Geomagnetically induced currents (GICs) in technological conductor systems are a manifestation of the ground effects of space weather. Large GICs are always associated with large values of the time derivative of the ge-omagnetic field, and especially with its horizontal component (dH/dt). By using the IMAGE magnetometer data from northern Europe from 1982 to 2001, we show that large dH/dt's (exceeding 1 nT/s) primarily occur during events governed by westward ionospheric currents. However, the directional distributions of dH/dt are much more scattered than those of the simultaneous baseline subtracted horizontal variation field vector ∆H. A pronounced difference between ∆H and dH/dt takes place at about 02-06 MLT in the au-roral region when dH/dt prefers an east-west orientation, whereas ∆H points to the south. The occurrence of large dH/dt has two daily maxima, one around the local magnetic midnight, and another in the morning. There is a single maximum around the midnight only at the southernmost IMAGE stations. An identical feature is observed when large GICs are considered. The yearly number of large dH/dt values in the auroral region follows quite closely the aa index, but a clear variation from year-to-year is observed in the directional distributions. The scattering of dH/dt distributions is smaller during descending phases of the sunspot cycle. Seasonal variations are also seen, especially in winter dH/dt is more concentrated to the north-south direction than at other times. The results manifest the importance of small-scale structures of ionospheric currents when GICs are considered. The distribution patterns of dH/dt cannot be explained by any simple sheet-type model of (westward) ionospheric currents, but rapidly changing north-south currents and field-aligned currents must play an important role.

The geomagnetically induced currents (GIC) in the Finnish natural gas pipeline have been measured at one site in southern Finland since November 1998. We give an overview of this long and uniform time series covering one sunspot maximum. We also briefly discuss the conventional magnetic activity indices K and Ak, and, additionally, the range of the magnetic field and of its time derivative, as indicators of the GIC activity. The measured GIC data formed the basis of the development of a nowcasting service (GIC Now!), which was implemented within the European Space Agency Space Weather Applications Pilot Project. The service provides a near‐real‐time view of the geomagnetic activity level and nowcast estimates of GIC and pipe‐to‐soil voltages in the Finnish natural gas pipeline system. We describe the operation and validation of this service in this paper.

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Abstract. We present the ground signatures of dynamic substorm features with particular emphasis on the event interpretation capabilities provided by the IMAGE magnetometer network. This array covers the high latitudes from the sub-auroral to the cusp/cleft region. An isolated substorm on 11 Oct. 1993 during the late evening hours exhibited many of well-known features such as the Harang discontinuity, westward travelling surge and poleward leap, but also discrete auroral forms, known as auroral streamers, appeared propagating westward along the centre of the electrojet. Besides the magnetic ®eld measurements, there were auroral observations and plasma¯ow and conductivity measurements obtained by EISCAT. The data of all three sets of instruments are consistent with the notion of upward ®eld-aligned currents associated with the moving auroral patches. A detailed analysis of the electrodynamic parameters in the ionosphere, however, reveals that they do not agree with the expectations resulting from commonly used simplifying approximations. For example, the westward moving auroral streamers which are associated with ®eld-aligned current ®laments, are not collocated with the centres of equivalent current vortices. Furthermore, there is a clear discrepancy between the measured plasma¯ow direction and the obtained equivalent current direction. All this suggests that steep conductivity gradients are associated with the transient auroral forms. Also self-induction eects in the ionosphere may play a role for the orientation of the plasma¯ows. This study stresses the importance of multi-instrument observation for a reliable interpretation of dynamic auroral processes.

Abstract. It is known that under certain solar wind (SW)/interplanetary magnetic field (IMF) conditions (e.g. high SW speed, low cone angle) the occurrence of groundlevel Pc3-4 pulsations is more likely. In this paper we demonstrate that in the event of anomalously low SW particle density, Pc3 activity is extremely low regardless of otherwise favourable SW speed and cone angle. We re-investigate the SW control of Pc3 pulsation activity through a statistical analysis and two empirical models with emphasis on the influence of SW density on Pc3 activity. We utilise SW and IMF measurements from the OMNI project and groundbased magnetometer measurements from the MM100 array to relate SW and IMF measurements to the occurrence of Pc3 activity. Multiple linear regression and artificial neural network models are used in iterative processes in order to identify sets of SW-based input parameters, which optimally reproduce a set of Pc3 activity data. The inclusion of SW density in the parameter set significantly improves the models. Not only the density itself, but other density related parameters, such as the dynamic pressure of the SW, or the standoff distance of the magnetopause work equally well in the model. The disappearance of Pc3s during lowdensity events can have at least four reasons according to the existing upstream wave theory: 1. Pausing the ion-cyclotron resonance that generates the upstream ultra low frequency waves in the absence of protons, 2. Weakening of the bow shock that implies less efficient reflection, 3. The SW becomes sub-Alfvénic and hence it is not able to sweep back the waves propagating upstream with the Alfvén-speed, and 4. The increase of the standoff distance of the magnetopause (and of the bow shock). Although the models cannot account Correspondence to: B. Heilig (heilig@elgi.hu) for the lack of Pc3s during intervals when the SW density is extremely low, the resulting sets of optimal model inputs support the generation of mid latitude Pc3 activity predominantly through upstream waves.