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Ann. Geophys., 22, 2037-2045, 2004
www.ann-geophys.net/22/2037/2004/
© European Geosciences Union 2004
Model results for the ionospheric lower transition height over mid-latitude
J. Lei1,2, L. Liu1, W. Wan1, and S.-R. Zhang3
1Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
2Wuhan Ionospheric Observatory, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China
3Haystack Observatory, Massachusetts Institute of Technology, Westford, Massachusetts, USA
Abstract. Theoretical calculations of the ionospheric lower
transition height (LTH), a level of equal O+ and molecular ion
densities, were performed and compared with empirical models by Zhang et
al. (1996). This paper represents a substantial extension of the prior work
by including the AE-C data of ion composition analysis and by detailed
quantitative studies of the LTH simulation, and by creating a new LTH empirical
model based on our simulations. Results show that: (1) the
calculated LTH, in general, is lowest near 11-13LT and reaches the diurnal
maximum after midnight (about 01~02LT). The local time asymmetry
becomes more evident in summer, when the time of minimum shifts to 16LT.
(2) The simulated LTH presents a dominant, semiannual variation
during nighttime, and a pronounced annual variation during daytime.
(3) The simulated LTH increases with solar activity at night and decreases
by day, while the standard IRI option has an opposite tendency at night in
summer and equinox. Therefore, the day-night difference of simulated LTH
significantly increases with solar activity. (4) Both daytime and
nighttime LTHs, tend to increase with the increasing geomagnetic activity Ap
index, with a mean slope about 0.1455km per Ap unit. (5) The
diurnal variation of LTH is found to be more than 20 km, which is much
larger than the seasonal variation under F107=100 and Ap=10. Thus, the
diurnal and solar activity variations of LTH are more pronounced than its
seasonal and magnetic activity variations.
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