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Anomalous dynamics of the extremely compressed magnetosphere during 21 January 2005 magnetic storm

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.