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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 18, issue 7
Ann. Geophys., 18, 789–798, 2000
https://doi.org/10.1007/s00585-000-0789-0
© European Geosciences Union 2000

Special issue: Lance Thomas

Ann. Geophys., 18, 789–798, 2000
https://doi.org/10.1007/s00585-000-0789-0
© European Geosciences Union 2000

  31 Jul 2000

31 Jul 2000

Yearly variations in the low-latitude topside ionosphere

G.J. Bailey1, Y. Z. Su1, and K.-I. Oyama2 G.J. Bailey et al.
  • 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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