A two-step ring current decay time model during the passive recovery phase of magnetic storms

Abstract

Magnetic storms are caused by changes in the currents flowing near the Earth, particularly by the enhanced ring current. The ring current dynamics is described through the energy balance equation, where a key role is played by the decay time parameter τ . Two kind of models depending on diverse geophysical parameters have been introduced for this important function. One of them, based on the generally observed dependence on Dst, and the other depending on the energy input related to the electric field vB S , where v is the solar wind velocity and B S is the magnitude of the southward component of the interplanetary magnetic field. A comparison of both models and a 1 h-forecast of Dst was done previously. In this work we further analyze them doing a longtime-forecast of the passive recovery phase of storms (almost null injection, vB S ∼ E c = 0.49 mV / m ) through diverse study cases from the OMNI database. We assume that the decay time depends on Dst and Dst-peak because during that phase the energy injection, usually considered as proportional to vB S , ceases. We show that the observed data during the early recovery is better described by the model based on Dst, but during the late recovery is better correlated with an exponential function. This indicates that the whole recovery phase of storms should be described by a two-step model which considers both approaches.