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Ann. Geophys., 25, 161-170, 2007
www.ann-geophys.net/25/161/2007/
© European Geosciences Union 2007
A comparison between FUV remote sensing of magnetotail stretching and the T01 model during quiet conditions and growth phases
C. Blockx1, J.-C. Gérard1, V. Coumans1, B. Hubert1, and M. Meurant2
1Laboratoire de Physique Atmosphérique et Planétaire, Université de Liège, Liège, Belgium
2Institute for Space Research – University of Calgary, Calgary, Canada
Abstract. In a previous study, Blockx et al. (2005) showed that the SI12 camera on board the IMAGE
spacecraft is an excellent tool to remotely determine the position of the
isotropy boundary (IB) in the ionosphere, and thus is able to provide a
reasonable estimate of the amount of stretching of the magnetic field lines
in the magetotail. By combining an empirical model of the magnetospheric
configuration with Sergeev's criterion for non-adiabatic motion, it is also
possible to obtain a theoretical position of IB in the ionosphere, for known
conditions in the solar wind. Earlier studies have demonstrated the
inadequacy of the Tsyganenko-1989 (T89) model to quantitatively reproduce
the field line stretching, particularly during growth phases. In this study,
we reexamine this question using the T01 model which considers the time
history of the solar wind parameters. We compare the latitude of IB derived
from SI12 global images near local midnight with that calculated from the
T01 model and the Sergeev's criterion. Observational and theoretical results
are found to frequently disagree. We use in situ measurements of the magnetic field
with the GOES-8 satellite to discriminate which of the two components in the
calculation of the theoretical position of the IB (the T01 model or
Sergeev's criterion) induces the discrepancy. For very quiet magnetic
conditions, we find that statistically the T01 model approximately predicts
the correct location of the maximum proton precipitation. However, large
discrepancies are observed in individual cases, as demonstrated by the large
scatter of predicted latitudes. For larger values of the AE index, the model
fails to predict the observed latitude of the maximum proton intensity, as a
consequence of the lack of consideration of the cross-tail current component
which produces a more elongated field configuration at the location of the
proton injection along the field lines. We show that it is possible to match
the observed location of the maximum proton precipitation by decreasing the
current sheet half-thickness D parameter. We thus conclude that
underestimation of the field line stretching leads to inadequately
prediction of the boundary latitude of the non-adiabatic proton
precipitation region.
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