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| Titel |
Transient ULF pulsations: time dependence of magnetic fields observed at the ground |
| VerfasserIn |
W. Allan |
| 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 ; 13, no. 9 ; Nr. 13, no. 9, S.938-945 |
| Datensatznummer |
250011978
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| Publikation (Nr.) |
 copernicus.org/angeo-13-938-1995.pdf |
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| Zusammenfassung |
| The continuum oscillation of a latitudinal
range of closed geomagnetic field lines or shells appears to be a basic feature
of the magnetosphere. Such oscillations are observed at the ground, and have
been termed transient ULF pulsations. Earlier modelling showed that the apparent
mean damping rate at the ground should be much greater than that in the
magnetosphere. This modelling is extended to examine the time dependence of the
magnetic field of transient pulsations as seen by a latitudinal chain of
magnetometers. It is found that there should be significant temporal variation
of both period and damping decrement observed at a given latitude, which could
help to identify transient events even when the period variation with latitude
is not obvious. Time-frequency analysis and analytical signal analysis do not
seem to be effective in determining temporal parameter variation for the short,
highly damped data segments typical of transient events. Least squares fitting
of two decaying sinusoids gives surprisingly good results, but seems to have no
physical basis, is difficult to interpret, and may be misleading. Least squares
fitting of a single sinusoid with time-varying period and damping rate gives
reasonably good fits. The resulting parameter variations with latitude may help
to determine the structures of ionospheric current systems associated with
transient ULF events. In particular, the time change of the period at a single
station can determine where that station is relative to the ionospheric current
maximum. |
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