Interplanetary and geomagnetic parameters during January 16–26, 2005

The interplanetary Bz(min) and geomagnetic Dst(min) are well related. There is no large negative Bz without causing large negative Dst. Similarly, large negative Dst is not expected to occur without a large negative Bz; but recently, Du et al. (2008) reported that the Dst storm of Jan. 21–22, 2005 was anomalous because the Dst storm main phase developed during northward interplanetary magnetic field (positive Bz). Here, we examined this event (storm 3) and compared it with earlier events of Jan. 16–20, 2005 (storms 1 and 2, in quick succession). It was noticed that storms 1 and 2 had large negative Dst deviations (−121 nT and −93 nT, separated by 26 hours) but the Bz fluctuated between +21 nT and −17 nT. For storm 3 where negative Dst deviations were large (−105 nT), the Bz was not completely positive (northward) as mentioned by Du et al. (2008) but had negative values (−7 nT) for an hour, followed by positive values (13 nT). Thus, the need of a negative Bz (albeit small) was satisfied.