Several corrections to the model proposed by [23] for Dst have been introduced trying to consider the contributions the dynamic pressure effect and the contribution of other currents (tail and magnetopause currents) that can also affect the magnetic field on the Earth surface [7, 17]. But a substantial modification was that of McPher- ron and O’Brien [26]. They assumed that the parameters involved in the model of Burton et al. have not a constant value, but depended on the dawn-dusk component of solar wind electric field. About the value of the decay time, it has varied from 7.7 hours (assumed by [23]) to a value which decreases in an exponential way as geomagnetic activity increases, varying nearly from the order of 15 hour for low geomagnetic activity to 5 hours for high level of activity.