Relation of PC index to the geomagnetic storm Dst variation

Relationships between the polar cap magnetic activity index PC and the magnetic storm Dst index have been studied for the magnetic storms with duration more 12 h and peak value Dst < -30 nT and, observed in 1998-2002 and 2004-2005. Along with PC index the geoeffective interplanetary electric field Em was also examined. It has been found that all examined storms, lying in range from -30 to -373 nT, started when the PC index and, correspondingly, the Em field firmly exceeded the threshold > 2 mV/m. In particular, the anomalous magnetic storm on January 21-22, 2005 occurring under conditions of northward IMF B-z (Du et al., 2008) is usual phenomena fitted well with the threshold restriction owing to the large IMF By component input. The maximal storm depression (the peak value of Dst) is linearly related to the quantities Em and PC, averaged for the time interval from the storm beginning to the storm maximum. The correlation between Dst and PC is more steady and larger than correlation between Dst and Em, the latter being dependent on Em value (effect of Dst saturation). The moment of the firm descent of the Em and PC quantities below the threshold level similar to 2 mV/m is indicative of the depression damping and transition to the recovery phase. The results are consistent with the similar peculiarities revealed for substorms development (Troshichev and Janzhura, 2009) and support the conclusion that the PC index is a reliable proxy characterizing the solar wind energy having been entered into the magnetosphere. (C) 2011 Elsevier Ltd. All rights reserved.