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Ann. Geophys., 18, 789-798, 2000
www.ann-geophys.net/18/789/2000/
© European Geosciences Union 2000
Yearly variations in the low-latitude topside ionosphere
G.J. Bailey1, Y. Z. Su1, and K.-I. Oyama2
1Department of Applied Mathematics, The University of Sheffield, Sheffield S3 7RH, UK
E-mail: g.bailey@sheffield.ac.uk
2Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229, Japan
Correspondence to: G. J. Bailey
Abstract. Observations made by the Hinotori satellite
have been analysed to determine the yearly variations of the electron density
and electron temperature in the low-latitude topside ionosphere. The
observations reveal the existence of an equinoctial asymmetry in the topside
electron density at low latitudes, i.e. the density is higher at one equinox
than at the other. The asymmetry is hemisphere-dependent with the higher
electron density occurring at the March equinox in the Northern Hemisphere and
at the September equinox in the Southern Hemisphere. The asymmetry becomes
stronger with increasing latitude in both hemispheres. The behaviour of the
asymmetry has no significant longitudinal and magnetic activity variations. A
mechanism for the equinoctial asymmetry has been investigated using CTIP
(coupled thermosphere ionosphere plasmasphere model). The model results
reproduce the observed equinoctial asymmetry and suggest that the asymmetry is
caused by the north-south imbalance of the thermosphere and ionosphere at the
equinoxes due to the slow response of the thermosphere arising from the effects
of the global thermospheric circulation. The observations also show that the
relationship between the electron density and electron temperature is different
for daytime and nighttime. During daytime the yearly variation of the electron
temperature has negative correlation with the electron density, except at
magnetic latitudes lower than 10°. At night, the correlation is positive.
Key words: Ionosphere (equatorial ionosphere;
ionosphere-atmosphere interactions; plasma temperature and density)
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