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| Titel |
Time derivative of the horizontal geomagnetic field as an activity indicator |
| VerfasserIn |
A. Viljanen, H. Nevanlinna, K. Pajunpää, A. Pulkkinen |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 19, no. 9 ; Nr. 19, no. 9, S.1107-1118 |
| Datensatznummer |
250014320
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| Publikation (Nr.) |
 copernicus.org/angeo-19-1107-2001.pdf |
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| Zusammenfassung |
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) |
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