Phase shift (time) between storm-time maximum negative excursions of geomagnetic disturbance index Dst and interplanetary Bz

The evolutions of severe geomagnetic storms (Dst<−200 nT) during solar cycle 23 were examined. For each storm, certain timing landmarks (starting of increases of interplanetary total field B, its Bz component, Dst changes, etc.) were noted and from these, various antecedence intervals were calculated. It was noticed that the various delays varied in a very wide range from storm to storm. Thus, some storms had a warning of only 4 h at the ACE location, while others had a warning of up to 30 h. These variations do not depend upon the Sun-Earth transit time. Also, faster interplanetary structures do not necessarily give quicker or stronger Dst evolutions, though larger negative Bz seems to give stronger negative Dst, but not necessarily earlier.