Combined TOPEX/Poseidon TEC and ionosonde observations of negative low-latitude ionospheric storms
Ionospheric storms showing a strong depression in daytime <I>fo</I>F2 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 pea...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2004-09-01
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| Series: | Annales Geophysicae |
| Online Access: | https://www.ann-geophys.net/22/2837/2004/angeo-22-2837-2004.pdf |
| Summary: | Ionospheric storms showing a strong depression in daytime <I>fo</I>F2 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 <I>fo</I>F2 observed by
the Southern Hemisphere ionosondes. The latter was associated with an
additional bite-out in both TEC and <I>fo</I>F2 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 <I>fo</I>F2
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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| ISSN: | 0992-7689 1432-0576 |