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

The dayside magnetosphere and proton radiation belt were analyzed during unusual magnetic storm on 21 January 2005. We have found that from 1712 to 2400 UT, the subsolar magnetopause was continuously located inside geosynchronous orbit due to strong compression. The compression was extremely strong from 1846 to 2035 UT when the dense plasma of fast erupting filament produced the solar wind dynamic pressure that peaked up to > 100 nPa, and during the first time, the upstream solar wind was observed at geosynchronous orbit for almost 2 h. Under the extreme compression, the outer magnetosphere at L > 5 was pushed inward, and the outer radiation belt particles moved earthward, became adiabatically accelerated, and accumulated in the inner magnetosphere at L 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. Key Points Extreme compression pushes the bow shock inside geosynchronous orbit for ~2 h Ring current moves to the inner magnetosphere (L~3) with betatron energization High He abundance in the erupting filament allows predicting the observations