Timing of substorm onset signatures on the ground and at geostationary orbit

[1] In order to study the relative timing of substorm onset signatures on the ground and at geostationary orbit we have used data of simultaneous dispersionless electron and proton injections from the LANL satellite 1991–80, located slightly westward of Scandinavia. Out of 9 years of data we have identified a number of events during which such injections occurred close to local magnetic midnight. By careful inspection of ground-based magnetograms from the Scandinavian magnetometer network, IMAGE, we then identified the location and time of the formation of a substorm current wedge (SCW) during these events. 40 clear cases of geostationary injections, which were clearly associated with the formation of SCWs, were found. A statistical study of these events reveals that there is a clear time delay of the order of several minutes in the occurrence of the substorm injection with respect to the first indication of the SCW measured on the ground.