Anomalous geomagnetic storm of 21-22 January 2005: A storm main phase during northward IMFs

The major (minimum Dst = -105nT) magnetic storm which occurred on 21-22 January 2005 is highly anomalous because the storm main phase (identified by the SYM-H indices) developed during northward interplanetary magnetic fields (IMFs). We believe this to be the first event of its type to be reported in the literature. Interplanetary ACE and Cluster C1 data are used for solar wind diagnostics, and LANL 90 and 97, GOES 10 and 12 and GEOTAIL data are used for magnetospheric diasgnostics. An unusually strong (magnetosonic Mach number equal to 5.4) shoch detected at similar to 1674 UT, 21 January 2005 by ACE causes a SI+ (of 57 nT) at similar to 1712 UT at Earth. Southward magnetic fields in the sheath following the shock caused a decrease of SYM-H with a peak value similar to-41 nT. A dynamic pressure jump across a double discontinuity in the solar wind at 1823 UT observed by ACE induced a second SI+ (of 25 nT) at 1847 UT at Earth. Southward magnetic fields following this event led to a second SYM-H decrease with peak intensity -2 nT. However, when the storm main phase developed starting at 1946 UT, the IMF Bz turned northward. The IMF was northward form the portion of the main phase from similar to 1946 UT to 0124 UT (almost 6 h). By comparing solar wind energy input current (represented by the integrated SYM-H), we arrive at a possible explanation that there is first energy storage in the magnetotail and the delayed energy injection (after storage in the magnetotail) into the magnetospphere. Other interpretations/mechanisms are possible.