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| Titel |
Combined TOPEX/Poseidon TEC and ionosonde observations of negative low-latitude ionospheric storms |
| VerfasserIn |
K. J. W. Lynn, M. Sjarifudin, T. J. Harris, M. Huy |
| 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 ; 22, no. 8 ; Nr. 22, no. 8 (2004-09-07), S.2837-2847 |
| Datensatznummer |
250014969
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| Publikation (Nr.) |
 copernicus.org/angeo-22-2837-2004.pdf |
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| Zusammenfassung |
| Ionospheric storms showing a strong depression in daytime foF2 values were
sought which penetrated to low-latitudes, as identified by vertical
ionosondes operating at Darwin and Townsville over the period 1992-1998. The
32 storms thus identified showed a seasonal occurrence peaking near the
equinoxes with a bias to the summer side. Of these storms, three (27 March
1995, 25 October 1997, 8 November 1997) combined Australian and South East Asian
ionosonde observations with local afternoon TOPEX/Poseidon measurements of
TEC. The equatorial anomaly is usually well developed at this time of day
and consequently these storms were chosen for detailed study. The
TOPEX/Poseidon satellite provided vertical profiles of the ionosphere across
both hemispheres, thus allowing the totality of storm behaviour to be
observed for the first time at low-latitudes and related directly to the
ionosonde observations. The three storms were remarkably consistent in their
behaviour, the negative ionospheric storm day followed some 24-36h
after the beginning of a magnetic storm and the development of the
equatorial anomaly was suppressed. However, the suppression of the equatorial
anomaly was not the main cause of the strong depression in foF2 observed by
the Southern Hemisphere ionosondes. The latter was associated with an
additional bite-out in both TEC and foF2 that occurred on the southern side
of the magnetic equator. None of the three storms produced any major
negative disturbance outside the range of normal variability of TEC and foF2
at the northern latitude sites for which data was available, despite the
absence of the anomaly. The satellite measurements show the strength of the
anomaly to be highly variable from day-to-day and anomaly peaks are
frequently not present even on magnetically quiet days. Thus, an absence of
anomaly peaks is contained within the normal variability of non-storm days.
The north-south asymmetry and seasonal occurrence are consistent with an
enhancement of the normal summer-to-winter system carrying compositional
changes induced by energy inputs at auroral latitudes to equatorial
latitudes not usually reached. The ability of associated atmospheric and/or
electric field changes to coincidentally switch off the equatorial E region
electrojet remains to be explained, as indeed does the large range of
variability in equatorial anomaly development from day-to-day evident in the
TEC measurements outside periods of geomagnetic disturbance. Some possible
positive storm effects occurring on the day preceding the negative storm
phase are also noted. |
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