An Approach to the Relationship Between Magnetic Clouds and the Recovery Phase of Geomagnetic Storms

Using an elliptical cross-section model for the study of the magnetic topology of magnetic clouds (MCs) in the interplanetary medium, we develop an analytical approach to their relationship with geomagnetic storms. Assuming an axially symmetric ring current and once we have obtained the disturbances produced in its current density due to the passage of a MC through it (whose axis has a latitude θ, a longitude φ, and its cross-section has an orientation ζ), then we determine the decrease in the value of the geomagnetic field at the Earth's equator, i.e., the D _st index. The D _st model presented allows us to estimate the physical parameters which characterize the symmetric ring current during the recovery phase of the storm time. The theoretical and experimental D _st indexes are compared for four intense geomagnetic storms (D _st<−100 nT), all of them associated with MCs. As seen in the figures presented, the fits are good for every storm. In view of these results we conclude that the effects of a MC over the symmetric ring current can explain the main profile of the recovery phase of a geomagnetic storm.