On the discrimination between magnetospheric and ionospheric contributions on the ground manifestation of sudden impulses

The definite identification of the characteristics of the geomagnetic response to solar wind pressure changes represents an interesting element of magnetospheric dynamics that is also important in the Space Weather context. In the present analysis the aspects of the global response in ground-based observations have been examined for three case events, discriminating between magnetospheric and ionospheric contributions in ground manifestations of sudden impulses (SI). The separation between the magnetospheric and ionospheric contributions is obtained by a comparison between the observations at geostationary orbit and the predictions of the Tsyganenko and Sitnov (2005) model for the different magnetospheric current systems (from the magnetopause, ring current, tail current, etc.). The magnetopause current is the key element for the SI variation observed at geosynchronous orbit in a wide local time sector and practically represents the DL field of magnetospheric origin. The expected DL field is then subtracted, at each ground station, from the experimental measurements, in order to obtain a confident estimate of the residual DP field at different latitudes and local times. After evaluating the contribution of the field-aligned currents, we estimate the ionospheric current flow pattern of the preliminary and main impulses (PIIC and MIIC). The patterns of PIIC and MIIC fields are consistent with those proposed by Araki (1994). Some anomalous ground manifestations can be interpreted in terms of the combined effect of the irregular configuration of the boundary of the vortices of the ionospheric currents, of the rapid temporal evolution of the entire pattern, and of the station rotation beneath the pattern.