Statistical investigation of heliospheric conditions resulting in magnetic storms

Time behavior of the solar wind and interplanetary magnetic field parameters is investigated for 623 magnetic storms of the OMNI database for the period 1976-2000. The analysis is carried out by the superposed epoch technique (the magnetic storm onset time is taken to be the beginning of an epoch) for five various categories of storms induced by various types of solar wind: CIR (121 storms), Sheath (22 storms), MC (113 storms), and uncertain type (367 storms). In total, the analysis conducted for all storms included 623 storms. The obtained data, on one hand, confirm the results obtained earlier without selecting the intervals according to the solar wind types, and, on the other hand, they indicate the existence of distinctions in the time variation of parameters for various types of solar wind. Though the lowest values of the B (z)-component of IMF are observed in the MC, the lowest values of the D (st)-index are achieved in the Sheath. Thus, the strongest magnetic storms are induced, on average, during the Sheath rather than during the MC body passage, probably owing to higher pressure in the Sheath. Higher values of nkT, T/T (exp), and beta parameters are observed in the CIR and Sheath and lower ones in the MC, which corresponds to the physical essence of these solar wind types.