Solar wind and interplanetary magnetic field features before magnetic storm onset

The presented results, concerning the features of the solar wind plasma structure as observed by spacecraft upstream of Earth, could be used for development of middle-term forecasts of magnetic storms. We have analyzed 1-hour data for 1995-2005 and a whole year of 1-min data during solar minimum (1995) and during solar maximum (2000) with 48 and 60 storms, respectively. The long-term statistical correlations between the solar wind/IMF parameters are found to vary during the solar cycle, and this fact should be taken into account for the prognostic aims. During solar maximum the yearly correlation of V with ground geomagnetic indices drops, and the correlation of N with these indices becomes significant during solar minimum only. Elevated solar wind density enhances statistically the IMF magnitude, but not the IMF Bz component. A remarkably high correlation exists between the low-frequency solar wind plasma turbulence with time scales 4-32 min and the IMF magnitude. It was shown that solar wind dynamic pressure variations are mainly determined not by speed, but density. The density changes play a significant geoeffective role. In many cases magnetic storms with -30 nT< Dst <-100 nT are the result of sharp increases in solar wind density with consequent negative Bz at the background of low and steady solar wind velocity. Besides, about 2 days before ∼80% of magnetic storm commencements a weak increase of density is observed. This increase of the solar wind density is irregular and accompanied by fluctuations with time scales ∼ 2− 120 min, on the average, 2 days prior to storm commencements. The possible mechanisms of these pre-storm solar wind/IMF variations have not been firmly established yet. Thus, variations of the solar wind plasma are a largely underestimated factor in magnetic storm triggering and could be effectively used for space weather forecasting.