The ULF magnetic and the dc electric antennas operating onboard GEOS have been used to study the origin and characteristics of short irregular pulsations (SIP's). Strong SIP's are always observed during substorm ''onsets'', which are characterized by an abrupt change of the GEOS dc magnetic field from a taillike configuration to a more dipolar one and which is known to be associated with rapid poleward displacement of aurorals forms in the vicinity of the GEOS field line. By applying to the three-component ULF signal a complex processing it is possible to demonstrate that most of SIP's are in fact the magnetic signatures of localized current structures passing by the spacecraft at a high velocity. Very intense spikes in the electric field (E≂3-25m V/m) are observed in connection with the passing over of such structures at the satellite location, but these E field spikes are generally observed 10-20 s earlier than the SIP's. On the average the E field direction is earthward with a smaller component towards dusk. Provided that the duration of the magnetic signature of the signal (2 &tgr;) is short enough (less than 2 s) it is possible to show that they correspond to field-aligned current tubes (FACT's) passing by the spacecraft. When 2 &tgr;>2 s, the signature of the SIP is still consistent with that of a field-aligned current tube, but this cannot be assessed without ambiguity from the ULF experiment alone. Nevertheless, the direction of the velocity of the moving structure can still be deduced from the magnetic signature. For structures detected during substorm onsets this direction is consistent (within 40¿) with the drift velocity direction as determined by E¿B. This situation occurs for 28 events out of a total of 42, which were analyzed in detail. For these events, and assuming that the amplitude of the velocity is given by E¿B/B2, it is possible to compute the characteristic parameters of the structure: current density J, radius R (by assuming a cyclindrical structure for simplification), and velocity v. The parameters that have thus been obtained are 6.10-9<J<3.10-/A/m2, 20<R<900 km, 15<v<170 km/s, with the following average values: 8¿10-8 A/m2, 215 km, and 70 km/s. When transposed to ionospheric altitudes these values are consistent with those found for localized field-aligned current structures by ground or low orbiting spacecraft observations. The origin and nature of these localized current structures are discussed. A simple theory based on the MHD stability of a field-aligned current structure is proposed to explain why the current density can hardly exceed in the equatorial plane a critical value that is of the order of 10-7 A/m2.