
  • Sector boundary: Area between sectors, which are large-scale features distinguished by the predominant direction of the interplanetary magnetic field, toward the sun (a negative sector), or away from the sun (a positive sector). The sector boundary separating fields of opposite polarity is normally narrow, passing the earth within minutes to hours as opposed to the week or so needed for passage of a typical sector.
  • Solar activity: Five terms are often used to describe the activity (see x-ray solar flare class).

    Very low ---- x-ray events less than C-solar flare class.

    Low ---- C-class x-ray events.

    Moderate ---- isolated (one to 4) M-solar flare class x-ray events.

    High ---- several ( 5 or more) M-class x-ray events, or isolated (one to 4) M5 or greater x-ray events.

    Very high ---- several (5 or more) M5 or greater x-ray events.

  • Substorm: A geomagnetic perturbation lasting one to two hours, which tends to occur during local post-midnight nighttime. A substorm corresponds to an injections of charged particles from the magnetotail into the auroral oval.
  • X-ray solar flare class: Rank of a flare based on its x-ray energy output.

    Class ----- Peak burst intensity (I) measured at the Earth in the 0.1-0.8 nm band (W m^-2)

    B ----- I< 10^-6

    C ----- 10^-6 <= I < 10^-5

    M ----- 10^-5 <= I < 10^-4

    X ----- I > = 10^-4