A study of magnetic storms development in two or more steps and its association with the polarity of magnetic clouds

The study of the response of the magnetosphere, measured by the Dst index, to different interplanetary magnetic field configurations observed in magnetic clouds has led us to conclude that about 20% of intense magnetic storms have the growth of their main phases in three or more steps. We also have found that the magnetic clouds with south-to-north magnetic field rotation tend to lead to moderate or intense magnetic storms with a two-step main phase development due to the closer southward magnetic fields in the sheath and in the cloud. On the other hand, magnetic clouds with a north-to-south magnetic field rotation mostly seem to lead to magnetic storms with one-step main phase development, due to the larger separation between the southward magnetic fields in the sheath and in the cloud. Furthermore, the magnetic clouds with a substantial tilt to the ecliptic plane appear to lead to intense magnetic storms when they have a prolonged southward axial field. (C) 2001 Elsevier Science Ltd. All rights reserved.