Severe geomagnetic storms and Forbush decreases: interplanetary relationships reexamined

Severe storms (Dst) and Forbush decreases (FD) during cycle 23 showed that maximum negative Dst magnitudes usually occurred almost simultaneously with the maximum negative values of the Bz component of interplanetary magnetic field B, but the maximum magnitudes of negative Dst and Bz were poorly correlated (+0.28). A parameter Bz(CP) was calculated (cumulative partial Bz) as sum of the hourly negative values of Bz from the time of start to the maximum negative value. The correlation of negative Dst maximum with Bz(CP) was higher (+0.59) as compared to that of Dst with Bz alone (+0.28). When the product of Bz with the solar wind speed V (at the hour of negative Bz maximum) was considered, the correlation of negative Dst maximum with V Bz was +0.59 and with V Bz(CP), 0.71. Thus, including V improved the correlations. However, ground-based Dst values have a considerable contribution from magnetopause currents (several tens of nT, even exceeding 100 nT in very severe storms). When their contribution is subtracted from Dst(nT), the residue Dst* representing true ring current effect is much better correlated with Bz and Bz(CP), but not with V Bz or V Bz(CP), indicating that these are unimportant parameters and the effect of V is seen only through the solar wind ram pressure causing magnetopause currents. Maximum negative Dst (or Dst*) did not occur at the same hour as maximum FD. The time evolutions of Dst and FD were very different. The correlations were almost zero. Basically, negative Dst (or Dst*) and FDs are uncorrelated, indicating altogether different mechanism.