Glossary of Solar Terrestrial Termsskip terms menu
Unit of charge of the Earth's magnetic field. 1 gamma = 1 nanotesla = 10 gauss.
Natural variations in the geomagnetic field classified into quiet, unsettled, active and storm conditions. Descriptive levels of activity such as these are determined by the A index defined as follows.
- quiet: A < 8
- unsettled: 8 <= A <= 15
- active: 16 <= A <= 24
- minor storm: 25 <= A <= 35
- major storm: A >= 36
Geomagnetic Dip Equator
Where the geomagnetic field is horizontal to the Earth. Where the inclination is zero.
The Earth's magnetic field.
The radio wave which propagates close to the Earth's surface. Severe signal losses due to ground resistance limit the range of ground waves to about 100 km over land and 300 km over sea for the lowest HF frequencies. The ground waves for the higher HF frequencies cover much shorter distances.