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In solar-terrestraial terms, a worldwide disturbance of the earth's magnetic field, distinct from regular diurnal variations. Minor Geomagnetic Storm: A storm for which the Ap index was greater than 29 and less than 50. Major Geomagnetic Storm: A storm for which the Ap index was greater than 49 and less than 100. Severe Geomagnetic Storm: A storm for which the Ap index was 100 or more. Initial Phase: Of a geomagnetic storm, that period when there may be an increase of the middle-latitude horizontal intensity (H). Main Phase: Of a geomagnetic storm, that period when the hori- zontal magnetic field at middle latitudes is generally decreasing. Recovery Phase: Of a geomagnetic storm, that period when the depressed northward field component returns to normal levels.
source: NOAA National Weather Service Glossary
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