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Ann. Geophys., 19, 219-228, 2001
www.ann-geophys.net/19/219/2001/
© European Geosciences Union 2001
The Mg II index for upper atmosphere modelling
G. Thuillier and S. Bruinsma
Service d’Aéronomie, BP 3, 91371 Verrières-le-Buisson, France
Correspondence to: G. Thuillier, (gerard.thuillier@aerov.jussieu.fr)
Abstract. The solar radio flux at
10.7 cm has been used in upper atmosphere density modelling because of its
correlation with EUV radiation and its long and complete observational record. A
proxy, the Mg II index, for the solar chromospheric activity has been derived by
Heath and Schlesinger (1986) from Nimbus-7 data. This index allows one to
describe the changes occurring in solar-activity in the UV Sun spectral
irradiance. The use of this new proxy in upper atmosphere density modelling will
be considered. First, this is supported by the 99.9% correlation between the
solar radio flux (F10.7) and the Mg II index over a period of 19 years with,
however, large differences on time scales of days to months. Secondly,
correlation between EUV emissions and the Mg II index has been shown recently,
suggesting that this last index may also be used to describe the EUV variations.
Using the same density dataset, a model was first run with the F10.7 index as a
solar forcing function and second, with the Mg II index. Comparison of their
respective predictions to partial density data showed a 3–8% higher precision
when the modelling uses the Mg II index rather than F10.7. An external
validation, by means of orbit computation, resulted in a 20–40% smaller RMS of
the tracking residuals. A density dataset spanning an entire solar cycle,
together with Mg II data, is required to construct an accurate, unbiased as
possible density model.
Key words. Atmospheric composition and structure
(middle atmosphere – composition and chemistry; thermosphere – composition
and chemistry) – History of geophysics (atmospheric sciences)
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