Time derivative of the horizontal geomagnetic field as an activity indicator
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 (<i>d&...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2001-09-01
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| Series: | Annales Geophysicae |
| Online Access: | https://www.ann-geophys.net/19/1107/2001/angeo-19-1107-2001.pdf |
| Summary: | 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 (<i>d<b>H</b>/dt</i>). By using the IMAGE magnetometer
data from northern Europe from 1982 to 2001, we show that large <i>d<b>H</b>/dt</i>’s
(exceeding 1 nT/s) primarily occur during events governed by westward
ionospheric currents. However, the directional distributions of <i>d<b>H</b>/dt
</i>are much more scattered than those of the simultaneous baseline subtracted
horizontal variation field vector Δ<b><i>H</i></b>. A pronounced
difference between Δ<b><i>H</i></b> and <i>d<b>H</b>/dt</i> takes
place at about 02–06 MLT in the auroral region when <i>d<b>H</b>/dt </i>prefers
an east-west orientation, whereas Δ<b><i>H</i></b> points to the
south. The occurrence of large <i>d<b>H</b>/dt</i> 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 <i>d<b>H</b>/dt</i> values in the auroral region follows quite closely
the <i>aa</i> index, but a clear variation from year-to-year is observed in the
directional distributions. The scattering of <i>d<b>H</b>/dt</i> distributions
is smaller during descending phases of the sunspot cycle. Seasonal variations
are also seen, especially in winter <i>d<b>H</b>/dt </i> 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 <i>d<b>H</b>/dt</i>
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.<br><br><b>Key words. </b>Geomagnetism and
paleomagnetism (geomagnetic induction; rapid time variations) - Ionosphere (ionospheric
disturbances) |
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| ISSN: | 0992-7689 1432-0576 |