The disturbances in the geomagnetic fields are caused by fluctuations in the solar wind impinging on the Earth. Such storms tend to be moderate.
Severe storms tend to be nonrecurrent and are difficult to predict.
These features are effective in causing geomagnetic disturbances and are said to be geoeffective. Currently, the criterion is that Dst remain below -80 nT for two hours or longer. The sporadic eruptions and the subsequent propagation of the ejecta cannot be predicted accurately at this time. The new prediction method takes advantage of the distinct physical attributes of geoeffective solar structures in the following way. They are magnetically well organized, so that the leading edge of a structure has clear relations to the solar wind that has yet to arrive at the observing platform.