  Records of Dst, Kp and Sym-H indexes are analyzed looking for evidences of
possible precursors for magnetic storms. To accomplish this task the main
magnetic storms were located and some periods immediately before storms and
some periods well before them studied. Statistical properties of both types of
periods were then compared. In particular, were compared the slopes of the
power laws that have been found for the distributions of indexes values. A
systematic deviation was found between both distributions for the Kp index. It
was no so for Dst and Sym-H indexes. For the three indexes it was found a
correlation between slopes and the corresponding storm intensity, which could
serve as a probabilistic approach to magnetic storms forecasting. The data for
the analysis was obtained at the World Data Centre for Geomagnetism at Kyoto.
