  Dynamics of the dayside magnetosphere and proton radiation belt was analyzed
during unusual magnetic storm on 21 January 2005. We have found that during the
storm from 1712 to 2400 UT, the subsolar magnetopause was continuously located
inside geosynchronous orbit due to strong compression. The compression was
found to be extremely strong from 1846 to 2035 UT when the dense plasma of fast
erupting filament produced the solar wind dynamic pressure Pd peaked up to >100
nPa and, in the first time, the upstream solar wind was observed at
geosynchronous orbit during almost 2 hours. Under the extreme compression, the
outer magnetosphere at L > 5 was pushed inward and the outer radiation belt
particles with energies of several tens of keV moved earthward, became
adiabatically accelerated and accumulated in the inner magnetosphere at L < 4
that produced the intensified ring current with an exceptionally long lifetime.
The observations were compared with predictions of various empirical and first
principles models. All the models failed to predict the magnetospheric dynamics
under the extreme compression when the minimal magnetopause distance was
estimated to be ~3 Re. The inconsistencies between the model predictions and
observations might result from distortions of plasma measurements by extreme
heliospheric conditions consisting in very fast solar wind streams (~1000 km/s)
and intense fluxes of solar energetic particles. We speculated that anomalous
dynamics of the magnetosphere could be well described by the models if the He
abundance in the solar wind was assumed to be >20%, which is well appropriate
for erupting filaments and which is in agreement with the upper 27% threshold
for the He/H ratio obtained from Cluster measurements.
