Classification and mean behavior of magnetic storms

The Dst index is used to identify more than 1000 storms which occurred in the time interval 1957 to 1993. Using the minimum Dst value as an indicator, we classify the storms as weak (482), moderate (346), strong (206), severe (45), and great (6). For each of these classes the mean time variation is determined. In contrast to the well‐known study of Sugiura and Chapman 1960, the Dst minimum is used as a common reference epoch. This leads to much better agreement between the average and the typical storm behavior. We also find that the maximum ap and AE activity precedes the Dst minimum by 1 to 2 hours. Finally, we demonstrate that both sudden commencement and gradual commencement storms are associated with a distinct decrease in the B z component of the interplanetary magnetic field.