Severe geomagnetic storms and Forbush decreases: interplanetary relationships reexamined

Abstract. 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 B_z component of interplanetary magnetic field B, but the maximum magnitudes of negative Dst and B_z were poorly correlated (+0.28). A parameter B_z(CP) was calculated (cumulative partial B_z) as sum of the hourly negative values of B_z from the time of start to the maximum negative value. The correlation of negative Dst maximum with B_z(CP) was higher (+0.59) as compared to that of Dst with B_z alone (+0.28). When the product of B_z with the solar wind speed V (at the hour of negative B_z maximum) was considered, the correlation of negative Dst maximum with VB_z was +0.59 and with VB_z(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 B_z and B_z(CP), but not with VB_z or VB_z(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.