FORECAST SOL: Disturbed red MAG: Normal green ION: Normal green
Space Weather

Glossary of Solar Terrestrial Terms

E Layer Screening

When a radio wave directed for F region communications is instead refracted from the E region. This is likely to occur when the take-off angles of the F mode and the E mode are similar and the operating frequency is below the E region MUF for the E mode. The radio wave is then likely to propagate via multiple E layer hops. The E modes can be heavily attenuated, especially when more than two hops occur.

Electromagnetic Radiation

Radiation which has both electric and magnetic properties. Examples: microwaves, light, infra-red, ultraviolet, X rays, gamma, radio and television. Travels at 300 million m/s.


An atomic particle with a fixed negative electric charge.

Electron Density

The number of electrons in a unit volume, e.g., 1 cubic centimetre.

Elevation Angle

Angle between the horizontal and the direction of concern.

Equatorial Anomaly

A depression in F region frequencies at the geomagnetic equator relative to frequencies at low latitudes. A daytime phenomenon.

Equatorial Electrojet

A thin electric current in the ionosphere over the magnetic equator around 100 to 115 km altitude normally flowing eastward during the day. The electrojet may reverse its direction during geomagnetically disturbed conditions and magnetically quiet times. The reversals during quiet times have been related to lunar tides. There is a strong correlation between the electrojet and equatorial sporadic E.


Time when the Sun crosses the geographic equator. Day and night are of equal lengths. Occurs in March and September.

E Region

A solar controlled ionospheric region around 90-140 km capable of refracting HF radio waves. This region is only significant for HF communications during daylight hours.

EUV Radiation

See Extreme Ultraviolet Radiation.

Extraordinary Wave

A radio wave may split into two oppositely polarised components due to, in the case of the ionosphere, the geomagnetic field. The ordinary wave obeys the laws of refraction and is unaffected by the magnetic field. The extraordinary wave travels a different path through the ionosphere.

Extreme Ultraviolet Radiation

Electromagnetic radiation at the high frequency end of the ultraviolet spectrum.

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