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Ann. Geophys., 24, 1343-1353, 2006
www.ann-geophys.net/24/1343/2006/
© European Geosciences Union 2006
Relationships between electron density, height and sub-peak ionospheric thickness in the night equatorial ionosphere
K. J. W. Lynn1, T. J. Harris2, and M. Sjarifudin3
1Ionospheric Systems Research, Noosaville, Australia
2Defence Science & Technology Organisation, P.O. Box 1500, Edinburgh, Australia
3National Institute for Aeronautics and Space (LAPAN), Bandung, Indonesia
Abstract. The development and decay of the southern equatorial anomaly
night-time peak in electron density as seen at a number of ionosonde
reflection points extending from New Guinea and Indonesia into northern
Australia was examined in terms of the characteristic rise and fall in
height associated with the sunset ionisation-drift vortex at the magnetic
equator. The observations relate to measurements made in November 1997.
Following sunset, the ionospheric profile was observed to narrow as the
maximum electron density increased during a fall in height that took the
peak of the layer at Vanimo and Sumedang down to some 240 km. The fall was
followed by a strong rise in which the electron density sub-peak profile
expanded from a slab width (as given by POLAN) of 20 km to over 100km with
no corresponding change in peak electron density. The post-sunset equatorial
fall in height and associated changes in profile density and thickness
continued to be seen with diminishing amplitude and increasing local time
delay in moving from the anomaly peak at Vanimo to the southernmost site of
observation at Townsville. Secondary events on a lesser scale sometimes
occurred later in the night and may provide evidence of the multiple
vortices suggested by Kudeki and Bhattacharyya (1999). Doppler measurements
of vertical velocity as seen at Sumedang in Java are compared with the
observed changes in electron density profile in the post-sunset period. The
normal post-sunset variation in ionospheric parameters was disrupted on the
night of 7 November, the night before a negative ionospheric storm was
observed.
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