FORECAST SOL: Moderate yellow MAG: Normal green ION: Normal green
Space Weather

Glossary of Solar Terrestrial Terms

Seasonal Anomaly

See Winter Anomaly.

Short Wave Fadeout

See Daylight Fadeout.


Sudden Ionospheric Disturbance. See Daylight Fadeout.

Signal-to-Noise Ratio

The ratio of the magnitude of a signal to that of noise.

Skip Distance

The distance between where the ground wave ends and the sky wave becomes detectable. The length of the skip distance is dependent on the operating frequency. Decreasing the operating frequency is likely to reduce the skip distance.

Sky Wave

The radio wave which propagates through the ionosphere. It is often called the ionospheric wave to distinguish it from the direct (line of sight) wave and ground wave.

Smoothed Sunspot Number

An average of monthly sunspot numbers centred on the month of concern. There are various formulas, however, the aim is to smooth discrete data points.

Solar Activity

Any change in the Sun's appearance or behaviour. The Sun's activity is described as being very low, low, moderate, high or very high. These terms refer to the strength and number of x-ray solar flares:

  • Very Low: x-ray events less than C-class
  • Low: C-class x-ray events
  • Moderate: isolated (1 to 4) M-class x-ray events
  • High: several (5 or more) M-class x-ray events, or isolated (1 to 4) M5 or greater x-ray events
  • Very High: several (5 or more) M5 or greater x-ray events

Solar Control

The term used to indicate that the behaviour of an ionospheric region is dominated by the Sun.

Solar Cycle

Solar activity changes over a period of, on average, 11 years. At solar maximum, solar activity is high and so too the EUV radiation output which affects the ionosphere. At solar minimum, the opposite is true. A solar cycle commences at solar minimum.

Solar Flux Unit

Unit of radio emission from the Sun, usually given as 10-22 watts per square metre.

Solar Maximum

The time at which the Sun reaches its highest activity as defined by the 12-month smoothed value of the sunspot number.

Solar Minimum

The time at which the Sun reaches its lowest point in solar activity as defined by the 12-month smoothed value of the sunspot number.

Solar Wind

The outflow of solar material from the hot, unstable corona. The solar wind blows into interplanetary space with a speed of about 400 km/s (this can vary dramatically), carrying with it the magnetic fields that originate in the Sun.


The times when the Sun reaches its greatest declination away from the Earth's equator. The times of longest day and shortest night, and vice versa. Occur in June and December.

Sporadic E

A thin ionised layer in the E region that occurs irregularly. Unlike the E region, sporadic E is not solar controlled. Sporadic E may occur day or night.

Spread F

Irregularities in the F region of the ionosphere which scatter radio signals causing a degradation in communications.


Severe departure from normal conditions in either the ionosphere or the geomagnetic field.

Sudden Commencement

A sudden impulse becomes a sudden commencement if the impulse is succeeded by a geomagnetic storm. In most cases, sudden commencements occur almost simultaneously around the world.

Sudden Impulse

Abrupt increase in the strength of the horizontal component of the geomagnetic field.

Sudden Ionospheric Disturbance

See Daylight Fadeout.

Sunspot Number

An index of solar activity related to the number of sunspots and sunspot groups present on the Sun.


Relatively cool regions in the solar photosphere that appear dark. They contain intense magnetic fields which provide the energy for solar flares. Sunspots occur in groups. They underlie plage areas.

go to top of page