Solar wind and geomagnetic activity

Interplanetary plasma and magnetic-field parameters, measured by the ESRO satellite HEOS-1, have been correlated with theD st , taken as an index of the overall geomagnetic activity. It is found that two different states exist for the interaction between the solar wind and the magnetosphere: they correspond to quiet periods (D st positive) and to disturbed conditions (main phase of geomagnetic storms,i.e. D st negative). In disturbed conditions the magnetosphere appears to be much more sensitive to variations of the solar-wind dynamical pressure than in periods of positiveD st .