Time derivative of the horizontal geomagnetic field as an activity indicator

Details Time derivative of the horizontal geomagnetic field as an activity indicator Time derivative of the horizontal geomagnetic field as an activity indicator

Sep 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001angeo..19.1107v&link_type=abstract

Annales Geophysicae, vol. 19, Issue 9, pp.1107-1118

Mathematics

Logic

24

Scientific paper

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 H. A pronounced difference between H and dH/dt takes place at about 02 06 MLT in the auroral 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

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