Abstract
Abstract. 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 geomagnetic 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 ΔH. A pronounced difference between ΔH and dH/dt takes place at about 02–06 MLT in the auroral region when dH/dt prefers an east-west orientation, whereas ΔH 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.Key words. Geomagnetism and paleomagnetism (geomagnetic induction; rapid time variations) - Ionosphere (ionospheric disturbances)
Highlights
Induced currents (GIC) in power systems, pipelines and other technological conductor systems on the ground occur during rapid changes in the Earth’s magnetic field (e.g. Boteler et al, 1998; Viljanen and Pirjola, 1994).little attention has been paid to the ionospheric current systems, which primarily produce Geomagnetically induced currents (GICs)
Little attention has been paid to the ionospheric current systems, which primarily produce GIC
Our starting point was the study of large events of geomagnetically induced currents (GICs)
Summary
Induced currents (GIC) in power systems, pipelines and other technological conductor systems on the ground occur during rapid changes in the Earth’s magnetic field (e.g. Boteler et al, 1998; Viljanen and Pirjola, 1994). A visual inspection of these days showed that the large morning hour events seem to be a mixture of a substorm and pulsation activity We repeated this test for 1994, which has a narrow dH/dt pattern at auroral latitudes (Fig. 2). MLT, a simple westward sheet current model can explain these patterns It produces a southward horizontal variation field, and amplitude variations make the time derivative to point to the north or south. The induced currents enhance the time derivative of the magnetic field perpendicular to the strike of the anomaly The GIC occurrence at Mäntsälä is quite similar to that of |dH/dt| at NUR This indicates that the sources of large dH/dt at the southern stations are partly different from those occurring in the average auroral region. At some stations (AND, LYC, MAS, TRO) the directional distribution of dH/dt is strongly rotated or scattered by internal currents in the Earth’s crust, but Geomagnetically induced currents and the geoelectric field
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